The Impact of Diet and Lifestyle on Gut Microbiota and Human Health
Added on 2023-06-07
28 Pages18639 Words305 Views
Nutrients 2015, 7, 17-44; doi:10.3390/nu7010017
nutrients
ISSN 2072-6643
www.mdpi.com/journal/nutrients
Review
The Impact of Diet and Lifestyle on Gut Microbiota and
Human Health
Michael A. Conlon * and Anthony R. Bird
CSIRO Food and Nutrition Flagship, Kintore Ave, Adelaide, SA 5000, Australia;
E-Mail: tony.bird@csiro.au
* Author to whom correspondence should be addressed; E-Mail: michael.conlon@csiro.au;
Tel.: +61-8-8303-8909; Fax: +61-8-8303-8899.
Received: 17 September 2014 / Accepted: 9 December 2014 / Published: 24 December 2014
Abstract: There is growing recognition of the role of diet and other environmental factors
in modulating the composition and metabolic activity of the human gut microbiota, which in
turn can impact health. This narrative review explores the relevant contemporary scientific
literature to provide a general perspective of this broad area. Molecular technologies have
greatly advanced our understanding of the complexity and diversity of the gut microbial
communities within and between individuals. Diet, particularly macronutrients, has a major
role in shaping the composition and activity of these complex populations. Despite the body
of knowledge that exists on the effects of carbohydrates there are still many unanswered
questions. The impacts of dietary fats and protein on the gut microbiota are less well defined.
Both short- and long-term dietary change can influence the microbial profiles, and infant
nutrition may have life-long consequences through microbial modulation of the immune
system. The impact of environmental factors, including aspects of lifestyle, on the microbiota
is particularly poorly understood but some of these factors are described. We also discuss
the use and potential benefits of prebiotics and probiotics to modify microbial populations.
A description of some areas that should be addressed in future research is also presented.
Keywords: diet; lifestyle; gut; microbiota; health
OPEN ACCESS
nutrients
ISSN 2072-6643
www.mdpi.com/journal/nutrients
Review
The Impact of Diet and Lifestyle on Gut Microbiota and
Human Health
Michael A. Conlon * and Anthony R. Bird
CSIRO Food and Nutrition Flagship, Kintore Ave, Adelaide, SA 5000, Australia;
E-Mail: tony.bird@csiro.au
* Author to whom correspondence should be addressed; E-Mail: michael.conlon@csiro.au;
Tel.: +61-8-8303-8909; Fax: +61-8-8303-8899.
Received: 17 September 2014 / Accepted: 9 December 2014 / Published: 24 December 2014
Abstract: There is growing recognition of the role of diet and other environmental factors
in modulating the composition and metabolic activity of the human gut microbiota, which in
turn can impact health. This narrative review explores the relevant contemporary scientific
literature to provide a general perspective of this broad area. Molecular technologies have
greatly advanced our understanding of the complexity and diversity of the gut microbial
communities within and between individuals. Diet, particularly macronutrients, has a major
role in shaping the composition and activity of these complex populations. Despite the body
of knowledge that exists on the effects of carbohydrates there are still many unanswered
questions. The impacts of dietary fats and protein on the gut microbiota are less well defined.
Both short- and long-term dietary change can influence the microbial profiles, and infant
nutrition may have life-long consequences through microbial modulation of the immune
system. The impact of environmental factors, including aspects of lifestyle, on the microbiota
is particularly poorly understood but some of these factors are described. We also discuss
the use and potential benefits of prebiotics and probiotics to modify microbial populations.
A description of some areas that should be addressed in future research is also presented.
