Nutritionists are especially uncertain about how the body reacts to processed protein isolates and powders, which have skyrocketed in popularity. (iStock) (Moussa81/Getty Images/iStockphoto)

If there's one claim that's almost certain to boost sales of a food these days, it's to say the item is high in protein.

Consumers cannot seem to get enough protein — they often turn to it because they've shunned carbohydrates, and also associate it with increased muscle mass. While many nutritionists say eating extra is usually harmless — if it's part of a balanced diet and doesn't all come from animal sources — and small increases can indeed help with weight control by increasing satiety, others are not convinced, citing the lack of long-term research on high-protein diets.

They're especially uncertain about how the body reacts to or uses processed protein isolates and powders, which have skyrocketed in popularity.

A growing body of evidence suggests that some segments of the population should be cautious about hopping on the high-protein bandwagon, infants and young children in particular. Some studies have linked high protein intake in early childhood to a risk of obesity later in life. Researchers are still trying to understand what accounts for that link.

Pregnant women, meanwhile, are commonly advised to boost protein intake. But in a recent study of a group of women who consumed relatively high amounts of protein, children born to the mothers who consumed the most during pregnancy were shorter at birth and through mid-childhood than children of mothers who consumed the least protein.

Karen Switkowski, lead author of the study, said that "while it's important for women to eat enough protein to support the growth of their baby, they might want to be cautious about going far beyond the recommended amounts." (She said there's not enough data yet, though, to set specific pregnancy-related recommended levels, adding, "I think that more research needs to be conducted in this area in different populations before translating the findings into any guidelines.")

Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health, extends that caution to people of all ages, citing protein's role in cell multiplication. He explains that protein — especially from animal sources, and in particular from dairy — boosts a growth-promoting hormone that makes cells multiply faster, which is vital early in life but not necessarily later on in life.

"Overly rapid cell multiplication is one of the underlying factors for cancer," Willett said. "It seems pretty clear that we don't want to have our cell-growth accelerator to the floor from the day we're born until the day we die."

Some studies on later-life protein consumption, meanwhile, have raised an important concern.

One preliminary study, which evaluated the self-reported diets of more than 100,000 women between ages 50 to 79, appeared to find a significantly higher rate of heart failure among those who ate a lot of animal protein than among those who ate less of it.

Older adults are often told to seek out extra protein, largely to help them maintain muscle mass, which deteriorates as one ages. Willett said that's not bad advice, but not to go overboard. "Having some hormonal boost from protein sources may not be a bad thing. It may be good — although the most important way to maintain muscle mass is resistance training," he said.

How much protein, then is good?

Most nutrition experts are reluctant to cite a single number because individual needs are so variable, but Willett offers some guidance — along with a few qualifications.

"I think a range for total protein between about 12 to 20 percent of calories is okay; pushing higher, especially with protein supplements, is certainly not necessary and has potential long-term hazards," he said.

"I am particularly concerned about adding protein supplements, such as whey protein, which has a strong effect on cell multiplication," he said, then added some practical advice: If protein is taking the place of foods high in sugar or refined starch — white bread, for example — it will benefit the body. But if it's replacing foods rich in whole grains and healthy fats, it won't.

John Swartzberg, a professor emeritus at the University of California at Berkeley School of Public Health, said certain groups definitely should ignore the ­increase-your-protein message.

It has long been known that too much protein is harmful for people with chronic kidney disease. (Kidneys are responsible for eliminating the products of protein metabolism, and those products accumulate in the blood when kidneys don't function well.) But it can also exacerbate damage to kidneys that someone may not yet know are already impaired, before clear evidence of poor kidney function is apparent.

Swartzberg said some studies show that about 1 in 9 Americans have impaired kidney function, many of them unaware of it. For such people, following the high-protein trend will accelerate a decline in kidney health. "It's an asymptomatic problem until it's mid-stage kidney disease," he said. People who want to assess their kidney status, he advises, can request a blood test for creatinine for initial screening.

Even for those with healthy kidneys, Swartzberg urges caution about excess protein. While some nutrition experts say there's no evidence suggesting eating even twice the recommended daily allowance for protein, Swartzburg says there isn't enough long-term data to conclude that such a high number is either good for you or safe. He suggests an amount somewhere between 100 and 150 percent of the recommended daily allowance, which is 0.8 grams per kilogram of body weight; that translates to between about 54 grams and 82 grams of protein for an adult who weighs 150 pounds. "I certainly would not eat excessive protein. I would never take any protein supplements. And I wouldn't advise my children to, either," he says.

Preventive cardiologist Stephen Devries, the executive director of the nonprofit Gaples Institute for Integrative Cardiology in Deerfield, Ill., recommends avoiding or only eating only minimal amounts of animal protein; he is also cautious about what he calls "artificially enhanced protein," such as protein powders, even ones derived from plants. He recommends getting your protein instead from beans, lentils, nuts and tofu. "These are terrific sources of protein, and they're the ones we should concentrate on, rather than the artificial sources, whether they come from animals or plants."

Some people think the benefits of extra protein give them a free pass to simply eat more — but protein calories are still calories.

Marion Nestle, a professor of nutrition, food studies and public health at New York University, said, "If you eat a lot of extra protein, you're either breaking it down for energy or you're turning it into sugar and into fat — one or the other."

Her general advice is for people to stop obsessing over protein. "It is most definitely not a nutrient of concern. Most people get twice as much as they need without thinking about it," she said. "My nutrition pet peeve is calling foods 'protein,' as in 'Would you like some protein with that salad?' If the salad has beans or grains or cheese, it already has protein."

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