Protein has become one of the biggest selling points in modern food.
Supermarkets now offer protein cereals, bars, drinks, yogurts and even protein-enriched coffee and water, while many people assume that getting more protein must always be healthier.
A major new scientific review questions that simple message. After examining more than 350 studies, researchers argue that some people—particularly adults who are not very physically active—may be eating more protein than they need, and that moderately reducing protein could have benefits for metabolism and healthy aging.
The review was published July 31 in the Cell Press journal Cell Press Blue. The authors examined evidence from laboratory animals, human studies and clinical trials to understand how the amount and type of protein in the diet may affect aging.
Protein is essential for life. The body uses it to build and repair muscle, skin and other tissues, and proteins are also needed to make enzymes, hormones and many parts of the immune system.
That means the goal is not to eliminate protein. The more interesting question is whether constantly eating well above the body’s needs provides extra benefits, especially for people who do little exercise.
Dudley Lamming of the University of Wisconsin–Madison, the review’s corresponding author, says protein clearly supports muscle growth and recovery in active people. But he argues that many sedentary adults may consume more than they need, potentially changing biological systems linked to aging.
Scientists have known for decades that calorie restriction can extend lifespan in many laboratory organisms. Eating substantially fewer calories over long periods, however, is difficult for most people and can carry risks if nutrition is inadequate.
Protein restriction may offer a different approach. Experiments in flies and rodents have found that lowering protein can sometimes extend lifespan even when the animals do not eat fewer calories overall.
Small human trials have also produced interesting results. In some studies, people who reduced protein lost body weight and fat and improved fasting blood sugar, even though their total calorie intake did not necessarily fall.
One possible explanation involves a hormone called FGF21. Levels of this hormone rise when protein intake is reduced, and research suggests it can influence energy use, blood sugar control and inflammation.
In mice, higher FGF21 activity has been linked to longer life, although the effect has not been identical in males and females. Lower-protein diets also increase FGF21 in humans, but researchers do not yet know whether this translates into a longer human lifespan.
The review also points to amino acids, the smaller building blocks that make up proteins. Researchers are especially interested in methionine, isoleucine and valine because high amounts may stimulate cell-growth signals that are useful at some times but potentially harmful when they remain highly active.
Growth signals help children develop and help adults repair tissues. Yet some of the same pathways are linked with aging when they are continuously stimulated, because cells may spend more energy growing and less energy on maintenance and repair.
This could help explain why lower protein intake sometimes reduces cellular damage and improves metabolic health in laboratory studies. It may also explain why the relationship between protein and health is more complicated than simply saying that more is better.
There is an important opposing side to the evidence. Higher protein intake can help people preserve or build muscle, and maintaining muscle becomes increasingly important with age because muscle loss can contribute to weakness, falls and loss of independence.
Older adults who eat too little, people recovering from illness, pregnant women and highly active people may therefore need more protein rather than less. Athletes also use protein to repair and build muscle after training.
Exercise may change the equation because active muscles can make productive use of amino acids. Lamming suggests that this may partly explain why athletes can consume relatively high amounts of protein without showing the same metabolic problems researchers worry about in sedentary populations.
This makes personalized advice especially important. Two people of the same age and weight may have very different protein needs if one performs strength training several times a week and the other spends most of the day sitting.
The review should not be interpreted as proof that eating less protein will make humans live longer. Much of the strongest lifespan evidence comes from animals, and human aging takes decades, making direct longevity trials extremely difficult.
Another limitation is that protein does not come in isolation. Someone who reduces meat and replaces it with beans, vegetables and whole grains is making several dietary changes at once, while someone who cuts protein but replaces it with highly processed carbohydrates may experience very different results.
Overall, the review raises an important challenge to today’s protein-heavy food culture. Protein is necessary, but more is not automatically better, and the ideal amount may depend heavily on age, health and physical activity.
The most useful message is therefore not that everyone should cut protein. Instead, the research suggests that nutrition advice may eventually need to move away from one-size-fits-all targets and consider whether a person’s protein intake actually matches what their body is using.
If you care about nutrition, please read studies about how Mediterranean diet could protect your brain health, and this plant nutrient could help reduce high blood pressure.
For more information about nutrition, please see recent studies that olive oil may help you live longer, and vitamin D could help lower the risk of autoimmune diseases.
Source: University of Wisconsin–Madison.
