- The gut bacteria at birth is mainly Bifidobacterium
- In an adult gut the main species are Bacteroidetes and Firmicutes
- The gut of an obese person often has decreased Bacteroidetes and increased Firmicutes compared to a lean person
- In children, low Bifidobacterium may predispose them to obesity
- Diet plays a big role in the composition of gut bacteria, with differences seen in diets high in animal fats versus plant fats or saturated fats. There is some discrepancy in the results of experiments so further research is required before any conclusions can be made.
- We can support a diverse range of good gut bacteria by adding prebiotics and probiotics to our diet, avoiding excess sugar, and unnecessary antibiotic use
The human gut is comprised of trillions of bacteria. In infants, the main bacteria are Bifidobacterium. Differences have been found depending on mode of birth and early nutrition, with caesarean born and formula fed babies having less Bifidobacterium compared to vaginally born and breastfed infants. The main bacteria in an adult gut belong to Bacteroidetes and Firmicutes. In general studies have shown obese adults have a decreased Bacteroidetes to Firmicutes ratio compared to lean adults, which is attributable to increased Firmicutes in obese people. Changes in diet produce rapid changes in the gut microbiota with animal proteins having a more profound effect than higher fibre plant based diets. A healthy and diverse gut composition can be supported through adding prebiotics, polyphenols and probiotics to the diet, and avoiding excess sugar and unnecessary antibiotic use.
IN THE BEGINNING:
The human microbiota consists of the 10-100 trillion symbiotic microbial cells (bacteria, fungi and viruses) within each person, primarily bacteria in the gut, and the human microbiome comprises the genes inside these cells (1)
Within the adult gut the microbes are dominated by four main phyla of bacteria: Firmicutes (includes genus Lactobacillus), Bacterioidetes (Bacteroides), Actinobacteria (includes genus Bifidobacterium), and Proteobacteria (includes genus Enterobacterium species), which represent more than 95% of the gut flora (2, 3)
When we are born the gut is essentially sterile but colonization starts immediately after birth. The composition of this emerging bacteria (or microflora) is the result of a number of factors, including the mode of delivery, diet, hygiene levels and medication. Babies born via caesarean section typically come into a more sterile environment and have significantly less Bacteroidetes compared to babies born vaginally. It has been shown that the gut flora may be disturbed for up to 6 months if born by caesarean section (4). In breastfed infants Bifidobacterium (Bifidobacteria) are the predominant species, while this is more variable in formula fed infants who have been shown to have much more complex microflora with facultative anaerobes, Bacteroides and Clostridia at higher levels and frequency, and lower levels of Bifidobacterium and Lactobacilli. (5-7). The introduction of solid foods leads to composition similar to the adult gut, particularly by increasing numbers of Firmicutes. It is generally assumed that the gut microbiota after 3 years of age closely resembles that of an adult (8).
HOW DOES THE GUT BACTERIA COMPARE IN LEAN AND OBESE ADULTS?
Overall the two predominant bacterial groups in the adult microbiota are Firmicutes and Bacteroidetes (9, 10).
Decreased Bacteriodetes and increased Firmicutes (a lower Bacteroidetes to Firmicutes ratio) have been found in genetically obese mice compared to lean mice (11- 15). These Firmicutes help the obese mice to draw more calories from the ingested diet which may lead to obesity (16). Studies in mice have also shown that obesity can be induced in lean individuals via faecal transplants from obese individuals (13, 16, 17). Similar ﬁndings were observed with obese people who had less of Bacteroidetes and more of Firmicutes in their gut (18, 19). When placed on either a fat-restricted or carbohydrate-restricted low calorie diet, their abundance of Bacteriodetes increased as their body weight decreased, transitioning from the signature ‘obese’ microbial community to a ‘lean’ community (19).
Overall, obese people have less microbial diversity in comparison with lean people (20) and dietary intervention may improve this diversity and associated clinical phenotypes (21).
WHAT ABOUT CHILDREN?
Obese children have also been found to have more of Firmicutes and less of Bacteroidetes in their gut. In fact, they also had higher short chain fatty acids (SCFAs) that were correlated with the development of obesity (22)
A prospective study in children showed that the risk of being overweight at seven years old could be predicted by the composition of gut microbiota at six months old, which was associated with lower prevalence of Bifidobacterium and higher of Staphylococcus aureus (23).
HOW CAN DIET AFFECT THE MICROFLORA?
A very recent review in the Journal of Translational Medicine in 2017 evaluated the current data regarding the effects of common dietary components on intestinal microbiota (24). They found
that the consumption of particular food types produces predictable changes in existing host bacteria.
