Comment Limitations of Study (Score 2) 477
A few notes to keep in mind with the interpretation of the results:
1. The macronutrient consumption data are based on food frequency questionaires, a somewhat unreliable means to measuring food consumption. How accurate do you think you'd be if asked about how much of each type of food you ate over the past few months? For more disucssion of problems with food frequency questionaires and other general issues with studies on nutrition see: http://fivethirtyeight.com/fea...
2. The study is an observational study that can only assess correlation, not causation. People who reported eating more carbohydrates had higher mortality. Was eating more carbohydrates the cause of the higher mortality, or were there other differences between people who ate more carbohydrates and those who ate less? A common problem with these studies is that people who follow dietary guidelines are more likely to follow other guidelines for healthy living, so one could just be picking up a signal from decreased mortality of those who pay attention to their health in general. Socioeconomic factors are another potential confounding factor in the results. In many of the third world countries studied, a diet higher in animal protein would likely be more expensive than a diet higher in carbohydrates. These confounding variables make inferring causation difficult. Randomized controlled trials would provide a gold standard for assessing whether there is a causal relationship between carbohydrate intake and mortality, though these are notoriously difficult to perform (how do you get a large cohort of people to change their diets for long periods of time?).
3. Even if the differences in diet are causally related to the changes in mortality and CV events, the exact mechanism is unclear. For example, in a commentary published in the Lancet along side the research paper (I would recommend reading the commentary if you are interested in the subject), the authors note:
Micronutrient malnutrition is an important problem in many of the countries included in PURE. Animal products are rich sources of zinc, bioavailable iron, vitamin K2, and vitamin B12, which might be suboptimal in populations consuming high carbohydrate diets. Therefore, one potential explanation for the PURE results is that nutrient-dense meats corrected one or more nutrient deficiencies
http://www.thelancet.com/journ...
If the results are partly due to consumption of animal products alleviating micronutrient malnutrition, it is unclear whether the results would be as applicable in populations where micronutrient malnutrition is not an issue.
Overall, the study is a very important piece of evidence in determining the best amount of carbohydrates, proteins and fats to include in one's diet. However, it is not a definitive study, so one needs to consider the entire body of evidence including observational studies (such as this one) done in a number of different populations, randomized clinical trials, and laboratory experiments that get at the mechanisms involved.