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Walter Willett is a professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health.
Q: I’ve been getting mixed messages about animal protein and plant protein. Is it true that animal protein is easier to absorb? Which is better for me?
A: The proteins in foods from animals, such as meat, milk and eggs, tend to be absorbed more easily than those from plant sources, such as nuts, beans and grains. This is partly because of the fibrous coatings that help protect plants from insects and diseases, and this shield can also reduce the rate of digestion.
But this isn’t a reason to choose animal proteins over plant proteins.
The difference in absorption is minor, typically about 10 to 20 percent lower from plants than from animals, and would only be a concern if our diets had barely enough protein to meet requirements — about 0.8 grams per kilogram of body weight, or about 10 percent of daily calories.
For adults in the United States and other affluent countries, protein intake is usually above what we need — about 15 percent of daily calories on average — so the difference in absorption between animal and plant proteins is largely inconsequential.
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One reason there is so much confusion surrounding this topic relates to amino acids. Proteins are strings of 20 amino acids, which are the building blocks for muscle, other body components, hormones, antibodies and enzymes that control our metabolism. Nine of these amino acids can’t be made by our body and are called essential because we need to get them from our diet.
A common argument for consuming animal proteins is that they are “high quality” and “complete” compared with plant protein, in terms of their essential amino acids.
Let’s start with the idea of protein quality. This nutritional concept is based on the mix of specific amino acids that maximize the growth of young mice and other mammals. But maximizing growth isn’t an issue for adults. By this definition of protein quality, eggs and milk come out on top but aren’t dramatically higher than most plant sources of protein, and beef protein is actually similar to soy protein.
You also don’t need to worry about whether plant proteins are “complete.” It’s a myth that plant foods don’t contain all the essential amino acids, and that we need to eat complementary proteins such as rice and beans in the same meal for optimal results. In fact, if we eat a variety of plant foods in our diet, the overall mix of amino acids isn’t substantially different from what we would get from eating animal protein.
Some concerns have been raised that anti-nutrients such as phytates, lectins and oxalates in plant foods may reduce the absorption of essential nutrients.
In poor populations with high intakes of starchy foods and low dietary diversity, eating a lot of phytates may contribute to deficiencies in some minerals. But within the context of a more diverse diet, as is the case in the United States, this doesn’t appear to be a problem.
Phytates may contribute to the many health benefits of plant foods because of their antioxidant activity, and higher phytate intake has been associated with overall good health and lower risk of kidney stones.
Focus on the protein package
Most Americans don’t need to worry about any of these issues — digestion efficiency, amino acid proportions, anti-nutrients — because we don’t consume protein in isolation or from a single food. These differences would only become important for someone on the cusp of protein deficiency.
For everyone else, the health effects of the whole protein package are more important.
When we eat beef, we get protein, essential minerals and vitamins, yes, but we also get hefty doses of saturated fat, cholesterol and other factors that increase the risk of heart disease, with very little beneficial polyunsaturated fat.
With plant proteins such as nuts or soy foods, we get good amounts of fiber and polyunsaturated fats, a different mix of essential minerals and vitamins, and many other compounds that appear to convey health benefits.
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Making room for more plants
We can best understand the health effects of protein-containing foods using randomized trials to assess their short-term impact on risk factors for disease, such as blood cholesterol and blood pressure, and epidemiologic studies to assess their long-term effects on risks of specific diseases and overall mortality.
Our research group conducted a prospective analysis of more than 130,000 men and women who were followed for up to three decades. The total amount of dietary protein was unrelated to overall mortality or other outcomes, but mortality increased with greater consumption of animal protein and decreased with greater amounts of plant protein.
Although some evidence suggests that total protein requirements may be greater at older ages, the same pattern favoring plant protein was seen among older adults when the outcome was frailty.
Protein and caloric requirements increase with pregnancy and among serious athletes, but without a clear advantage of animal vs. plant sources. For those who feel “better” with more animal protein, I suggest slowly weaving more nuts and soy foods into meals, which can also be satisfying and delicious.
The benefits go beyond improving your health. At this time in human history, it’s also important to consider the role of food choices in preserving a viable planet for future generations. Although eliminating use of fossil fuels is the highest priority, we have little hope of avoiding disaster if we don’t also shift our diets to be more plant-centric.
While this would be a major shift for many Americans, the traditional diets of Asia, Africa, Latin America and the Mediterranean region provide abundant flavorful and attractive examples.
It doesn’t mean we must eliminate steak or Parmesan cheese from our diet, or become a vegan, although that is an option some may wish to pursue. Studies have shown that diets with about two servings of animal-sourced foods per day can be both healthy and sustainable. That can mean a cup of yogurt plus 3 to 4 ounces of chicken or fish.
So instead of having large portions of animal protein at every meal, focus on adding more plant proteins to your plate, such as lentils, tofu, chickpeas, peanuts, nuts and beans. Make your diet as diverse as possible, incorporating a wide range of fruits, vegetables and whole grains. This not only is smart eating but can also unlock new flavors and exciting meals. The advantages include better personal health now — and hope for future generations.
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