There has been a lot of hype and a lot of confusion when it comes to what type of diet is best for you. Low carb? Low fat? High protein? When the Atkins diet was all the craze, concern was raised that the high amount of fat in that diet would have an adverse effect on heart health.
An article in this week’s New England Journal of Medicine helps to allay that concern. The results of the study, titled, “Low-Carbohydrate-Diet Score and the Risk of Coronary Heart Disease in Women,” suggest that diets lower in carbohydrates and higher in protein and fat do not increase the risk of coronary artery disease. If vegetable sources of fat (e.g., olive oil) and protein (e.g., soy) are used, these diets may actually reduce the risk of heart disease to a moderate extent.
How the study was done
Thomas Halton, ScD, and colleagues used data from the 82,802 women who completed the “food frequency” portion of the Nurses’ Health Study questionnaire to look at the relationship between dietary choices and heart health.
The Nurses Health Study was started in 1976 when nurses (age 30-55) were asked to complete periodic questionnaires that asked about many different aspects of their health and lifestyle. A “semi-quantitative food frequency” questionnaire was used to obtain detailed information about what types and how much of different foods they ate. This group of women has been followed, evaluated, and resurveyed every two to four years since then. Therefore, this is a rich source of information about the impact of diet over a large part of a woman’s lifespan.
The researchers calculated a “low-carbohydrate-diet” score by looking at the percentage of total energy that was consumed as carbohydrate, fat, or protein. Scores ranged from 0 (the lowest fat and protein intake and the highest carbohydrate intake) to 30 (the highest protein and fat intake and the lowest carbohydrate intake). In addition, they calculated two additional scores that took into account whether the protein and fat in the diet came from an animal or a vegetable source. They also calculated the glycemic index and the glycemic load of the different diets.
They divided the women into ten categories (deciles) based on their low-carbohydrate-diet scores. Women with the lowest scores (higher carb intake) were in decile 1 and women with the highest scores (low carb intake) were in decile 10. They then tried to determine if there was a statistical correlation between the type of diet and whether women developed heart disease or not. Because diets can change over time, they used the average scores from the survey administered within a four-year time frame of the heart disease event to determine if there was a correlation.
And the results?
There were 1994 new cases of coronary heart disease during the 20 years of the study. The researchers found the following relationships between the presence or absence of heart disease and different diets as follows:
- There was no statistically significant impact of low carbohydrate diet on the risk of coronary heart disease.
- If fat and protein in the diet came from vegetable sources, as opposed to animal sources, there was a moderately lower risk of coronary heart disease.
- When the glycemic index of the carbohydrates was taken into account, there was an association with increased risk of coronary heart disease, but it was not statistically significant.
- Women in the study were not trying to lose weight on the low carbohydrate diets in this study. Overall, the mean body mass index increased to the same degree in all of the groups of women, regardless of the low carbohydrate score.
So what does this mean for your everyday life?
Low carbohydrate diets, at least in groups of women who were not using them over prolonged periods to lose weight, do not appear to increase the risk of heart disease. So, it may be ok to get relatively more of your calories from protein and fat. However, if you want a diet that protects you from heart disease, make sure that fat and protein come from largely from vegetable sources (e.g., olive oil) instead of animal sources (meat).