You probably know by now how bad smoking is for your health. But this shocking new study suggests that eating animal-based proteins may be just as harmful!

The study was published in the journal Cell Metabolism. Researchers studied data from the National Health and Examination Survey III (NHANES III), the only nationally representative dietary survey in the U.S. They divided participants into three categories: Those who consumed no more than 10 percent of their daily calories from protein, those who consumed 10-20 percent of their daily calories from protein, and those who consumed more than 20 percent of their daily calories from protein.

They then used the National Death Index to track participants' mortality over the 18 years following when data for NHANES III was collected.

It turns out that people who consumed the highest level of protein and were 50-65 when NHANES III was conducted were 74 percent more likely to die for any reason during the 18-year span of the study. They were also more than four times more likely to die of cancer during that period than other groups. The correlation held true even after researchers controlled for other factors such as socio-economic status, diseases, waist circumference, saturated fat intake, total fat intake and carbohydrate intake.

However, when researchers controlled for participants' intake of animal-based protein, the correlation all but disappeared, suggesting that animal proteins may be responsible for a significant portion of these relationships - when researchers controlled for plant-based protein intake, the correlation remained, suggesting they don't have the same effect.

Although this study didn't look at why this relationship might exist, lead study author Morgan Levine Canon, a Ph.D. student in gerontology at the University of Southern California, says a growth hormone receptor called insulin-like growth factor 1 (IGF-1) may play a role: "For most people, at some point in their life, they will probably have a cancer cell or pre-cancer cell in their body," says Canon. "But what determines whether that cell progresses into a full-fledged cancer cell might be some of these growth hormones since cancer cells are very sensitive to growth hormones; that's what causes them to proliferate and grow out of control."

Meanwhile, animal studies suggest that low levels of IGF-1 have been associated with a longer life, because they may allow cells to devote more energy to cell maintenance and repair, rather than growth.

So how does eating meat and dairy products translate into being as bad for you as smoking?

Valter Longo, director of the University of Southern California's Longevity Institute, told, "Cancer mortality was higher for high-protein [eaters] compared to current smokers."

The survey didn't distinguish between farm-raised meat consumption and factory-farmed meat consumption. "We know that there's a lot of hormones injected in animals, but unfortunately, the data we have doesn't provide information on that," says Canon. "I think down the road that's another study that people can look at in more detail."

Canon emphasized that the findings aren't a reason to continue or start smoking. She just hopes it will encourage people to cut back on their animal-based protein intake. "This is an association we're finding right now, and we recognize more work needs to be done with this," she says. "But there's a lot of evidence that probably eating plant-based is healthiest."