Switching to a plant-based diet can change more than what’s on your plate. Over time, your body may also change how it handles some of those foods and nutrients.
That’s the idea behind a new review published in Biology. Researchers looked at studies involving iron, protein, creatine and carnosine, fiber and plant compounds called polyphenols. They found evidence that the body can adjust in several ways, such as absorbing more iron when less is readily available and becoming better able to tolerate higher amounts of fiber. But the review does not suggest that the body can compensate for every nutritional gap or that plant-based diets automatically meet everyone's needs.
The paper is a narrative review, meaning the authors brought together selected studies to explore a broader idea rather than systematically weighing all available evidence or combining study results in a meta-analysis. That makes it useful for identifying patterns, but it cannot tell us exactly how much adaptation occurs, how long it takes or whether everyone responds the same way.
Iron provides one of the clearest examples.
Plant foods contain a form of iron that generally is not absorbed as easily as the iron found in animal foods. But the percentage of iron the body absorbs is not fixed.
When the body has less iron available, it can adjust the systems that control absorption. Several studies included in the review found that people absorbed a greater share of iron after regularly eating diets in which iron was harder to absorb. One recent study also found that long-term vegans absorbed more iron from the same test meal than omnivores.
That does not mean iron deficiency stops being a concern on a plant-based diet.
The authors point out that the body's ability to compensate has limits, particularly during times when iron needs are higher or absorption is impaired. Pregnancy, menstruation, adolescence, chronic inflammation and certain digestive disorders can all increase the need to pay closer attention to iron intake and status. Some people may still need fortified foods, supplements or individualized nutrition guidance.
Protein tells a similar story.
Some plant proteins are less easily digested or contain smaller amounts of certain essential amino acids than animal proteins. Short-term studies have sometimes found differences in how strongly individual plant and animal proteins stimulate muscle building after a meal.
But longer studies suggest those differences do not always translate into worse outcomes over time.
The review cites controlled studies in which well-planned plant-based diets providing enough total protein supported muscle protein production, muscle mass and strength similarly to omnivorous diets. The body also continually reuses amino acids and adjusts how much protein it breaks down or loses, which may help it make efficient use of the protein that is available.
Again, adaptation is not the same thing as immunity from inadequate intake. If someone consistently eats too little protein, or has an illness or condition that greatly increases protein needs, the body cannot simply make up the difference indefinitely.
Creatine is another example of why dietary intake does not always tell the whole story.
Creatine is found mainly in animal foods, so vegetarians generally have lower amounts stored in their muscles. But the body can also make creatine on its own, and the review notes that healthy vegetarians typically maintain normal function despite those lower stores.
Vegetarians also tend to see larger increases in muscle creatine when they take supplements, which the authors say probably reflects starting with lower stores rather than having a deficiency. Research on carnosine, another compound found largely in animal foods, is more limited.
The easiest form of dietary adaptation to recognize may be what happens after someone suddenly starts eating a lot more beans, whole grains and other high-fiber foods.
At first, that increase can mean more gas, bloating or digestive discomfort. Part of the reason is that gut bacteria ferment carbohydrates that reach the large intestine.
But several studies included in the review found that digestive symptoms often became less noticeable after people continued eating legumes or other fermentable foods.
Interestingly, that did not always mean the gut stopped producing gas. People sometimes simply became more comfortable with the same foods over time, suggesting that both gut bacteria and the digestive system may adjust with repeated exposure.
That experience is not universal. People with conditions such as irritable bowel syndrome or inflammatory bowel disease may have more persistent symptoms and may need a different approach.
The review also looked at polyphenols, compounds naturally found in foods such as soy, whole grains, nuts, fruits and vegetables.
Gut bacteria help break many of these compounds into forms the body can use. People who regularly eat plant-based diets often have different levels of these breakdown products than people eating omnivorous diets.
But this is one area where the evidence is less clear. The authors acknowledge that people may have more of these compounds simply because they eat more plant foods, not necessarily because their gut bacteria have become better at processing them. More research is needed to separate those possibilities.
That uncertainty applies to the review's broader idea as well.
There is no single point at which someone can say the body has fully “adapted” to a plant-based diet. Different systems may adjust at different speeds, and age, health, activity level, previous diet and other factors may affect what happens.
The authors also stress that adaptation should not be interpreted to mean that nutrient intake no longer matters.
Instead, their argument is that nutrition is more complicated than simply comparing how much of a nutrient enters the diet. What happens after that nutrient is eaten, absorbed, stored, reused or processed also matters, particularly when a dietary pattern is followed for months or years.
That may help explain why the body's response to one meal or the first few days of a dietary change does not always predict what happens after someone has been eating that way for much longer.
The review received no external funding. One author reported previous advisory and consulting work for Danone and Foods For Tomorrow that was unrelated to the review and is no longer ongoing. Another reported unrelated grant funding from the Karuna Foundation and financial support as a writer and speaker for Soy Nutrition Institute Global.