WASHINGTON, DC — A new peer-reviewed review published in the journal Biology argues that much of the debate over whether plant-based diets deliver enough iron, protein, creatine, and other nutrients rests on a flawed assumption: that eating less of something automatically means the body has less of it available. Drawing on human intervention and observational studies across five biological systems, the review concludes that the body actively adapts to sustained dietary change, absorbing iron more efficiently, synthesizing more of its own creatine and carnosine, using protein more economically, and remodeling gut bacteria to better ferment fiber, and that these adaptations, not just intake numbers, determine long-term nutritional status.
The review was co-authored by Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee for Responsible Medicine, alongside lead author Miguel López-Moreno of the Universidad Francisco de Vitoria in Madrid and colleagues from the University of British Columbia, the University of Granada, and the Instituto de Investigación Nutricional in Lima.
"For years, nutrition science has treated dietary intake as if it were a straight line to physiological status—eat less iron, absorb less iron; eat less creatine, have less creatine," said Dr. Kahleova. "That's not how human physiology works. The body is not a passive pipe. When intake changes and stays changed, regulatory systems like the hepcidin-ferroportin axis in the gut, or the enzymes that make our own creatine and carnosine, adjust to hold function steady. As a clinician, I see this in my patients: The digestive symptoms that come with a new plant-based diet in week one are rarely the same story at month three, and a single blood draw or a single test meal doesn't tell you what a person's physiology looks like after they've actually adapted."
The review synthesizes evidence across five areas where plant-based diets are most often flagged as "nutrients of concern":
Iron. Vegans and vegetarians rely on less efficiently absorbed non-heme iron, but longer-term studies show the gut compensates by absorbing a greater fraction of it, partly explained by lower circulating hepcidin, the hormone that gates iron absorption. Total iron intake among vegetarians and vegans is also often as high as, or higher than, that of omnivores, since staple plant foods such as legumes, whole grains, and fortified cereals are iron-dense.
Creatine and carnosine. These compounds, found almost exclusively in animal foods, are also made by the body from amino acid precursors. Vegetarians run lower baseline muscle stores but generally maintain normal function and respond more robustly to creatine supplementation when it's used, evidence the authors say reflects lower baseline saturation rather than deficiency.
Protein. Well-planned plant-based diets that meet total protein recommendations preserve lean mass and strength comparably to omnivorous diets in controlled trials.
Fermentable fiber. Early bloating and gas from a higher-fiber diet typically ease over weeks as gut bacteria shift toward taxa that ferment complex carbohydrates more efficiently, a pattern documented since early controlled feeding studies of legume consumption.
Polyphenols. Gut bacteria convert plant compounds, from soy isoflavones to walnut ellagitannins, into more bioactive metabolites; long-term vegans show markedly higher levels of several of these microbial byproducts than people eating a standard omnivorous diet.
The authors are careful to note that adaptation has limits. Pregnancy, chronic inflammation, inflammatory bowel disease, celiac disease, and severe or prolonged dietary insufficiency can all outpace the body's regulatory capacity, which is why the review calls for individualized counseling rather than either blanket reassurance or blanket concern.
"This isn't an argument that nutrient composition doesn't matter, it clearly does, especially for patients whose adaptive capacity is already stretched thin," Dr. Kahleova said. "It's an argument for asking a better clinical question: not just what is this person eating, but how long have they been eating this way, and does their physiology show signs of having adapted? In my own randomized trial of a fiber-rich, plant-based diet in adults with type 1 diabetes, we saw insulin requirements fall in ways that track with exactly this kind of adaptive, whole-body response rather than a simple nutrient swap. That's the kind of clinical population where testing the limits of adaptation matters most, and where I think this framework will do the most good."
The review calls for longitudinal studies that track adaptation as it unfolds, rather than comparing only a baseline and endpoint, and for biomarkers that can distinguish a temporary response to dietary change from a stable, adapted state. The authors argue the same physiological principle extends well beyond plant-based diets, to any nutrition research that infers long-term status from short-term intake data.
About Hana Kahleova, MD, PhD
Hana Kahleova, MD, PhD, is director of clinical research at the Physicians Committee for Responsible Medicine, where she leads randomized controlled trials on plant-based diets, diabetes, and cardiometabolic health. She published over 100 papers in peer-reviewed journals, including JAMA Network Open, Diabetologia, and BMJ Nutrition, Prevention & Health.
Founded in 1985, the Physicians Committee for Responsible Medicine is a nonprofit organization that promotes preventive medicine, conducts clinical research, and encourages higher standards for ethics and effectiveness in education and research.