Keywords: diet; lifestyle; gut; microbiota; health
OPEN ACCESS
Nutrients 2015, 7 18
1. Introduction
There are approximately 10 times as many microorganisms within the gastro-intestinal (GI) tract of
humans (approximately 100 trillion) as there are somatic cells within the body. While most of the
microbes are bacteria, the gut can also harbor yeasts, single-cell eukaryotes, viruses and small parasitic
worms. The number, type and function of microbes vary along the length of the GI tract but the majority
is found within the large bowel where they contribute to the fermentation of undigested food components,
especially carbohydrates/fiber, and to fecal bulk. Some of the most commonly found or recognized genera
of gut bacteria in adults are Bifidobacterium, Lactobacillus, Bacteroides, Clostridium, Escherichia,
Streptococcus and Ruminococcus. Approximately 60% of the bacteria belong to the Bacteroidetes or
Firmicutes phyla [1]. Microbes which produce methane have been detected in about 50% of individuals
and are classified as Archaea and not bacteria [2]. Although individuals may have up to several hundred
species of microbes within their gut, recent findings from The Human Microbiome Project and
others [3,4] show that thousands of different microbes may inhabit the gut of human populations
collectively and confirm a high degree of variation in the composition of these populations between
individuals. Despite this variation in taxa the abundance of many of the microbial genes for basic or
house-keeping metabolic activities are quite similar between individuals [3]. There is growing evidence
that imbalances in gut microbial populations can be associated with disease, including inflammatory
bowel disease (IBD) [5], and could be contributing factors. Consequently, there is increased awareness
of the role of the microbiota in maintaining health and significant research and commercial investment
in this area. Gut microbes produce a large number of bioactive compounds that can influence health;
some like vitamins are beneficial, but some products are toxic. Host immune defenses along the intestine,
including a mucus barrier, help prevent potentially harmful bacteria from causing damage to tissues. The
maintenance of a diverse and thriving population of beneficial gut bacteria helps to keep harmful bacteria
at bay by competing for nutrients and sites of colonization. Dietary means, particularly the use of a range
of fibers, may be the best way of maintaining a healthy gut microbiota population. Strategies such as
ingestion of live beneficial bacteria (probiotics) may also assist in maintaining health. In this review, we
will expand upon these subjects relating to diet and lifestyle, the gut microbiota and health, and provide
some indication of opportunities and knowledge gaps in this area.
2. Microbial Products that Impact Health—Beneficial and Harmful
Microbial mass is a significant contributor to fecal bulk, which in turn is an important determinant of
bowel health. Consumption of dietary fibers reduces the risk of colorectal cancer (CRC) [6] at least
partly as a consequence of dilution and elimination of toxins through fecal bulk, driven by increases in
fermentative bacteria and the presence of water-holding fibers [7–9]. Aspects of this will be discussed
in more detail later in the review.
Gut microbes are capable of producing a vast range of products, the generation of which can be
dependent on many factors, including nutrient availability and the luminal environment, particularly
pH [10]. A more in-depth review of gut microbial products can be found elsewhere [11]. Microbial
products can be taken up by GI tissues, potentially reach circulation and other tissues, and be excreted
in urine or breath. Fermentation of fiber and protein by large bowel bacteria results in some of the most
1. Introduction
There are approximately 10 times as many microorganisms within the gastro-intestinal (GI) tract of
humans (approximately 100 trillion) as there are somatic cells within the body. While most of the
microbes are bacteria, the gut can also harbor yeasts, single-cell eukaryotes, viruses and small parasitic
worms. The number, type and function of microbes vary along the length of the GI tract but the majority
is found within the large bowel where they contribute to the fermentation of undigested food components,
especially carbohydrates/fiber, and to fecal bulk. Some of the most commonly found or recognized genera
of gut bacteria in adults are Bifidobacterium, Lactobacillus, Bacteroides, Clostridium, Escherichia,
Streptococcus and Ruminococcus. Approximately 60% of the bacteria belong to the Bacteroidetes or
Firmicutes phyla [1]. Microbes which produce methane have been detected in about 50% of individuals
and are classified as Archaea and not bacteria [2]. Although individuals may have up to several hundred
species of microbes within their gut, recent findings from The Human Microbiome Project and
others [3,4] show that thousands of different microbes may inhabit the gut of human populations
collectively and confirm a high degree of variation in the composition of these populations between
individuals. Despite this variation in taxa the abundance of many of the microbial genes for basic or
house-keeping metabolic activities are quite similar between individuals [3]. There is growing evidence
that imbalances in gut microbial populations can be associated with disease, including inflammatory
bowel disease (IBD) [5], and could be contributing factors. Consequently, there is increased awareness
of the role of the microbiota in maintaining health and significant research and commercial investment
in this area. Gut microbes produce a large number of bioactive compounds that can influence health;
some like vitamins are beneficial, but some products are toxic. Host immune defenses along the intestine,
including a mucus barrier, help prevent potentially harmful bacteria from causing damage to tissues. The
maintenance of a diverse and thriving population of beneficial gut bacteria helps to keep harmful bacteria
at bay by competing for nutrients and sites of colonization. Dietary means, particularly the use of a range
of fibers, may be the best way of maintaining a healthy gut microbiota population. Strategies such as
ingestion of live beneficial bacteria (probiotics) may also assist in maintaining health. In this review, we
will expand upon these subjects relating to diet and lifestyle, the gut microbiota and health, and provide
some indication of opportunities and knowledge gaps in this area.
