Skip to main content

Verified by Psychology Today


A Trillion Little Reasons Why You Can’t Lose Weight

Scientists find that gut microbes affect weight gain.

Key points

  • Obesity and depression are linked and gut microbes contribute to both.
  • Changing gut microbes can help with both weight loss and mood problems.
  • Specific foods, especially those with fiber, can improve gut microbes.

For the first 50 years of your life the food industry is trying to make you fat. Then, the second 50 years, the pharmaceutical industry is treating you for everything. —Pierre Dukan


We get energy to run our bodies from the calories in our food. What we don’t use to keep the life-fires burning, we store as fat. That is the unassailable physics of the calories-in, calories-out theory of weight gain.

You won’t win by betting against physics, but this is biology, so there are bound to be some wrinkles. The missing part of the equation is the microbiota, the microbial metropolis that resides in the gut. How important are these microbes? Think of this: A 1,500-pound horse gets all its impressive muscle from grassy roughage. You and I would die on that diet.

The difference is that a horse has microbes that can ferment grass into fatty acids, accounting for the bulk of its energy requirements. If we had those microbes, we could get buff on lawn clippings.

Clearly, our microbes can affect what we get out of our food. The more efficient a microbe is, the more energy it gets out of dinner. A slacker microbiota, on the other hand, can mean that many calories are flushed down the toilet. That’s the biological wrinkle: calories-in/calories-out doesn’t account for calories flushed.

And that’s what a new Danish study looked at. When they examined the residual energy in the poop of lean people, they found they left a lot behind. Lean people have a gut microbiota characterized by relatively lazy Ruminococcaceae bacteria. Overweight people, on the other hand, scrounged much more from their food and left little behind. Their highly efficient gut microbiota is characterized by Bacteroides bacteria. Bacteroides are not bad, per se, but they are excessively exuberant when it comes to digesting starchy foods.

Fecal transplants

Some of this isn’t totally new. Researchers have known for years that microbes can make you fat. Many of these studies use fecal transplants to impose a new microbiota on animal subjects. When researchers fed lean mice some poop from fat mice, they gained weight. Sure enough, when they fed fat mice some poop from lean mice, they lost weight.

That showed that the microbiota isn’t just associated with weight gain, it can cause it. In mice, at least. Mice are great research animals, and a lot of preclinical studies depend on them. But mouse studies are not always applicable to humans, so some caution is advised before we draw conclusions. However, the Danish study presents a clever way to track calories, and the human results align well with the mice studies.

If microbes are causing obesity, does that mean that weight is set in stone? There is a certain reasonable pessimism about this because our microbiota, once established, is hard to change. In fact, the main way we know to make a permanent change is through fecal transplants: First a massive dose of antibiotics is used to destroy the existing microbiota, and then donor poop is injected via enema. These transplants have been found to hold for up to six years, which is heartening.

People who have had these transplants, typically to cure hospital infections, experience immediate relief. They may also pick up some characteristics of the donor, including mood and body shape. That’s a humbling acknowledgement of the power of our microbiota. We don’t need a fecal transplant though: we can encourage a lean microbiota with diet.

The miracle of dietary fiber

An excellent option is to add fiber to our diet. This may be the best leverage we have against weight gain.

Manufacturers processed fiber out of our food several decades ago. Intriguingly, that coincides with the start of the obesity epidemic. The bacteria that make us lean enjoy fiber which they ferment into short-chain fatty acids like butyrate. This wonder molecule both heals and nourishes the gut lining, keeping it impermeable to pathogens. Butyrate thus tamps down the inflammation that underlies most diseases, including arthritis, heart disease, Alzheimer’s, Parkinson’s, and diabetes. It shouldn’t be surprising that inflammation is also associated with obesity.

Thus, adding fiber to your diet, not calorie-counting, may help with weight loss. A new study from The Chinese University of Hong Kong looked at adding back some fiber using a mix of four prebiotics: pea, orange, barley, and inulin. That dietary change altered gut microbes to favor those with enzymes involved in glucose metabolism and genes associated with leanness in humans.

Obesity and depression

The stereotype of the jolly fat person is not on the mark. Obesity and depression are surprisingly well-correlated, and the association is bidirectional. Obese people are significantly more likely to be depressed than the general population, and depressed people are more likely to develop obesity.

Many antidepressants cause weight gain, and weight gain can decrease the effectiveness of antidepressants. Worst of all, depression and obesity are strongly associated with diseases including type 2 diabetes, heart disease, and cancer.

What to do

We are not helpless. Our microbes outnumber us, but humans have been known to summon a little will power. Here’s what we can do:

  • Eat more fiber, especially veggies like onions, peas, lentils, and asparagus. The lack of fiber in our diet is likely how we got into this mess in the first place. If you can’t get enough veggies in your diet, look to prebiotics.
  • Eat ferments like yogurt and sauerkraut. They contain a multitude of beneficial bacteria that can help diversify your microbiota. Diversity is key to a healthy microbiota.
  • Exercise is less effective than diet for weight loss, but it can help offset the danger of being overweight. It can also improve your microbiota.
  • Drink plenty of water. Your microbes get thirsty too.

Don’t expect to lose weight quickly. Changing your microbiota can be like nailing jello to the wall. But by reversing decades of lost fiber, we are at least going in the right direction.

More research is needed, but we have enough now to rebuff some simplistic notions of weight gain. The theory that all you need to do is eat less and exercise more is missing the bacterial angle. When it comes to biology, one should never be surprised by complexity.

Knowledge is power, and we can use this research to tame our microbes, lose some weight and improve our moods. What a great way to start the year!


Boekhorst, Jos, Naomi Venlet, Nicola Procházková, Mathias L. Hansen, Christian B. Lieberoth, Martin I. Bahl, Lotte Lauritzen, et al. “Stool Energy Density Is Positively Correlated to Intestinal Transit Time and Related to Microbial Enterotypes.” Microbiome 10, no. 1 (December 12, 2022): 223.

She, Junjun, Chi Chun Wong, and Jun Yu. “Targeted Prebiotics Alter the Obese Gut Microbiome in Humans.” Signal Transduction and Targeted Therapy 6, no. 1 (October 8, 2021): 1–2.

Valentino, Taylor R., Ivan J. Vechetti Jr, C. Brooks Mobley, Cory M. Dungan, Lesley Golden, Jensen Goh, and John J. McCarthy. “Dysbiosis of the Gut Microbiome Impairs Mouse Skeletal Muscle Adaptation to Exercise.” The Journal of Physiology 599, no. 21 (2021): 4845–63.

Lai, Zi-Lun, Ching-Hung Tseng, Hsiu J. Ho, Cynthia K. Y. Cheung, Jian-Yong Lin, Yi-Ju Chen, Fu-Chou Cheng, et al. “Fecal Microbiota Transplantation Confers Beneficial Metabolic Effects of Diet and Exercise on Diet-Induced Obese Mice.” Scientific Reports 8, no. 1 (October 23, 2018): 15625.

More from Scott C. Anderson
More from Psychology Today