I’ve received a lot of emails about an article that appeared in the July 17, 2008 issue of the NEJM. The article reports on a two-year study comparing weight loss results between a low-fat diet, a low-carbohydrate diet, and the Mediterranean diet. I asked the lead author to do an interview and she agreed to answer some questions by email. I attached the questions I sent her to the end of this blog post. I have several additional questions, but I didn’t want to push my luck.
The study suffers from the same problem that most studies comparing weight loss diets suffer from: the researchers relied entirely on study subjects reporting what they were eating. This means that the data is suspect. The fact that there was “no statistical difference” between the amount of calories cut from the baseline diet by each group and no statistical difference between the amount of calories expended with exercise makes it curious as to how there was a difference between the groups regarding weight lost.
And, by the way, there was not a significant difference in weight lost between the low-carb diet and the Mediterranean diet.
I’ll go into more detail with my podcast discussion to be posted in a few days - podcast 107.
My questions to the lead author of the paper:
1. You mention that people in the low-carb group were advised to get most of their protein and fat from vegetable sources (page 231, first column under the heading “Low-Carbohydrate Diet”). I could not find a table in the paper or in the attached appendices that gives data on what foods people in any of the three groups actually ate. Did the people in the low-carb group eat most of their fat and protein from plants as opposed to animals?
2. If the people in the low-carb group did eat mostly a vegetarian diet, how is that different from a Mediterranean diet? Is it only the olive oil?
3. Is there data available for what foods each group actually ate?
4. Was there an emphasis on whole grain products vs refined grain products in all three groups?
5. Were the carbohydrates consumed by the low-fat and Mediterranean diet group mostly in the form of white bread and other refined carbohydrates, or were they from a variety of whole grain products, beans, fruits and vegetables?
6. The difference in decreased calorie consumption between groups did not reach statistical significance. But, the Mediterranean group had a decrease in calorie consumption of about 100 calories per day less than the other two groups. Multiplying this 100 calorie/day difference by 730 days (2 years) gives a difference of 73,000 calories or about 21 pounds of weight loss that should have occurred in both the low-fat and low-carb groups compared to the Mediterranean group – yet this did not happen. Why?
7. How do you account for any difference in weight loss between the three groups if your data suggests there was no statistical difference in the amount of decrease in calorie consumption and no difference in the amount of calories burned with exercise?
8. People have suggested that possibly dietary choices influence metabolic rate and therefore a “calorie may not be a calorie.” If there is a change in metabolic rate that accounts for different amounts of weight loss between different groups why didn’t you measure resting metabolic rate in your subjects before and after the diet changes?
9. The low-carb group had the biggest increase in HDL. They also had a relative and an absolute increase in the consumption of saturated fat by the data provided in table 2. Saturated fat consumption has been shown to increase the “wrong kind” of HDL – a form of HDL that is actually proinflammatory and proatherogenic. I think it would be important to mention this, but you didn’t. Why? See reference: http://content.onlinejacc.org/cgi/content/full/48/4/715
10. The Mediterranean diet was not developed by an entrepreneur looking to cash in on the obesity epidemic like the Atkins diet was. The Mediterranean diet is a traditional eating pattern that has been consistently linked with lower rates of cardiovascular disease, type 2 diabetes, and cancer over decades of research. The Mediterranean diet was not “created” as a weight loss diet. It was also a traditional eating pattern associated with a lifestyle that included a lot of physical activity, less stressful work and family settings, and moderate alcohol consumption. I think it is misleading and confusing to drag the Mediterranean diet into the ridiculous debate between low-fat and low-carb zealots because of this very fundamental difference in the origin of the various diets. The Mediterranean diet is really much more about a lifestyle than a diet – and it certainly isn’t a quick weight loss program. Your thoughts?
Sincerely,
Monte Ladner, M.D.
www.fitnessrocks