2. Microbial Products that Impact Health—Beneficial and Harmful
Microbial mass is a significant contributor to fecal bulk, which in turn is an important determinant of
bowel health. Consumption of dietary fibers reduces the risk of colorectal cancer (CRC) [6] at least
partly as a consequence of dilution and elimination of toxins through fecal bulk, driven by increases in
fermentative bacteria and the presence of water-holding fibers [7–9]. Aspects of this will be discussed
in more detail later in the review.
Gut microbes are capable of producing a vast range of products, the generation of which can be
dependent on many factors, including nutrient availability and the luminal environment, particularly
pH [10]. A more in-depth review of gut microbial products can be found elsewhere [11]. Microbial
products can be taken up by GI tissues, potentially reach circulation and other tissues, and be excreted
in urine or breath. Fermentation of fiber and protein by large bowel bacteria results in some of the most
Nutrients 2015, 7 19
abundant and physiologically important products, namely short chain fatty acids (SCFA) which act as
key sources of energy for colorectal tissues and bacteria, and promote cellular mechanisms that maintain
tissue integrity [12–14]. SCFA can reach the circulation and impact immune function and inflammation
in tissues such as the lung [15]. However, some protein fermentation products such as ammonia, phenols
and hydrogen sulphide can also be toxic. There are many other products which deserve mention for their
influence on health. Bacteria such as Bifidobacterium can generate vitamins (e.g., K, B12 , Biotin, Folate,
Thiamine) [11]. Synthesis of secondary bile acids, important components of lipid transport and turnover
in humans, is mediated via bacteria, including Lactobacillus, Bifidobacterium and Bacteroides [11].
Numerous lipids with biological activity are produced by bacteria, including lipopolysaccharide (LPS),
a component of the cell wall of gram negative bacteria that can cause tissue inflammation [16]. Also,
many enteropathogenic bacteria (e.g., some E. coli strains) can produce toxins or cause diahorrea under
the right conditions, but under normal circumstances other non-pathogenic commensal bacteria with
similar metabolic activities outcompete and eventually eliminate them [17]. Bacteria such as
Bifidobacterium can also help prevent pathogenic infection through production of acetate [18].
Many enzymes produced by microbes influence digestion and health. Indeed, much of the microbial
diversity in the human gut may be attributable to the spectrum of microbial enzymatic capacity needed
to degrade nutrients, particularly the many forms of complex polysaccharides that are consumed by
humans [19]. Some bacteria such as Bacteroides thetaiotamicron have the capacity to produce an array
of enzymes needed for carbohydrate breakdown [20], but in general numerous microbes appear to be
required in a step-wise breakdown and use of complex substrates. Bacterial phytases of the large intestine
degrade phytic acid present in grains, releasing minerals such as calcium, magnesium and phosphate that
are complexed with it [21], making these available to host tissues (e.g., bone). Enzymes which degrade
mucins help bacteria meet their energy needs and assist in the normal turnover of the mucus barrier
lining the gut.
Competition between bacteria for substrates has a significant influence on which products are
generated. Hydrogen is used by many bacteria and there is a hydrogen economy within the gut based
around production by some bacteria and its use by others, including methanogens and sulphate-reducing
bacteria (SRB) [22,23]. The use of hydrogen for production of methane by methanogenic Archaea may
limit acetate production by other microbes, thereby potentially limiting production of beneficial butyrate
and impacting health [2,23]. The role of methanogens in health is not yet clear. Breath methane correlates
with levels of constipation in irritable bowel syndrome (IBS) [24] but methanogens numbers are depleted
in IBD [2].
Production of gases such as methane, hydrogen, hydrogen sulphide and carbon dioxide is associated
with digestion and fermentation within the GI tract. While excess production may cause GI problems
such as bloating and pain, the gases may serve useful purposes. However, there is debate over whether
hydrogen sulphide is largely beneficial or detrimental [23].
