There is good news for smokers; a cigarette is apparently no more harmful for us than a chicken wing.

Or it's bad news for those Paleo diet people - they might as well be smoking cigarettes.

Or if you have seen scare journalism and miracle vegetable claims based on population statistics for more than a few years, you just take the whole thing with a grain of salt (but not too much salt!) and keep doing what you are doing.

A longevity analysis may not be drawing conclusions you like - if you eat meat in middle age you are 4X more likely to get cancer practically practically screams for a mainstream media headline - but that is because the methodology is rather weak. We can make correlations for almost anything in health, and that includes finding that eating meat is as likely to kill you as smoking is. Just find a population and a diet you don't like and see how many more people died than were on a diet you do like.  The authors suggest rates of cancer and death did not seem to be affected by controlling for carbohydrate or fat consumption, that animal protein is the main culprit. 

Do you see anything scary in this picture? It isn't just your imagination, meat and cheese are bad for you. Until next year, when they are not. Credit and link: Pamela Graham Photography.

It may be true that meat is bad. Smoking is obviously bad, but once the cultural vitriol snowballed people stopped asking the awkward questions about suspect claims. So we get stories about third-hand smoke killing us while the fact that only 10 percent of smokers get lung cancer and 50 percent of lung cancer patients never smoked gets ignored. Eating meat may be as bad as smoking because smoking is exaggerated. 

It isn't just meat that is the problem in the new study; milk and cheese are also implicated. The only thing that gets the protein stamp of approval are beans. 

The authors declare that those on higher-protein diets were 74 percent more likely to die (of any cause) than their more low-protein counterparts. They were also several times more likely to die of diabetes.

Yet we need protein. So how much is too much? How much is enough?  That is the diet book of the week. And this look at population statistics gives some insight but won't settle any debates.

Rather than look at adulthood as one phase of life, they break it into biological stages - and what works at one period may not work in another. In other words, what's good for you at one age may be damaging at another. Protein controls the growth hormone IGF-I, which helps our bodies grow but has been linked to cancer susceptibility. Levels of IGF-I drop off dramatically after age 65, leading to potential frailty and muscle loss. The study shows that while high protein intake during middle age is very harmful, it is protective for older adults: those over 65 who ate a moderate- or high-protein diet were less susceptible to disease.

The latest paper draws from past research on IGF-I by corresponding author Valter Longo, Professor of Biogerontology at USC and director of the USC Longevity Institute, including on an Ecuadorian cohort that seemed to have little cancer or diabetes susceptibility because of a genetic mutation that lowered levels of IGF-I; the members of the cohort were all less than five-feet tall.

"The research shows that a low-protein diet in middle age is useful for preventing cancer and overall mortality, through a process that involves regulating IGF-I and possibly insulin levels," said co-author Eileen Crimmins, the AARP Chair in Gerontology at USC. "However, we also propose that at older ages, it may be important to avoid a low-protein diet to allow the maintenance of healthy weight and protection from frailty."

The researchers define a "high-protein" diet as deriving at least 20 percent of calories from protein, including both plant-based and animal-based protein. A "moderate" protein diet includes 10-19 percent of calories from protein, and a "low-protein" diet includes less than 10 percent protein.

Even moderate amounts of protein had detrimental effects during middle age, the researchers found. Across all 6,318 adults over the age of 50 in the study, average protein intake was about 16 percent of total daily calories with about two-thirds from animal protein — corresponding to data about national protein consumption. The study sample was representative across ethnicity, education and health background.

"The majority of Americans are eating about twice as much proteins as they should, and it seems that the best change would be to lower the daily intake of all proteins but especially animal-derived proteins," Longo said. "But don't get extreme in cutting out protein; you can go from protected to malnourished very quickly."

People who ate a moderate amount of protein were still three times more likely to die of cancer than those who ate a low-protein diet in middle age, the study shows. Overall, even the small change of decreasing protein intake from moderate levels to low levels reduced likelihood of early death by 21 percent.

Credit and link: 10.1016/j.cmet.2014.02.006

For a randomly selected smaller portion of the sample – 2,253 people – levels of the growth hormone IGF-I were recorded directly. The results show that for every 10 ng/ml increase in IGF-I, those on a high-protein diet were 9 percent more likely to die from cancer than those on a low-protein diet, in line with past research associating IGF-I levels to cancer risk.

The researchers also extended their findings about high-protein diets and mortality risk, looking at causality in mice and cellular models. In a study of tumor rates and progression among mice, the researchers show lower cancer incidence and 45 percent smaller average tumor size among mice on a low-protein diet than those on a high-protein diet by the end of the two-month experiment.

"Almost everyone is going to have a cancer cell or pre-cancer cell in them at some point. The question is: Does it progress?" Longo said. "Turns out one of the major factors in determining if it does is is protein intake."