The ins and outs of the new dietary guidelines for Americans in light of the gut microbiome

The Gut Microbiota for Health acknowledges the release of the 2025-2030 Dietary Guidelines for Americans. Many recommendations align with long-standing nutritional science, including more attention to the health of fiber and the microbiome. However, leading food groups (including Harvard TH Chan School of Public Health And Stanford Medicine) highlight that the new guidelines notably deviate from the evidence-based recommendations of the Dietary Guidelines Advisory Committee, which may hinder actionable insights for physicians and nutrition policymakers. Ultimately, this can lead to nutritional decisions by the general public and patients who are vulnerable to political and food industry influences^1,2.

The new guidelines provided limited nuance and clarity on how the types and amounts of fats and proteins make it difficult to consume enough fiber, which ultimately does not benefit the gut microbiome.

A diet high in saturated fats promotes microbiome-derived bile acids involved in colorectal cancer

The inverted image of the food pyramid accompanying the new guidelines placed sources of animal protein and saturated fats in the top left position, interpreted as staples that we should prioritize and eat more often. Although red meat and cheese have received a lot of attention, this is not supported by current scientific evidence, which recommends prioritizing dietary patterns consisting primarily of whole foods, to improve cardiovascular and metabolic health by beneficially influencing the composition and function of the gut microbiome.^3.4.

The new guidelines recommended limiting highly processed foods, added sugars and saturated fat. However, the food pyramid appears tolerant of foods high in saturated fat, such as butter, beef tallow, red meat and full-fat dairy productswhich is inconsistent with the recommendation to limit saturated fat to 10% of total calories. While full-fat dairy was heavily promoted without giving credit to people who are lactose intolerant or don’t consume much dairy based on personal preference, plant-based foods and oily fish were notably left off the list of ‘healthy fats’, despite showing positive effects on the gut microbiome.

When it comes to the gut microbiome, a diet primarily containing animal products can increase bile acids and promote bile-tolerant microorganisms (Alistipes, BilophilaAnd Bacteroids), and reducing microorganisms that break down dietary fiber from vegetables, fruits and legumes (Rozeburia, Eubacterium rectaleAnd Ruminococcus bromii). In the colon, the microbial conversion of primary bile acids to secondary bile acids (e.g. deoxycholic acid, lithocholic acid) is strongly implicated in the development of colorectal cancer, supported by causal evidence from porcine and mouse models.⁵.

Animal protein sources are overemphasized compared to plant proteins

In another departure from the science, the new guidelines prioritize animal sources over a plant-forward pattern, even though 85% of the US population consumes more than the recommended daily allowance of 0.8 g/kg/day.⁶. Also, there is no benefit to consuming more than 1.6 g of protein per kg of body weight per day, which is twice the recommended daily allowance, for gains in muscle mass and strength.⁷. High protein intake can accelerate atherosclerosis and increase cardiovascular risk⁸ and is linked to increased production of imidazole propionate by intestinal bacteria, which promotes the formation of plaque in the arteries.

The heavy emphasis on protein intake, especially on red meat, can easily lead to exceeding the recommended limits for saturated fat and sodium. When protein displaces fermentable carbohydrates in the dietmore amino acids reach the large intestine. As a result Microbial metabolism may shift towards using proteins as the main fuelincreasing metabolites such as ammonia, phenols and hydrogen sulfide, which negatively affect the integrity of the intestinal barrier and promote intestinal inflammation, increasing the risk of colorectal cancer⁹.

In general, the “more protein is better” message is a concern, especially when fiber intake is low, as in Westernized societies. While ensuring a balance between protein and fiber intake is important for better gut and metabolic health¹⁰The new guidelines’ protein intake advice overlooked the role of the source of protein and replaced fermentable carbohydrates from diets in favor of animal protein for gut health and overall health.

Fiber was downplayed in favor of protein and fat

The new guidelines water down the position on plant-forward diets. Despite clear evidence that these diets prevent chronic diseases such as diabetes and cancer, the new guidelines have softened their language and removed the specific details needed for effective public health action. As a result, fiber is downplayed in favor of protein and fat. Recognition of dietary fiber as an essential nutrient for humanssuch as certain amino acids and vitamins, would help boost fiber intake, which is a simple and effective approach to reducing chronic diseases diseases, as it claims in a recent commentary Natural food¹¹.

