Low protein, longer life — for some

No one knows the precise secret to a long life. But new research serves up one strong clue: Eat less animal protein.

Consuming a diet low in meat, eggs and dairy products may extend one’s life, according to two new studies. While one study was conducted in mice and the other on people, both reached the same conclusion. And both appear in the same March 4 issue of Cell Metabolism.

Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and other tissues. Protein is also an essential nutrient. The new studies suggest people should pay attention to how much protein they eat, as well as where it comes from. For example, both a juicy steak and a bowl of lentil soup contain lots of protein.

In one of the new studies, the scientists found a higher risk of early death among middle-aged people who ate lots of protein. The link seemed strongest for diets rich in protein from animals, not plants. (So vegans, you’re off the hook.)

In that study, an international team of scientists surveyed data on the diets of more than 6,300 people, all age 50 or older. The diners had been followed for 18 years. During that time, those younger than 65 who got less than 10 percent of their calories from protein were less likely to die from cancer or diabetes than were people who ate more protein.

However, eating less protein wasn’t good for everyone. Among people older than 65, the reverse was true. Here, people who got at least 20 percent of their calories from protein had lower risks of dying from cancer.

“A high-protein diet is one of the worst things you can do up to age 65,” Valter Longo told Science News. Longo works at the University of Southern California in Los Angeles, where he studies aging.  He says his team’s new data show eating too much protein is nearly as dangerous to health as smoking is.

Not all scientists, however, are ready to declare protein the enemy. Edward Giovannucci studies diet and health at the Harvard School of Public Health in Boston. He told Science News he was puzzled by the reversal of the protein risks that showed in people over age 65.

The results “are sort of intriguing, but I would view them as not really definitive,” he said. Researchers need to study this more to better understand the results, he concluded.

In a second study, scientists fed different diets to 858 mice. Those diets included various amounts of proteins, fats and carbohydrates. In general, mice that ate diets high in protein were more likely to die younger. Those that ate less protein and more carbohydrates (foods rich in sugars and starches) lived longest.

Earlier studies had suggested that animals — including fruit flies, dogs and mice — live longer when they ate about 25 percent fewer calories than normal. The new study paints a different picture. It suggests the key to living longer is eating less protein, and not necessarily fewer calories.

“We think that calorie restriction works not by restricting the amount of energy, but by restricting protein,” David Le Couteur told Science News. He studies the science of aging at the University of Sydney in Australia. He also worked on the mouse study. Le Couteur says this and other studies could help scientists understand exactly why eating fewer calories has appeared beneficial.

Protein is an important part of any diet, and growing kids need to make sure they get enough. But adults — they tend to get plenty. Longo, who worked on the new human study, recommends that adults replace some animal proteins in their diets with more plant-based ones, such as those proteins found in beans and nuts. After all, he argues, “If we’re wrong, there’s no negative side effect.” However, he points out, “If we’re right, it means a reduction in cancer and diabetes.”

Power Words

calorie   The amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is typically used as a measurement of the energy contained in some defined amount of food.

cancer   Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. It can lead to tumors, pain and death.

carbohydrates   Any of a large group of compounds occurring in foods and living tissues, including sugars, starch and cellulose. They contain hydrogen and oxygen in the same ratio as water (2:1) and typically can be broken down to release energy in the animal body.

diabetes   A disease where the body either makes too little of the hormone insulin (known as type 1 disease) or ignores the presence of too much insulin when it is present (known as type 2 diabetes).

fat    A natural oily or greasy substance occurring in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat is also a vital nutrient, though it can be harmful to one’s health if consumed in excess amounts.

metabolism  The set of life-sustaining chemical reactions that take place inside cells. These reactions enable organisms to grow, reproduce, move and otherwise respond to their environments.

proteins   Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells.