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Weight Loss, Not Carb-to-Fat Ratio, Shifts Gut Bacteria in Adults

A feeding trial finds losing weight changes gut bacteria, while low-carb versus high-carb maintenance diets make no detectable difference.

By Dr. Josh Axe · November 9, 2026 · 1 source · Watch

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At a glance

What they found: Once adults had lost weight, switching their maintenance diet between low, moderate and high carbohydrate levels did not measurably change their gut bacteria.
Who was studied: Adults with overweight and obesity who lost weight, then followed one of three maintenance diets for 20 weeks.

A study published November 6, 2026 in Cell Reports Medicine found that losing weight shifted the community of microbes living in the gut, known as the gut microbiome, in adults with overweight and obesity, but adjusting the carbohydrate-to-fat ratio of a weight-maintenance diet afterward produced no detectable difference in that community.

20-weekfeeding trial

3carb-to-fat diets

The study, an ancillary analysis of the Framingham State Food Study, put adults with overweight and obesity through a calorie-restriction phase to induce weight loss, then randomly assigned them to one of three diets for 20 weeks of maintenance: low-carb (20% carbohydrate, 60% fat), moderate-carb (40% carbohydrate, 40% fat) or high-carb (60% carbohydrate, 20% fat). Weight loss itself produced shifts in gut bacteria that matched patterns seen in earlier research. Once participants were on their assigned maintenance diets, the researchers found no detectable difference in the overall mix of gut bacteria between the three carbohydrate-to-fat groups, after accounting for other dietary factors that could confound the comparison. One signal stood out: changes in the abundance of a bacterial group called Shuttleworthia tracked positively with how well people maintained their total daily energy burn after losing weight, which the authors say suggests a link between gut bacteria and energy metabolism.

Body composition and metabolic state appear to shape gut bacteria more than the specific carb-to-fat ratio in a maintenance diet, once weight has already changed. In general, gut bacteria respond to many inputs beyond macronutrient ratios, including total calorie intake, fiber, and the act of losing body fat itself, which changes the gut environment. This study did not test those individual mechanisms directly.

20weeks

Length of the weight-loss maintenance phase in which participants followed one of three randomized carb-to-fat diets (20% or 40% or 60% carbohydrate)

Cell Reports Medicine, 2026, Kohli et al.

The abstract did not report participant numbers, ages, sex split, how the microbiome was measured, or how large the weight loss was. Whether the Shuttleworthia finding reflects a real metabolic link or coincidence isn't established here; a trial testing that link directly would clarify it.

Dr. Axe’s Take

Losing the weight did more to these people's gut bacteria than the carb-to-fat split they ate afterward, whether that plate leaned steak or sweet potatoes. I still think the carb-to-fat ratio matters for blood sugar control, hunger and energy, but this data says don't expect one specific split to remodel your microbiome on its own. What actually moves gut bacteria is fiber variety: different colored vegetables, beans, lentils, and fermented foods like sauerkraut and kefir feeding different bacterial species. This week, keep the calorie deficit if weight loss is the goal, and build plates around fiber-rich plants regardless of which macronutrient camp you prefer.

— Dr. Axe

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Study summary

Who: Adults with overweight and obesity enrolled in the Framingham State Food Study; number, age and sex split not reported in the abstract.
Design: Ancillary analysis of a controlled, randomized feeding trial: calorie restriction for weight loss, then random assignment to one of three maintenance diets for 20 weeks.
Dose: Low-carb: 20% carbohydrate, 60% fat. Moderate-carb: 40% carbohydrate, 40% fat. High-carb: 60% carbohydrate, 20% fat.
Primary result: No detectable difference in gut microbiome composition across the three carb-to-fat diet groups after controlling for other dietary factors; weight loss itself produced significant microbiome shifts.
Secondary: Changes in Shuttleworthia abundance correlated positively with maintenance of total energy expenditure after weight loss.
Funding / conflicts: Funders listed in the journal record: National Institute of Diabetes and Digestive and Kidney Diseases; American Gastroenterological Association Research Foundation; Blue Cross Blue Shield of Massachusetts Foundation; New Balance Foundation; North American Society for Pediatric Gastroenterology Hepatology and Nutrition Foundation; National Institute of Child Health and Human Development. Competing interests: too long to show here; see the source paper.
Date: Nov 6, 2026