Although the new guidelines recognize fiber and fermented foods and recommend whole foods, the visual and textual emphasis on protein and fat makes it difficult to increase fiber intake. At the level of the gut microbiome, fiber fermentation leads to the production of short-chain fatty acids, which have metabolic and immunological benefits¹². Furthermore, the reduction in intestinal pH due to high fermentation of fermentable carbohydrates may suppress the formation of secondary bile acids and shift intestinal microbial metabolism away from metabolites such as branched chain fatty acids and trimethylamine N-oxide, which are associated with deleterious effects on intestinal and metabolic health.¹³.

While it is welcome that the new dietary guidelines recognize the role of the gut microbiome, it is unfortunate that they also place too much emphasis on diets rich in protein and fat that favor a gut microbiome associated with colorectal cancer. As an editorial by Marion Nestle, professor emeritus of nutrition, nutrition studies and public health at New York University, titled “Politics Trumps Science in New US Dietary Guidelines” highlights, the new guidelines are confusing and represent a regression in evidence-based public health policy that undermines public trust and is likely to lead to preventable harm to public health.¹⁴.

References:

1. Advisory Committee on Dietary Guidelines. Scientific report of the Advisory Committee on Dietary Guidelines 2025. Accessed on February 2, 2026. https://www.dietaryguidelines.gov/2025-advisory-committee-report
2. Williams Sr KA, Dastmalchi LN, Barnard ND. When nutrition science is ignored: potential public health costs of the 2025 dietary guidelines. JAMA. 2026. doi: 10.1001/jama.2026.0832.
3. Shi W, Huang X, Schooling CM, et al. Red meat consumption, cardiovascular disease and diabetes: a systematic review and meta-analysis. Eur Hart J. 2023; 44(28):2626-2635. doi: 10.1093/eurheartj/ehad336.
4. Gu X, Bui LP, Wang F, et al. Global adherence to a healthy and sustainable diet and possible reduction in premature mortality. PNAS. 2024; 121(50):e2319008121.
5. Osswald A, Wortmann E, Wylensek D, et al. Secondary bile acid production by intestinal bacteria promotes Western diet-related colorectal cancer. Intestine. 2025. doi: 10.1136/gutjnl-2024-332243.
6. National Academy of Medicine. 2005. Dietary reference intakes for energy, carbohydrates, fiber, fats, fatty acids, cholesterol, proteins and amino acids. Washington, DC: The National Academies Press.
7. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018; 52(6):376-384. doi: 10.1136/bjsports-2017-097608.
8. Zhang X, Kapoor D, Jeong SJ, et al. Identification of a leucine-mediated threshold effect regulating macrophage mTOR signaling and cardiovascular risk. Night Metab. 2024; 6(2):359-377. doi:10.1038/s42255-024-00984-2.
9. Davis RH, Bryant RV, Gibson PR, et al. The fate of dietary proteins in the gastrointestinal tract and the implications for intestinal diseases. Nat Rev Gastroenterol Hepatol. 2026. doi: 10.1038/s41575-026-01173-0.
10. Jardon KM, Canfora EE, Goossens GH, et al. Dietary macronutrients and the gut microbiome: a precision nutritional approach to improving cardiometabolic health. Intestine. 2022; 71(6):1214-1226. doi: 10.1136/gutjnl-2020-323715.
11. Reynolds AN, Cummings J, Tannock G, et al. Dietary fiber as an essential nutrient. Wet food. 2026; 7(1):4-5. two: 10.1038/s43016-025-01282-0.
12. Armet AM, Deehan EC, O’Sullivan AF, et al. A new look at healthy eating in the light of the intestinal microbiome. Cell host microbe. 2022; 30(6):764-785. doi: 10.1016/j.chom.2022.04.016.
13. Brinck JE, Sinha AK, Laursen MF, et al. Intestinal pH: a key driver of human gut microbiota composition and metabolism. Nat Rev Gastroenterol Hepatol. 2025; 22(9):639-656. two: 10.1038/s41575-025-01092-6.
14. Nestlé M. Politics trumps science in new US dietary guidelines. BMJ. 2026; 392:s143. doi: 10.1136/bmj.s143.