There is a strong interaction between the host immune system and the microbiota, with both producing
compounds that influence the other. Some bacteria such as the key butyrate-producer Faecalibacterium
prausnitzii may produce anti-inflammatory compounds [25]. Microbes also produce substances that
allow communication between each other.
abundant and physiologically important products, namely short chain fatty acids (SCFA) which act as
key sources of energy for colorectal tissues and bacteria, and promote cellular mechanisms that maintain
tissue integrity [12–14]. SCFA can reach the circulation and impact immune function and inflammation
in tissues such as the lung [15]. However, some protein fermentation products such as ammonia, phenols
and hydrogen sulphide can also be toxic. There are many other products which deserve mention for their
influence on health. Bacteria such as Bifidobacterium can generate vitamins (e.g., K, B12 , Biotin, Folate,
Thiamine) [11]. Synthesis of secondary bile acids, important components of lipid transport and turnover
in humans, is mediated via bacteria, including Lactobacillus, Bifidobacterium and Bacteroides [11].
Numerous lipids with biological activity are produced by bacteria, including lipopolysaccharide (LPS),
a component of the cell wall of gram negative bacteria that can cause tissue inflammation [16]. Also,
many enteropathogenic bacteria (e.g., some E. coli strains) can produce toxins or cause diahorrea under
the right conditions, but under normal circumstances other non-pathogenic commensal bacteria with
similar metabolic activities outcompete and eventually eliminate them [17]. Bacteria such as
Bifidobacterium can also help prevent pathogenic infection through production of acetate [18].
Many enzymes produced by microbes influence digestion and health. Indeed, much of the microbial
diversity in the human gut may be attributable to the spectrum of microbial enzymatic capacity needed
to degrade nutrients, particularly the many forms of complex polysaccharides that are consumed by
humans [19]. Some bacteria such as Bacteroides thetaiotamicron have the capacity to produce an array
of enzymes needed for carbohydrate breakdown [20], but in general numerous microbes appear to be
required in a step-wise breakdown and use of complex substrates. Bacterial phytases of the large intestine
degrade phytic acid present in grains, releasing minerals such as calcium, magnesium and phosphate that
are complexed with it [21], making these available to host tissues (e.g., bone). Enzymes which degrade
mucins help bacteria meet their energy needs and assist in the normal turnover of the mucus barrier
lining the gut.
Competition between bacteria for substrates has a significant influence on which products are
generated. Hydrogen is used by many bacteria and there is a hydrogen economy within the gut based
around production by some bacteria and its use by others, including methanogens and sulphate-reducing
bacteria (SRB) [22,23]. The use of hydrogen for production of methane by methanogenic Archaea may
limit acetate production by other microbes, thereby potentially limiting production of beneficial butyrate
and impacting health [2,23]. The role of methanogens in health is not yet clear. Breath methane correlates
with levels of constipation in irritable bowel syndrome (IBS) [24] but methanogens numbers are depleted
in IBD [2].
Production of gases such as methane, hydrogen, hydrogen sulphide and carbon dioxide is associated
with digestion and fermentation within the GI tract. While excess production may cause GI problems
such as bloating and pain, the gases may serve useful purposes. However, there is debate over whether
hydrogen sulphide is largely beneficial or detrimental [23].
There is a strong interaction between the host immune system and the microbiota, with both producing
compounds that influence the other. Some bacteria such as the key butyrate-producer Faecalibacterium
prausnitzii may produce anti-inflammatory compounds [25]. Microbes also produce substances that
allow communication between each other.
Nutrients 2015, 7 20
3. Lifestage and Lifetstyle Impacts on the Microbiota and the Influence of Nutrition
3.1. Lifestage
Microbes colonise the human gut during or shortly after birth. The fact that babies delivered naturally
have higher gut bacterial counts at 1 month of age than those delivered by caesarean section [26] suggests
gut colonization by microbes begins during, and is enhanced by, natural birth. The growth and
development of a robust gut microbiota is important for the development of the immune system [27] and
continues during breast-feeding, a stage which seems to be important for the long-term health of the
individual. Oligosaccharides present in breast milk promote the growth of Lactobacillus and
Bifidobacterium, which dominate the infant gut, and this can strengthen or promote development of the
immune system and may help prevent conditions such as eczema and asthma [28–30]. These bacteria
are undetectable in the stool of preterm infants in their first weeks of life [31]. A significant shift in the
populations of gut microbes occurs when infants switch to a more solid and varied diet, including a
decline in populations of Lactobacillus and Bifidobacterium to only a small percentage of the large bowel
microbiota [32]. A wide diversity of microorganisms is needed to utilize the many fibers and other
nutrients present in adult diets [19,33]. Functional maturation of the human microbiota, including the
capacity to produce vitamins, increases during the early years of life [34].
