A new study in Cell Metabolism about the benefits of a low-protein diet has some media outlets reporting that a high protein diet is as harmful as smoking, suggesting a Paleo lifestyle is not good for people. I have been asked by several people to comment on this study, so here is my take.
First, a quick summary. The authors used data from a NHANES III, a national US nutrition cross sectional survey, to estimate the usual intake of protein from animal sources and the risk of dying from specific diseases like diabetes, heart disease, and cancer. The data was analyzed for those between the ages of 50 and 65, versus those over 65 years old.
The subjects were grouped according to whether their diets were low, moderate, or high protein. The study found that overall, high protein intake was associated with a higher risk of diabetes mortality. More specifically, those in the 50 to 65 age group had a higher risk of death on a high protein diet, while those over the age of 65 had a lower risk of death. The authors then used mice studies that looked at protein intake, insulin like growth factor 1 (IGF-1), and insulin levels and mortality and found that increased protein intake, and increased IGF-1 and insulin levels in mice, were associated with increased mortality from cancer.
Is this big news?
Not so fast. It’s easy to make headlines by oversimplifying studies, so let’s look a little more closely at the limitations of this particular study. First of all, the authors note that the study is based upon one 24-hour recall to determine the food intake of the participants. In other words, they asked each person to remember what they ate in the last 24 hours, which isn’t necessarily indicative of what they eat all the time, or enough information to classify them into a protein group that is accurate over a lifetime.
In the animal portion of the study, the level of insulin and IGF-1 was associated with the amount of protein in the diet and increased risk of cancer. However, carbohydrates also increase insulin and IGF-1. Without looking at how carbohydrates also have the potential to have negative impacts on insulin and IGF-1, it is very difficult to know what effect protein alone has. Protein was the only macronutrient they studied.
Also not included in the data was vegetable and fruit consumption. The high flavonoid content in vegetables and fruits protects against the very diseases the study was analyzing (diabetes, heart disease, and cancer). For these reasons, isolating protein without assessing vegetable and fruit intake or carbohydrate intake is not very useful in understanding how to eat for optimal health.
And I haven’t even started in on how the study didn’t look at toxin exposure (such as smoking), activity level, socioeconomic status, social and family relationships, and stress. We also know that some societies do quite well on high-protein diets. There are two traditional societies that have thrived, eating almost exclusively animal products — the Arctic Inuit and the African Maasai. Both societies have minimal plant material in their diets along with plenty of physical activity, rich social networks, and minimal exposure to toxins.
Again, there are many factors that contribute to health and vitality which must be considered as you are trying to create your optimal eating and living plan to achieve more energy and vitality in your life. We know all of these things impact chronic disease rates and death rates.
Obviously, studies are easier to conduct if they isolate one aspect. Without a randomized prospective trial that controls for all these known lifestyles factors that influence health, you don’t know the impact of a eating a high animal protein diet. It is critical that we remember that epidemiologic data that show association is not the same as data that show causation. Rather, the epidemiologic data raise interesting questions or hypotheses that need to be tested prospectively.
To understand causation, it is far more useful to look at a prospective study that uses randomized controls, such as “Flavonoid-rich fruit and vegetables improve microvascular reactivity and inflammatory status in men at risk of cardiovascular disease.” In this trial, subjects were randomized and then the intervention group was given six servings of vegetables and berries a day.
Both groups were asked to maintain their usual lifestyle factors that influence health (e.g., physical activity, smoking pattern, and stress reduction activities) throughout the study, and the study controlled for these elements. Numerous biologic assessments were made of blood vessel function in both the group that had the intervention as well as the control group that continued their usual eating pattern.
The intervention group had healthier measures on many fronts, but researchers were able to conclude that consuming six or more servings of vegetables and berries improved blood vessel health and many biologic markers of risk for heart disease.
So what is my interpretation of this study?
That diets (whatever they might be) that lead to high insulin and IGF-1 increase the risk of cancer. Mouse studies demonstrate this. We don’t know if the mouse model would also suggest a higher risk for diabetes and heart disease because that experiment hasn’t been conducted and or reported yet. And that is all this study really says to me… see more