Diet-driven changes in the human intestinal microbiota have been shown to occur in as little as three to four days (25). One clinical study induced changes in the microbiota that would be better suited to an entirely animal or plant diet in just five days (26). It has been demonstrated that a diet rich in animal fats and low in fibre administered to mice results in a reduction of the quantity of Bacteroidetes strains and the growth of Firmicutes (27, 28). There is also evidence to show an animal-based diet has a greater impact on the gut microbiota than a plant based diet (26). That study found an animal-based diet increased the abundance of Bacteroidetes and decreased the levels of Firmicutes. They also noted a significant increase in diversity that was unique to the animal-based diet. This change occurred a single day after the diet reached the large intestinal gut microbiota which reverted back to the original composition 2 days after the animal-based diet ended (26).
Different formulations of high-fat diet, with different percentages of saturated and polyunsaturated fatty acids, seem to have different effects on the gut microbiota (29). Feeding mice diets with a higher percentage of saturated fatty acids not only seemed to be associated with worse weight gain and hepatic steatosis, it also induced more profound changes in the microbiota, with a decrease in diversity and an increase in the Firmicutes/Bacterioidetes ratio (30)
SO HOW CAN WE NOURISH GOOD BACTERIA AND MANIPULATE GUT BACTERIA FOR WEIGHT LOSS?
From the evidence gathered thus far it would seem a good goal would be to try and increase Bacteroidetes and decrease firmicutes in the gut. In general, Bacteroidetes need high fibre and Firmicutes need high fat to thrive. But rather than kill off Firmicutes, it may be better to support Bacteriodetes to preserve the diversity of other good bacteria in the gut.
INCLUDE PREBIOTICS IN YOUR DIET
A prebiotic is a ”non-digestible compound that, through its metabolisation by microorganisms in the gut, modulates composition and/or activity of the gut microbiota, thus conferring a beneficial physiological effect on the host” (31). They can be thought of as food for the “good” bacteria in the large intestine. Prebiotic rich foods include all vegetables (especially peas, broccoli, brussel sprouts and artichokes), fruits, beans and oats.
Polyphenols are phytochemicals, meaning compounds found abundantly in natural plant food sources that have antioxidant properties. The best whole food sources of polyphenols are dried spices, fruits, vegetables, red wine, green tea, olive oil, and cocoa. Dark chocolate is another great source. Most polyphenols pass through the small intestine without being absorbed, thus encountering the gut microbiota which colonizes the colon (32). The benefits of this are two fold. First, the gut microbiota act on the polyphenols increasing the bioavailability or amount of active compounds the body can absorb. Secondly, polyphenols modify the composition of the gut microbiota mostly by inhibiting pathogenic (“bad”) bacteria and stimulating beneficial bacteria by acting as a prebiotic (33). Resveratrol, a polyphenol found in high amounts in blueberries, peanuts, red wine and grapes, has been found to increase the ratio of Bacteroidetes to Firmicutes in rats (34).
ADD PROBIOTICS TO YOUR DIET
Probiotics are defined by the World Health Organisation as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host” (35). Some of the beneficial effects of probiotic consumption include improvement of intestinal health by the regulation of microbiota, and stimulation and development of the immune system, synthesizing and enhancing the bioavailability of nutrients, reducing symptoms of lactose intolerance, and reducing the risk of certain other diseases (36).
Probiotics generally come in the form of encapsulated powders or fermented foods. The most common probiotic is Lactobacillus (37). Fermented foods may be defined as those foods or beverages made through controlled microbial growth and enzymatic conversions of major and minor food components (38). Examples include raw sauerkraut, kimchi, kombucha, kefir and yoghurt. The ingestion of fermented foods potentially increases the numbers of microbes in the diet by up to 10 000-fold (39). Consuming ‘living’ fermented foods on a daily basis could be equivalent to introducing new microbes into the intestinal microbiota (40). To get probiotic benefit from a yoghurt it needs to contain bacteria that will survive passage through the intestine as well as refrigeration. Lactobacillus bulgaricus and Streptococcus thermophilus which are used to make yoghurt do not meet this criteria so Lactobacillus acidophilus and Bifidobacterium lactis are added (41). Large cohort investigations have revealed strong associations between consumption of yoghurt and weight maintenance (less weight gain) (42). Evidence is also accumulating for anti-diabetic and anti-obesity benefits of kimchi (43).
EAT LESS SUGAR
Firmicutes feed off sugar (44) so it makes sense to reduce sugar to reduce firmicutes in the gut. In rats, Bacteroidetes, Lactobacillus and Bifidobacterium have been shown to be elevated by sugar (45).
TAKE ANTIBIOTICS ONLY WHEN NECESSARY
Antibiotics kill many bad and good bacteria in the gut microbiome, possibly contributing to weight gain and antibiotic resistance. Repeated use of antibiotics, particularly broader-spectrum drugs, at younger than 24 months old is a risk factor for later obesity in children (46).
There is much more research to be carried out in the field of microbiota and obesity, and the impact diet can have on this relationship. There is the potential to avoid obesity in predisposed individuals from a young age. Overall the future looks promising that we could modify an individual’s unique microbiome through faecal transplants, diet and supplements, to make weight loss and management easier.
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