Does your craving for chocolate come from messages from the microbes in your GI tract? Do they secretly hijack your willpower to convince you to eat what might be best for them, but not for you?
The determination to survive (and ultimately, reproduce) drives much of our and other animals’ behavior. National Geographic specials reinforce this concept nightly. But it may be less apparent that similar pressures exist in the microbial world. Microbes can impact host behavior: some probiotics can reduce anxiety and lower pain perception; certain microbial population patterns are associated with altered taste receptors; and some microbes produce neurochemicals that mimic hormones involved in mood and behavior.
Alcock et al. (2014) wrote a provocative essay hypothesizing that microbes in the gastrointestinal tract manipulate their host’s eating behavior to increase their chances of survival. And they may do this at the expense of their host. Microbes may manipulate host eating behavior by influencing satiety, mood and taste perception. They may generate a state of ‘dissatisfaction’ in the host that is resolved once we eat foods that increase the microbe’s survival.
What are the implications?
This is still a hypothesis and much basic science is needed to determine if microbes do indeed change eating behavior. But if they do, perhaps manipulation of the gut microbiota may improve the situation. Of course, microbial influence over our eating behavior may have positive effects, too.
The authors note that lower microbial diversity in the gut has been associated with obesity. Individuals with reduced microbial gene richness are more likely to have a dysregulated metabolic profile. An energy-restricted, high-protein diet changed the gut microbes and improved the metabolic profile of obese study subjects (Cotillard et al. 2013). Could it be that highly diverse populations of gut microbes are more likely to spend their energies competing with other microbes rather than trying to exert negative influences on their host?
Delzenne and colleagues (2011) list many possible targets for colonizing gut microbes to influence our health, including gut barrier function, endocrine functions, lipid and glucose homeostasis (through influencing metabolism of cells in the liver and adipose tissue) and systemic inflammation.
If microbes are found to influence eating behavior, it is likely one of many, many factors. However, an implication is that there could be an ‘infectious’ component to obesity. Along these lines, researchers (Ridaura et al. 2013) showed that when lean and obese mice were co-housed, the obese mice become lean. Lean microbiota seems ‘dominant’.
Might probiotics play a role?
A few studies suggest that the right probiotic may reduce body fat and improve metabolic profiles. Feeding mice the 8-strain probiotic (known for extending remission in people with pouchitis), VSL#3, reduced body weight gain, reduced food intake, suppressed insulin resistance, improved glucose tolerance and altered gut microbiota (Yadav et al. 2013). Two human studies used this same probiotic mixture. Alisi et al. 2014 conducted a randomized, controlled trial (RCT) with children with non-alcoholic fatty liver disease, a condition associated with obesity. A reduction in BMI (a secondary outcome in this study) was observed in the group consuming VSL#3. Rajkumar 2014 conducted a RCT in overweight adults and found that VSL#3 improved insulin sensitivity and blood lipid profiles.
Other interesting studies include one where mice fed a high fat diet with Bifidobacterium breve B-3 demonstrated lower body fat and improved fasting glucose and insulin compared to mice fed just the high fat diet (Kondo et al. 2010). In humans, Lactobacillus paracasei subsp. paracasei W8 reduced overall energy intake, but had no effect on appetite or insulin response (Bjerg et al. 2014). In two RCTs (2010 and 2013), Kadooka and colleagues demonstrated a reduction in abdominal adiposity when a fermented milk containing Lactobacillus gasseri SBT2055 was consumed by healthy, overweight Japanese subjects.
These findings in human subjects are suggestive that probiotics may have some value in weight management, but dietary intake is an important factor and these studies all relied on self-reporting of diet. Most people do not accurately report what they eat (Nestle 2012). Further studies are clearly needed to better understand the extent to which probiotics may help. Also, not all live microbes or probiotics will necessarily have the desired effects – they likely need to be carefully chosen.
At this point, there are many more questions than answers. As concluded by Delzenne et al. 2011, the gut microbiota is “a potential nutritional and pharmacological target in the management of obesity and obesity-related disorders.”
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