Increase legumes, whole grains and vegetables and, at least initially, your intestines may notice before you: more fermentation, bloating, some discomfort. Then weeks pass and those symptoms often subside. The dish remained rich in fibre, it was the body that in the meantime began to work with it differently.
It also happens with iron, with some aspects of protein metabolism, and with compounds such as creatine and carnosine. A review published on Biology try to put these adaptations together and look at plant-based nutrition from an often overlooked angle: the body doesn’t simply receive what we eat and always use it in the same way. It regulates absorption, internal production, recycling and even the work of the microbiota based on what it usually finds on the plate.
The revision, however, is only the framework. The studies behind this idea tell a much more concrete, and at times even counterintuitive, story: the same food can produce different responses in people who have been used to eating differently for years. It happened with the iron in pistachios, with muscle proteins and even with intestinal bacteria.
The iron in pistachios and that tap called hepcidin
In 2025, a group of researchers gave 150 grams of pistachios to 27 young adults, 14 vegans and 13 omnivores, and followed what happened to the iron in the blood in the following hours.
In vegans the increase was greater. Furthermore, in their group, the response was associated with the levels of hepcidin, the hormone that regulates the availability of iron. The study published on Molecular Nutrition & Food Research therefore offers a rather precise example of what it might mean to adapt over time to a less readily available source of iron.
Hepcidin works much like a faucet. When it increases, it limits the passage of iron into the blood; when it decreases, it facilitates its availability. Those who follow a plant-based diet consume almost exclusively non-heme iron, which is generally less bioavailable than the heme iron present in animal foods. If availability is reduced, the organism can then react by trying to make better use of what arrives.
Those pistachios, however, are not enough to close the discussion. The sample was small, and the researchers measured the change in serum iron after the meal, without directly quantifying intestinal absorption through isotopes. It is an interesting physiological clue, still far from a valid rule for anyone.
A single meal and ten days can tell two different stories
With proteins the matter becomes even more curious. In a study of 16 adults aged 65 to 85, two meals with the same amount of protein and calories gave different results over the next six hours. The one containing beef produced a higher concentration of essential amino acids in the blood and a higher synthesis of muscle proteins compared to the completely plant-based meal.
If the photograph ended there, it would look pretty simple. However, a randomized crossover trial published in 2025 followed 34 physically active adults, with an average age of approximately 72 years, during ten days of a controlled vegan diet and ten days of an omnivorous diet.
When the researchers measured muscle protein synthesis throughout the day, the values were very similar: 1.23% per day during the vegan diet and 1.29% during the omnivore diet, without a statistically significant difference.
A plant-based meal can therefore cause a lower anabolic response in the hours immediately following, while the daily balance, within an adequate diet, can be much closer.
The same can be seen by extending the times even further. In a 12-week study, 19 young vegans and 19 omnivores followed the same strength training program with protein intake increased to approximately 1.6 grams per pound of weight per day. Muscle mass and strength increased in both groups without significant differences.
That number matters. Here the proteins were sufficient and planned. Adaptation can help the organism to use what it receives; if the raw material is scarce, the margins narrow a lot.
The microbiota can change jobs without completely changing its face
The microbiota adds another piece. A multi-omics study of 62 vegans and 33 omnivores found significant differences in only 14.8% of identified bacterial genera. Looking instead at what those bacteria produced, the distinction became much more evident: 43.3% of the metabolites identified in the feces differed from 55.8% of those detected in the blood.
In vegans, several carbohydrate fermentation products were more abundant and some compounds derived from amino acid fermentation were less abundant.
Said in a less laboratory way: the microbiota does not necessarily have to completely change its face to change jobs. A work published in 2025 on Gut Microbesconducted on 100 vegans and 73 omnivores and then verified in a second independent cohort, identified a microbial and metabolic signature associated with vegan nutrition. Some characteristics linked to the omnivorous diet also decreased as the years spent following the vegan diet increased.
Here time seems to enter directly into the photograph, even if the study is observational and cannot demonstrate on its own that it is the duration of the diet that produces those changes.
A randomized trial on 53 people helps to put a stop to too easy conclusions: after four weeks of a vegan or meat-rich diet, the overall composition of the microbiota had not changed drastically. The individual imprint of each person continued to weigh heavily. Four weeks, for an ecosystem that has been with us for a lifetime, could simply be too little.
And the beans? The first week they make themselves felt
Then there is adaptation which does not require metabolomics, test tubes or particularly sophisticated laboratories. In a randomized study of 180 adults, participants ate meals containing beans, peas or rice for six weeks, five times a week.
During the first week, those who ate beans reported bloating and flatulence more frequently than the rice group. Over the next few weeks those differences became much less apparent.
The famous phrase “then the intestine gets used to it” therefore has something more solid than the oral tradition of those who have exaggerated on chickpeas. The microbiota and digestive system can adapt to more fermentable fiber and the perception of symptoms can also change. Times and intensity, however, remain very personal. The intestine does not read manuals and above all it does not look at the calendar.
However, adaptation has a limit
All this does not transform the organism into a machine capable of compensating for any deficiency. The same review as Biology indicates conditions in which the margins may narrow: pregnancy, chronic inflammation, intestinal pathologies, celiac disease, absorption problems and serious or prolonged deficiencies. And there is at least one nutrient that helps us see very clearly where adaptation ends: vitamin B12.
In a study of 53 omnivorous adults assigned to a vegan diet with no supplements or a high-meat diet for four weeks, holotranscobalamin, the biologically available form of vitamin B12, significantly decreased in the vegan group.
The body can regulate iron, change amino acid metabolism, and learn to live better with more fiber. It cannot produce on its own a nutrient that the diet does not provide in adequate quantities. For those who follow a vegan diet, B12 must therefore be guaranteed through fortified foods or appropriate supplementation.
There is also an important piece missing: we still don’t know precisely how long each adaptation takes. A hormone can change rapidly, the microbiota follows different times, the muscle metabolism still others. And two people who eat the same way can start from completely different situations.
This is why knowing what’s on your plate tells a lot, but not everything. One person has been eating legumes for three days, another for ten years. The dish can be identical. The intestine definitely less so.