The complexities and variability of adult gut microbial populations have become increasingly evident
in recent years. The variability may relate to the influence of numerous factors, including diet and host
genetics. The composition and activity of gut bacteria can vary according to (and possibly a result of)
life events, including puberty, ovarian cycle, pregnancy and menopause [11]. The diets of children being
weaned may have particular influence on microbial diversity in later life. Another broad shift in gut
microbe populations occurs with age. The Bacteroidetes phylum bacteria tend to dominate numerically
during youth but numbers decline significantly by old age, whereas the reverse trend occurs for bacteria
of the Firmicutes phylum [11]. The consequences and reason for this change are not yet clear. However,
the gut microbiota profiles of the elderly may not be optimal. One study found a high prevalence of
potentially toxic Clostridium perfringens and lower numbers of Bifidobacterium and Lactobacillus in
those in long-term care [35]. The latter also have a reduced microbial diversity compared to the elderly
living in the community and this is related to increased frailty and changes in nutrition [36].
3.2. Lifestyle
The impact of non-dietary lifestyle factors on the gut microbiota has been largely ignored. Smoking
and lack of exercise can significantly impact the large bowel (and potentially the microbiota) as they are
risk factors for CRC [37]. Indeed, smoking has a significant influence on gut microbiota composition,
increasing Bacteroides-Prevotella in individuals with Crohn’s Disease (CD) and healthy individuals [38].
Smoking-induced changes in microbial populations could potentially contribute to increased risk of CD.
Air-borne toxic particles can reach the large bowel via mucociliary clearance from the lungs, and
increased environmental pollution associated with industrialization could contribute to concomitant
increases in IBD cases [39].
Another lifestyle factor, stress, has an impact on colonic motor activity via the gut-brain axis which
can alter gut microbiota profiles, including lower numbers of potentially beneficial Lactobacillus [40].
3. Lifestage and Lifetstyle Impacts on the Microbiota and the Influence of Nutrition
3.1. Lifestage
Microbes colonise the human gut during or shortly after birth. The fact that babies delivered naturally
have higher gut bacterial counts at 1 month of age than those delivered by caesarean section [26] suggests
gut colonization by microbes begins during, and is enhanced by, natural birth. The growth and
development of a robust gut microbiota is important for the development of the immune system [27] and
continues during breast-feeding, a stage which seems to be important for the long-term health of the
individual. Oligosaccharides present in breast milk promote the growth of Lactobacillus and
Bifidobacterium, which dominate the infant gut, and this can strengthen or promote development of the
immune system and may help prevent conditions such as eczema and asthma [28–30]. These bacteria
are undetectable in the stool of preterm infants in their first weeks of life [31]. A significant shift in the
populations of gut microbes occurs when infants switch to a more solid and varied diet, including a
decline in populations of Lactobacillus and Bifidobacterium to only a small percentage of the large bowel
microbiota [32]. A wide diversity of microorganisms is needed to utilize the many fibers and other
nutrients present in adult diets [19,33]. Functional maturation of the human microbiota, including the
capacity to produce vitamins, increases during the early years of life [34].
The complexities and variability of adult gut microbial populations have become increasingly evident
in recent years. The variability may relate to the influence of numerous factors, including diet and host
genetics. The composition and activity of gut bacteria can vary according to (and possibly a result of)
life events, including puberty, ovarian cycle, pregnancy and menopause [11]. The diets of children being
weaned may have particular influence on microbial diversity in later life. Another broad shift in gut
microbe populations occurs with age. The Bacteroidetes phylum bacteria tend to dominate numerically
during youth but numbers decline significantly by old age, whereas the reverse trend occurs for bacteria
of the Firmicutes phylum [11]. The consequences and reason for this change are not yet clear. However,
the gut microbiota profiles of the elderly may not be optimal. One study found a high prevalence of
potentially toxic Clostridium perfringens and lower numbers of Bifidobacterium and Lactobacillus in
those in long-term care [35]. The latter also have a reduced microbial diversity compared to the elderly
living in the community and this is related to increased frailty and changes in nutrition [36].
3.2. Lifestyle
The impact of non-dietary lifestyle factors on the gut microbiota has been largely ignored. Smoking
and lack of exercise can significantly impact the large bowel (and potentially the microbiota) as they are
risk factors for CRC [37]. Indeed, smoking has a significant influence on gut microbiota composition,
increasing Bacteroides-Prevotella in individuals with Crohn’s Disease (CD) and healthy individuals [38].
Smoking-induced changes in microbial populations could potentially contribute to increased risk of CD.
Air-borne toxic particles can reach the large bowel via mucociliary clearance from the lungs, and
increased environmental pollution associated with industrialization could contribute to concomitant
increases in IBD cases [39].
