Plenty of factors can impede or interfere with weight loss — including, perhaps, the “health” of your gut microbiome. If you’ve run the numbers, tracked your macros diligently, and are physically active but still can’t seem to move the needle on the scale, it certainly seems plausible that your gut is the culprit.
Gut health seems to be linked to everything else these days, so why wouldn’t it play a role in fat loss? The short answer is that it does, but not in the way you might think. Your gut microbiome plays an important role in appetite management, satiety, and weight regulation.
Here’s what the science currently says about how your gut health affects weight loss, and how you can take meaningful steps to stay on track or kickstart your progress.
Editor’s Note: The content on BarBend is meant to be informative in nature, but it should not be taken as medical advice. When starting a new training regimen and/or diet, it is always a good idea to consult with a trusted medical professional. We are not a medical resource. The opinions and articles on this site are not intended for use as diagnosis, prevention, and/or treatment of health problems. They are not substitutes for consulting a qualified medical professional.
What Is Gut Health?
It’s important to keep in mind that “gut health” is a colloquial term without a formal scientific definition, so it can mean just about anything. In most cases, people are probably referring to one or all of the following factors when they discuss the health of their gut:
- How well they digest and absorb nutrients, how they feel after eating, and their bowel habits.
- The management or prevention of gastrointestinal (GI) diseases (and how they can affect digestion and absorption).
- The diversity of their gut microbiome, meaning the tens of trillions of microbes and millions of microbial genes that inhabit the digestive tract.
These factors are related, but research is just beginning to uncover what might link the gut microbiome to digestion, disease progression, and, of course, weight management.
Understanding Your Gut Health
GI diseases that damage the intestinal lining — like inflammatory bowel diseases — directly affect nutrient absorption, which can in turn affect body weight (as well as many other factors) and often require medical interventions. Your gut microbiome, on the other hand, appears to play a much more complex and, thus far, poorly-understood role in human health and disease.
There is currently no way to conclusively determine whether your gut microbiome is “healthy,” and even terms used in the literature — like diversity or dysbiosis — have a number of different definitions.
While microbial diversity can be measured, dysbiosis is often considered to be a problematic term because it’s simply assumed to be present when someone has a disease. As it stands, there are no direct causative links between gut microbiome and any disease. (1)(2)(3)
How Gut Health Impacts Weight Loss
Early research suggested that a person’s gut microbes could predispose them to develop obesity. For a short time, researchers used the ratio of two bacterial phyla, Bacteroidetes and Firmicutes, as a biomarker for obesity; a lower ratio was assumed to predict obesity. (4)(5)
This so-called “B/F ratio” was quickly revealed to be unreliable, in part because a phylum contains hundreds or thousands of microbial species that all behave differently. Though thousands of fecal samples have been analyzed, no specific microbe or microbiome composition has emerged as a clear predictor of body weight or obesity.
For as many studies that suggest people with obesity have less diverse gut microbiomes, there is almost an equal number that have found no association between body weight and microbiome diversity. (4)(5)
In short, findings are mixed. However, gut health and weight-related outcomes are an extremely active area of research for the scientific community, and there have been some extremely interesting findings:
Gut Health and Appetite
As your gut ferments dietary carbohydrates, it produces short-chain fatty acids (SCFAs) such as butyrate, acetate, and propionate. These SCFAs can be absorbed like dietary fats, but they can also bind to receptors on intestinal cells, and unlike larger fatty acids, they can circulate freely in the bloodstream and cross the blood-brain barrier.
This allows them to bind receptors on a variety of cells, including muscle and fat cells, where they act more like chemical messengers than absorbable nutrients. (6)(7)(8)(9)
The mechanisms at play aren’t fully understood, partially because much of this science is conducted on rodents instead of humans. Regardless, this means that the behavior of your gut may impact your hunger and appetite by:
- Modifying the release of hunger and satiety hormones (some of which are also produced by your digestive tract without the help of gut microbes). (6)(7)(8)
- Affecting how quickly food travels through the digestive tract (also called transit time). (6)(7)(8)
- Acting on areas of the brain, such as the hypothalamus, to reduce appetite or food reward sensations. (9)
The science is still a long way from directly implicating any specific gut microbes or SCFAs with regard to hunger and appetite. There appears to be a relationship at play, but as of now, it’s mostly conjecture.
Gut Microbiome and Energy Balance
One of the many functions of the gut microbiome is energy harvesting, or converting one source of energy (calories) that isn’t accessible to humans (like dietary fiber) into a source that can be used by the human host (like butyrate.)
This process essentially “harvests” extra calories from your diet that you’d otherwise not utilize, which can impact energy balance. (7)(10) Energy balance refers to the ratio of calories consumed to calories burned.
Research in this area is still very new, and it’s unclear whether the conversion of fiber to SCFAs always results in higher energy absorption. There also isn’t a strong correlation between energy harvesting and body weight, (7) so it likely isn’t worth paying much attention to just yet.
Your Gut Health Might Affect Calorie Absorption
One recent study — which is still awaiting peer review — was able to shed some light on how much energy the gut microbiome might harvest based on someone’s diet. (7) Participants were fed calorie-matched diets with different amounts of dietary fiber, each for six days with a two-week break in-between dietary protocols.
They were also observed in metabolic chambers, which allowed researchers to very closely track how many calories they burned or expelled through body waste. Here’s the important part: Despite matching participants’ macronutrient and calorie content, there was a small difference in body weight at the conclusion of the study.
