Think twice before you pick up that hamburger or glass of milk. According to a new study that tracked a large number of adults for almost two decades, eating a high-protein diet could be as deadly as smoking cigarettes. While some people use high-protein diets—such as Atkins or Paleo—for short-term weight loss, they may have negative effects on health as you get older.

“There’s a misconception that because we all eat, understanding nutrition is simple,” said study author Valter Longo, a professor of gerontology and biological sciences at the University of Southern California in a press release. “But the question is not whether a certain diet allows you to do well for three days, but can it help you survive to be 100?”

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In the new study, which was published today in Cell Metabolism, researchers found that eating a high-protein diet during middle age increased the risk of dying from any cause by 74 percent and the risk of dying from cancer by more than four times, compared to eating a low-protein diet. The study included 6,381 adults ages 50 and older who were followed for 18 years. 

This puts high-protein diets right up there with smoking, which according to the Centers for Disease Control and Prevention, increases the overall risk of death by three times, compared to not smoking.

While the largest difference in the study was between the high- and low-protein diet groups, people who ate moderate amounts of protein during middle age were still three times more likely to die of cancer later on than low-protein eaters.

As many vegetarians and vegans are quick to point out, animal products are not the only sources of protein. But when the researchers took into account how much of the participants' diets was made up of plant-based protein, the results changed very little. The researchers say this suggests that animal-based proteins are the driving force behind the increased risk of death seen in the study.

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The findings for middle-aged participants fit with other research on the health effects of saturated fats found in meat and dairy products, which include an increased risk of chronic conditions like heart disease. 

In this study, though, the story was different for older adults. People over the age of 65 who ate a high-protein diet had a 28 percent lower risk of death from any cause and a 60 percent decreased risk of dying from cancer, compared to those eating low amounts of protein. Eating high amounts of protein at any age still increased the risk of dying from diabetes.

The apparent protective effect of high-protein diets in older adults, say the researchers, may have something to do with the growth hormone IGF-1. The level of this hormone decreases with age, which may contribute to the loss of muscle mass and increased susceptibility to disease. Eating more protein, then, could help stimulate production of this hormone and maintain overall health as people age.

Because this is the first study to show this type of age-related variation, it may be too soon for people to radically alter their diets. 

“It’s an observational study of a large population,” says Dr. Keith-Thomas Ayoob, an associate clinical professor of pediatrics at the Albert Einstein College of Medicine. “As such it generates a hypothesis, but we shouldn't conclude too much without additional clinical research to confirm the results, especially since things seem to flip-flop after age 65.”

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While more research is needed to understand why protein affects middle-aged and older adults differently, Ayoob says that some things about protein are clear.

“Most people need to eat more plant proteins, as well, and it isn't an either/or thing,” he says. “If people included beans, either alone or with lean protein, three times per week, there's good clinical evidence it would be a heart-healthy change.”

This is equally true for older adults.

“People over age 65 tend to lose muscle,” says Ayoob, “and with that they can lose strength, neither of which we want to happen, so they can need more protein, as well as physical activity to preserve muscle.”

Unless you are ready to give up animal products altogether, aim to reduce your intake of saturated fats from animal products. This can be done by choosing leaner cuts of meat, removing the skin from chicken and turkey, and choosing low- or no-fat dairy products. 

Also, if you tend to fall into the high-protein diet group, cutting back on your intake will also make room for other nutritious foods.

This may be especially useful advice for pregnant women. In a new study of 66,000 women, published online today in the medical journal BMJ, researchers found that women who ate a diet rich in vegetables, fruits, and whole grains had a significantly reduced risk of preterm delivery.

Although the study cannot prove that diet was directly responsible for the decrease in preterm births, it reinforces the benefits of healthy diets for pregnant women—and everyone else, over the long term.