Another lifestyle factor, stress, has an impact on colonic motor activity via the gut-brain axis which
can alter gut microbiota profiles, including lower numbers of potentially beneficial Lactobacillus [40].
Nutrients 2015, 7 21
Stress may contribute to IBS, one of the most common functional bowel disorders, and the associated
changes in microbial populations via the central nervous system (CNS). The gut-brain axis is bi-directional,
involving both hormonal and neuronal pathways [41], and so changes in the gut microbiota may influence
brain activity, including mood [42]. Autism, a neurodevelopmental disorder, is associated with significant
shifts in gut microbiota populations [43–45].
Obesity is associated with excess energy intakes and sedentary lifestyles. Exercise (or rather a
lack of it) may be an important influence on any shifts in microbial populations that are associated
with obesity. This is highlighted by a recent study that showed an increase in the diversity of gut
microbial populations in professional athletes in response to exercise and the associated diet [46].
In humans and animal models with obesity, shifts in gut microbial populations occur, with increases in
the Firmicutes and decreases in the Bacteroidetes, which could potentially contribute to adiposity through
greater energy harvest [47–49]. However, other data suggests the shifts in microbial populations are
driven primarily by the high fat obesogenic diets [50,51]. Irrespective of the cause, there are associated
increases in gut bacteria linked with poor health outcomes (e.g., Staphylococcus, E. coli,
Enterobacteriaceae) [52,53]. Dietary saturated fats may increase numbers of pro-inflammatory gut
microbes by stimulating the formation of taurine-conjugated bile acids that promotes growth of these
pathogens [54].
Geography also has a strong bearing on the composition of gut microbial populations. The diversity
of fecal microbes in children from rural Africa is greater than that of children of developed communities
in the EU, as is the number of bacteria associated with breakdown of fiber [55], suggesting dietary
differences contributes significantly to the microbial differences. In another study, the type of fecal
bacteria and their functional genes differed between individuals in the USA and in rural areas of
Venezuela and Malawi [34].
Other environmental factors may also influence health via gut microbes. Travel, particularly to
overseas destinations, increases the risk of contracting and spreading infectious diseases, including those
causing diarrhoea. Some infections may go undiagnosed but result in long-term GI problems, including
IBS [56]. Poor sanitary conditions in developing countries, and poor personal hygiene, can facilitate the
spread of infectious agents. Circadian disorganization, occurring because of travel, shift work or other
reasons, also impacts gut health and alters gut microbial populations [57].
4. Impacts of Macronutrients on the Gut Microbiota and Relevance to Health
4.1. Substrate Supply to the Colonic Microbiota
An adult colon contains approximately 500 g of contents, most of which is bacteria [58], and
about 100 g/day is voided as stool. A typical western type diet supplies the colonic microbiota with
about 50 g daily of potentially fermentable substrate, predominantly dietary fiber (DF). Non-starch
polysaccharides (NSP) are major components of DF and account for 20%–45% of the dry matter
supplied to the colon. Simple sugars and oligosaccharides each represent a further 10% whereas starch
(and starch hydrolysis products) supplies less than 8% of dry matter. Some sugar alcohols also escape
small intestine (SI) absorption and are minor dietary substrates for the colonic microbiota [59].
About 5–15 g of protein and 5–10 g of lipid passes into the proximal colon daily, largely of dietary
Stress may contribute to IBS, one of the most common functional bowel disorders, and the associated
changes in microbial populations via the central nervous system (CNS). The gut-brain axis is bi-directional,
involving both hormonal and neuronal pathways [41], and so changes in the gut microbiota may influence
brain activity, including mood [42]. Autism, a neurodevelopmental disorder, is associated with significant
shifts in gut microbiota populations [43–45].
Obesity is associated with excess energy intakes and sedentary lifestyles. Exercise (or rather a
lack of it) may be an important influence on any shifts in microbial populations that are associated
with obesity. This is highlighted by a recent study that showed an increase in the diversity of gut
microbial populations in professional athletes in response to exercise and the associated diet [46].
In humans and animal models with obesity, shifts in gut microbial populations occur, with increases in
the Firmicutes and decreases in the Bacteroidetes, which could potentially contribute to adiposity through
greater energy harvest [47–49]. However, other data suggests the shifts in microbial populations are
driven primarily by the high fat obesogenic diets [50,51]. Irrespective of the cause, there are associated
increases in gut bacteria linked with poor health outcomes (e.g., Staphylococcus, E. coli,
Enterobacteriaceae) [52,53]. Dietary saturated fats may increase numbers of pro-inflammatory gut
microbes by stimulating the formation of taurine-conjugated bile acids that promotes growth of these
pathogens [54].
Geography also has a strong bearing on the composition of gut microbial populations. The diversity
of fecal microbes in children from rural Africa is greater than that of children of developed communities
in the EU, as is the number of bacteria associated with breakdown of fiber [55], suggesting dietary
differences contributes significantly to the microbial differences. In another study, the type of fecal
bacteria and their functional genes differed between individuals in the USA and in rural areas of
Venezuela and Malawi [34].
Other environmental factors may also influence health via gut microbes. Travel, particularly to
overseas destinations, increases the risk of contracting and spreading infectious diseases, including those
causing diarrhoea. Some infections may go undiagnosed but result in long-term GI problems, including
IBS [56]. Poor sanitary conditions in developing countries, and poor personal hygiene, can facilitate the
spread of infectious agents. Circadian disorganization, occurring because of travel, shift work or other
reasons, also impacts gut health and alters gut microbial populations [57].
4. Impacts of Macronutrients on the Gut Microbiota and Relevance to Health
4.1. Substrate Supply to the Colonic Microbiota
An adult colon contains approximately 500 g of contents, most of which is bacteria [58], and
about 100 g/day is voided as stool. A typical western type diet supplies the colonic microbiota with
about 50 g daily of potentially fermentable substrate, predominantly dietary fiber (DF). Non-starch
polysaccharides (NSP) are major components of DF and account for 20%–45% of the dry matter
supplied to the colon. Simple sugars and oligosaccharides each represent a further 10% whereas starch
(and starch hydrolysis products) supplies less than 8% of dry matter. Some sugar alcohols also escape
small intestine (SI) absorption and are minor dietary substrates for the colonic microbiota [59].
About 5–15 g of protein and 5–10 g of lipid passes into the proximal colon daily, largely of dietary
Nutrients 2015, 7 22
origin. Various other minor dietary constituents, including polyphenols, catechins, lignin, tannins and
micronutrients also nourish colonic microbes. About 90% of the approximately 1 g/day of dietary
polyphenols escapes digestion and absorption in the SI [60,61] and can have significant influence on
microbial populations and activities [62–64].
4.2. Carbohydrates—Importance for Large Bowel Fermentation and Health
Carbohydrates are the principal carbon and energy source for colonic microbes. Collectively,
they have an immense capacity to hydrolyse a vast range of these nutrients, especially complex
polysaccharides [65].
DF is integral to a healthy diet and Australian adults consume ~27 g each day [66], which is greater
than in other high income countries, including the USA (<20 g/day). Epidemiological and experimental
studies show that DF is both preventative and therapeutic for many large bowel disorders and other
conditions or diseases, including cardiovascular diseases, type II diabetes and obesity [67–71].
One mechanism by which fiber promotes and maintains bowel health is through increasing digesta
mass. Incompletely fermented fiber (e.g., insoluble NSP such as cellulose), increases digesta mass
primarily though its physical presence and ability to adsorb water. An increase in digesta mass dilutes
toxins, reduces intracolonic pressure, shortens transit time and increases defecation frequency. Fibers
can also increase fecal mass to a lesser degree by stimulating fermentation, which leads to bacterial
proliferation and increased biomass [7].
Many of the health benefits ascribed to fiber are a consequence of their fermentation by the colonic
microbiota and the metabolites that are produced. Carbohydrates are fermented to organic acids that
provide energy for other bacteria, the bowel epithelium and peripheral tissues. SCFA are the major
endproducts of carbohydrate fermentation. These weak acids (pKa ~4.8) help lower the pH within the
colon thereby inhibiting the growth and activity of pathogenic bacteria. Other minor organic acids
produced include lactate, succinate and formate. Branched-chain SCFA (e.g., isobutyrate and isovalerate)
results from fermentation of branched chain amino acids [72].
There are spatial gradients in microorganisms along the length of the gut. Bacterial growth and
metabolic activity (fermentation) is greatest in the proximal colon where substrate availability is at a
maximum [13,73]. Accordingly, pH progressively increases as stool progresses from the proximal to
distal colon (from 5.8 to 7.0–7.5), largely because of the progressive depletion of carbohydrate substrates
and absorption of SCFA, and increasing efficiency of protein fermentation and production of alkaline
metabolites [72]. Total SCFA concentrations are highest in the proximal colon (~100 mM) and decline
progressively toward the distal colon. Acetate, propionate and butyrate are the major individual SCFA,
accounting for 90% of the total, with molar ratios approximating 65:20:15 [74].