Editor’s Note: If you’re interested in monitoring your caloric intake more closely, check out BarBend‘s scientifically-developed, in-house calorie calculator:
The high-fiber, minimally-processed dietary protocol resulted in a marginal amount of extra weight loss when compared to a traditional Westernized diet containing more processed foods.
Your Microbes Could Add Calories You Haven’t Counted
Another study fed participants resistant starch, which is another preferred energy source for gut microbes. The study found that some gut microbiomes can extract as many as 4 extra calories from each gram of resistant starch. (10)
Resistant starch is indigestible in humans, but your gut microbes are powerful enough to pull out as many calories as you’d get from standard carbohydrates in some cases. The study in question didn’t monitor body weight, but some participants absorbed as much as 200 extra calories per day.
These processes aren’t intrinsically harmful, but the literature does shed some light on why your pen-and-paper nutritional tracking may not align with your real-world results on the scale.
Gut Health and Weight Loss Maintenance
While your gut microbiome doesn’t predict weight loss (or gain), there is some evidence that it could play a role in the common phenomenon of weight regain. Certain microbes in your gut respond more strongly than others to changes in exercise habits, diet, or medication (11)(12)(13) — all of which affect whether your weight loss efforts are temporary or permanent.
Your Gut Changes When You Lose Weight, Sometimes
Some data has shown that the gut microbiome can change its composition and behavior over the course of a weight-loss program. However, over time — and even if you continue to follow a diet — your gut can revert to its “baseline” configuration. (11) Other researchers have theorized that the “temporary” gut microbiome might actually help with weight loss maintenance. (13)
But It’s Not Predictive
Fluctuations in your gut health as you proceed through a diet may not be as big of a deal as you’d think. One study found that there wasn’t any specific microbiome “composition” that predicted weight loss success.
Rather, it may be that the ebb and flow of the microbiome itself better facilitates weight loss, even if those internal changes are temporary. (12) The important takeaway here is that you can’t “fix” your gut health to prevent, or encourage, weight loss directly.
There’s Still No Gut Health Fat Loss “Hack”
These exciting advancements toward understanding the role of the gut microbiome in weight regulation have led researchers to test various interventions, with mostly unimpressive results. So far, nothing beats the basics: a prudent diet and regular physical activity are the best ways to lose weight and keep it off.
There are a host of gut-related treatments that promise to deliver dramatic, noticeable weight loss, ranging from fiber supplementation to fecal transplants. Here’s what the science has to say on these interventions.
Skip the Supplements and Stool Testing
You may be tempted to undergo a stool analysis or fecal transplant in an attempt to “solve” the relationship between your gut health and weight management. However, the jury is very out on the value of these interventions. Research currently suggests that you probably won’t get anything meaningfully predictive from a single analysis anyway. (12)(14)(15)
Probiotics are the most promising, but that’s not saying much just yet. They may lead to small reductions in waist and hip circumference — of less than a centimeter — and were only associated with small amounts of weight loss (less than two pounds after more than eight weeks) in some studies. (16)(17)
There’s also butyrate supplementation, which has had some promising results in rodent studies. These findings haven’t translated to human beings yet, though. As of now, it may not do much more than mildly affect how you think about food. (9)(18)
Focus on Diet and Exercise
Research suggests that there’s some relationship between exercise, diet, and the microbiome. Exercise and cardiovascular fitness are associated with a more diverse gut microbiome enriched with beneficial microbes. It seems that exercise and dietary fiber may have the greatest impact on the gut microbiome when they’re provided simultaneously. (19)(20)(21)
For example, in one study, bodybuilders whose diets were fiber-deficient had gut microbiomes that resembled the sedentary participants, while those eating adequate fiber had more diverse microbiomes (which agrees with other findings that show physically-active people generally have more diverse microbiomes.) (19)(20)
In another study that recruited people with overweight or obesity, researchers found that certain gut microbes only responded to prebiotic supplementation in participants who increased their physical activity over the course of the trial. (21)
In real-world terms, the rules don’t change if you’re trying to change your weight: Commit to an exercise routine, not only for the caloric expenditure, but for the associated productivity in your gut as well.
If your head is spinning a bit, don’t fret. Gut health is an emergent area of health science and is an extremely intricate topic to explore. It may not be exciting, but the current science on gut health and fat loss is still informative. Here’s what you need to keep in mind:
- If your diet is high in refined carbs and highly-processed foods, switching to a minimally-processed, plant-rich diet could reduce the amount of energy (or calories) you absorb, even if you continue to eat the same number of calories.
- Shifting to a higher-fiber, less-processed diet could put you in a small energy deficit without making you feel hungrier.
- Though your gut microbes can influence appetite-modifying hormones and energy absorption, they’re simply a variable to consider at this point, not a shortcut to rapid fat loss.
- You don’t need to “fix” or “reset” your gut to lose fat; you still just need to be in an energy deficit through a combination of dietary control and physical activity.
- Calorie and macronutrient calculators are just a starting point. Be prepared to adjust your “prescribed” intake a bit if the scale isn’t moving.
Health From the Inside Out
Despite there not being a clear-cut definition for it, gut health seems to be all the rage in the wellness industry. Your gut is a delicate, finely-calibrated ecosystem that affects much more than how well you digest your last meal. However, the science has yet to clearly define the relationship between gut health and weight management.
While the jury is still out — although it’s undeniable that your gut microbiome plays some — role, your best bet for achieving any weight change-related goal is to stick to the basics. Eat well, move regularly, track your progress, and be patient.
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