Giving up meat does not necessarily mean that the body will have less iron, protein or creatine in the long term. An international scientific review, published in the journal Biology, analyzes how the body responds when a person maintains a diet based on plant foods over time and concludes that the body can adapt different processes related to the absorption, production and use of nutrients. The work reviews more than one hundred references and brings together evidence from intervention studies in humans and observational studies.
The research, led by Miguel López-Moreno, from the Francisco de Vitoria University, and with the participation of researchers from the United States, Spain, Canada and Peru, suggests that part of the debate on plant diets is based on a premise that the authors consider insufficient: assuming that a lower intake of a nutrient automatically translates into a lower availability for the body. The review studies this question in five areas: iron, creatine and carnosine, proteins, fermentable fiber and polyphenols.
"For years, nutritional science has treated dietary intake as if it were a straight line influencing physiological state: less iron, less iron absorbed; less creatine, less creatine in the body," says Hana Kahleova, one of the authors. The researcher adds: "That's not how human physiology works. The body is not a passive conduit. When intake changes and stays that way, regulatory systems, such as the hepcidin-ferroportin axis in the intestine or the enzymes that produce our own creatine and carnosine, adjust to maintain stable function."
One of the examples analyzed is iron. Vegetarians and vegans depend mainly on non-heme iron, whose absorption is less efficient than that of heme iron present in foods of animal origin. However, according to the review, long-term studies indicate that the body can compensate by increasing the proportion of iron it absorbs. The authors link this adaptation, in part, to lower levels of hepcidin, a hormone that regulates intestinal absorption of the mineral. The review also notes that the total iron intake of vegetarians and vegans is usually equal to or higher than that of omnivores.
Creatine reserves, under study
Creatine has another different mechanism. Although it is present almost exclusively in foods of animal origin, the body can also produce it from amino acids. The review notes that vegetarians tend to have lower basal muscle creatine stores, but generally maintain normal function. Additionally, they respond more markedly to creatine supplementation when it is used. For the authors, these results reflect a lower initial saturation rather than a deficiency situation.
Regarding protein, the review maintains that a well-planned plant-based diet that meets protein recommendations can preserve lean muscle mass and strength comparable to an omnivorous diet in controlled trials. The work also describes changes in the intestinal microbiota: initial discomfort, such as gas and bloating associated with an increase in fiber, usually decrease over the weeks, while intestinal bacteria adapt to ferment complex carbohydrates more efficiently.
Researchers also analyze polyphenols, compounds present in numerous plant foods. According to the review, intestinal bacteria transform substances such as isoflavones from soy and ellagitannins from walnuts into more bioactive metabolites, and people who maintain a vegan diet for long periods have higher levels of some of these products derived from microbial activity. The work thus suggests that adaptation to a plant diet is not limited to the amount of nutrients ingested, but involves different physiological processes.
However, the authors warn that this ability to adapt has limits. Pregnancy, chronic inflammation, certain diseases that affect absorption, and severe or prolonged nutritional insufficiency can overwhelm the body's regulatory capacity. "This does not mean that nutritional composition does not matter; it clearly matters, especially for patients whose adaptive capacity is already at the limit," says Kahleova. Therefore, the review calls for longitudinal studies that allow us to observe how adaptation develops and biomarkers capable of differentiating temporary responses from an adapted stable state.
