Fats are good for you. So says yet another medical study that threatens to shake up the world of nutrition and dietary guidelines.
The headlines from PURE study, published in The Lancet, seem to suggest that there a benefit to eating a high fat diet. But a deeper dive into the details of the study’s results may result in a bit of caution about how we interpret this latest information.
The PURE study followed 135,335 participants from 18 countries over about 7 years. Their dietary intake was evaluated using self-reported food questionnaires. Their diets were categorized based on the percentage of total calories provided by carbohydrates, fats and proteins. The effects of their diets were determined on the basis of total mortality as well as major cardiovascular events such as heart attacks, strokes and heart failure.
The authors found that high carbohydrate intake was associated with a higher overall risk of mortality. Those participants with the highest carbohydrate intake, at about 77% of their daily calories, were 28% more likely to have died than those with the lowest carbohydrate intake at about 46% of their calories.
The opposite was true for fat intake. Those with the top intake of dietary fat, at about 35% of their daily calories, were 23% less likely to have died than those with only about 10% of their daily calories coming from fat. Additionally, the authors stated that higher saturated fat intake was associated with a lower risk of strokes.
So, is it time to upend dietary guidelines that recommend law fat diets as the best path to reducing cardiovascular risk?
There are some problems with the PURE study that makes interpretation of the results a bit problematic.
The study found a beneficial effect from eating more fruits, vegetables and legumes. But the maximum benefit was seen at three to four servings per day. Higher intake of these foods was not associated with any additional benefit. And, the benefit was greater when these foods were eaten raw instead of cooked.
The design of the study itself also raises concerns.
The diet of the participants was determined using food questionnaires. But these types of questionnaires are notoriously unreliable. Besides being based on recall which can be less than accurate, the mere act of asking people to report what they eat can both influence their diet and also lead them to try to report their intake in a way that they think sounds healthier than it really is.
There is also the problem of adjusting for confounding variables. Some experts have criticized the study design for failing to take into account that a very high carbohydrate diet is often associated with very high levels of poverty. That may have a bigger effect on health outcomes than the diet itself. Total mortality is also a problem in a study such as this one. While it is an outcome that is highly objective and easy to quantify, there are so many ways that overall mortality may be affected by a plethora of dietary, lifestyle and health problems that it is difficult to ensure that the change in mortality is solely related to the dietary variables being measured.
In the United States, increasing obesity and the prevalence of highly refined sugars and processed foods is clearly leading to an epidemic of type 2 diabetes and its precursors such as Metabolic Syndrome. There are well established physiologic mechanisms that appear to increase the risk of cardiovascular disease from the biochemical changes associated with diabetes and the years that lead up to it. So it does seem highly likely that over-emphasis on removing fat from your diet, which by default leads to a diet that is much more dependent on carbohydrates as a calorie source, can have deleterious effects on our health.
Multiple studies seem to imply that saturated fat is more harmful than unsaturated fats. Limiting total calories from saturated fat to less than 10% of the diet continues to seem like a reasonable recommendation. The benefits of limiting unsaturated fats is less clear. Limiting carbohydrates, especially those based on refined sugars or high in fructose, while eating a predominantly plant-based diet with more vegetables and moderate intake of fruit also seems prudent.