Butyrate has attracted significant attention because it serves as the principal source of metabolic
energy for the colonocytes [75], is instrumental in maintaining mucosal integrity, modulates intestinal
inflammation and promotes genomic stability. The capacity of butyrate to regulate colonocyte
differentiation and apoptosis, promoting removal of dysfunctional cells, underscores its potential to
protect against colon cancer [76].
The SCFA also have roles beyond the gut and may improve risk of metabolic and immune system
diseases and disorders, such as osteoarthritis, obesity, type II diabetes and cardiovascular disease [13,76].
origin. Various other minor dietary constituents, including polyphenols, catechins, lignin, tannins and
micronutrients also nourish colonic microbes. About 90% of the approximately 1 g/day of dietary
polyphenols escapes digestion and absorption in the SI [60,61] and can have significant influence on
microbial populations and activities [62–64].
4.2. Carbohydrates—Importance for Large Bowel Fermentation and Health
Carbohydrates are the principal carbon and energy source for colonic microbes. Collectively,
they have an immense capacity to hydrolyse a vast range of these nutrients, especially complex
polysaccharides [65].
DF is integral to a healthy diet and Australian adults consume ~27 g each day [66], which is greater
than in other high income countries, including the USA (<20 g/day). Epidemiological and experimental
studies show that DF is both preventative and therapeutic for many large bowel disorders and other
conditions or diseases, including cardiovascular diseases, type II diabetes and obesity [67–71].
One mechanism by which fiber promotes and maintains bowel health is through increasing digesta
mass. Incompletely fermented fiber (e.g., insoluble NSP such as cellulose), increases digesta mass
primarily though its physical presence and ability to adsorb water. An increase in digesta mass dilutes
toxins, reduces intracolonic pressure, shortens transit time and increases defecation frequency. Fibers
can also increase fecal mass to a lesser degree by stimulating fermentation, which leads to bacterial
proliferation and increased biomass [7].
Many of the health benefits ascribed to fiber are a consequence of their fermentation by the colonic
microbiota and the metabolites that are produced. Carbohydrates are fermented to organic acids that
provide energy for other bacteria, the bowel epithelium and peripheral tissues. SCFA are the major
endproducts of carbohydrate fermentation. These weak acids (pKa ~4.8) help lower the pH within the
colon thereby inhibiting the growth and activity of pathogenic bacteria. Other minor organic acids
produced include lactate, succinate and formate. Branched-chain SCFA (e.g., isobutyrate and isovalerate)
results from fermentation of branched chain amino acids [72].
There are spatial gradients in microorganisms along the length of the gut. Bacterial growth and
metabolic activity (fermentation) is greatest in the proximal colon where substrate availability is at a
maximum [13,73]. Accordingly, pH progressively increases as stool progresses from the proximal to
distal colon (from 5.8 to 7.0–7.5), largely because of the progressive depletion of carbohydrate substrates
and absorption of SCFA, and increasing efficiency of protein fermentation and production of alkaline
metabolites [72]. Total SCFA concentrations are highest in the proximal colon (~100 mM) and decline
progressively toward the distal colon. Acetate, propionate and butyrate are the major individual SCFA,
accounting for 90% of the total, with molar ratios approximating 65:20:15 [74].
Butyrate has attracted significant attention because it serves as the principal source of metabolic
energy for the colonocytes [75], is instrumental in maintaining mucosal integrity, modulates intestinal
inflammation and promotes genomic stability. The capacity of butyrate to regulate colonocyte
differentiation and apoptosis, promoting removal of dysfunctional cells, underscores its potential to
protect against colon cancer [76].
The SCFA also have roles beyond the gut and may improve risk of metabolic and immune system
diseases and disorders, such as osteoarthritis, obesity, type II diabetes and cardiovascular disease [13,76].
End of preview
Want to access all the pages? Upload your documents or become a member.
Related Documents
Short-Chain Fatty Acids as Key Bacterial Metaboliteslg...
|14
|14145
|378
Advantages of Microbiotalg...
|5
|1218
|289
Diets and Intestinal Health : Assignmentlg...
|5
|1132
|244
Effect of Refined Carbohydrates on Gut Microbiotalg...
|14
|3115
|39
Microbiology Critique Paper Essay 2022lg...
|5
|1035
|22