Jul 17 2008
A new study just published in the New England Journal of Medicine followed three diets for two years: a low-fat diet, a low-carb diet, and a so-called Mediterranean diet which is moderately low fat and replaces red meat with poultry, fish, and nuts. The result are being touted in the media as vindication for the Atkins style low-carb diet, but a careful look at the study tells a more complex story.
There are different ways to assess the healthfulness of a diet. Most Americans are primarily interested in weight loss, but there is also heart-health, reduction in risk for DM, and overall nutrition to consider as well. But let’s start with weight loss.
The low-carbohydrate (sugars and starches) diet is promoted primarily as a weight loss strategy. Proponents sometimes claim or infer that you can lose weight with a low-carb diet without reducing total calories – or, at any level of caloric intake, you will lose more weight. This has never been established and this new study does not establish it either. The real (and more plausible) question is whether or not a low-carb diet or a low-fat diet helps dieters achieve and maintain lower caloric intake to aid in weight loss.
This study looked at three groups of employees at an Israeli research center. They were given lunch according to which diet they were on (lunch is the biggest meal of the day in Israel) and then were counseled how to eat for breakfast and dinner. The study reports that exercise was similar among the groups. The low-fat diet restricted fat to 30% and total calories to 1500 per day for women and 1800 per day for men. The Mediterranean diet restricted fat to 35% with the same calorie restrictions and replaced red meat with poultry and fish and included things like nuts and olive oil. The low-carb diet restricted carbohydrates to 20g per day for a two month induction period then 120g per day after that, without other restrictions.
The results indicate that the low-fat dieters after two years lost an average of 2.9kg (6.5 lbs), Mediterranean dieters 4.4 kg (10 lbs), and low-carb dieters 4.7kg (10.3 lbs).
A couple of things to note from this – the first of which is that (as is typical of such studies) this is very modest. The people in the study had a BMI of 27 at least at the start (25 is overweight, 30 is obese). So they had the weight to lose (I understand the limitations of BMI, but on average this is likely true). A healthy diet and exercise program should be able to shed 10 pounds in 4-5 weeks.
But the real story comes from looking at the weight loss curves in the study. We see that all of the weight loss for all three groups occurred in the first 5 months. After that the low-carb and low-fat groups gained back some weight and then stabilized. Both of these groups gained back weight and stabilized along very similar curves. The only real difference between the two is that the low-carb diet lost more weight in the first 5 months – after that there was no advantage to low carb. This is consistent with earlier studies.
The Mediterranean diet also only resulted in weight loss for about 5 months but then stabilized without a period of weight gain.
To my understanding of the relevant literature, this study changes nothing and only confirms prior studies. Most people who diet will initially lose weight, and most will gain some or all of that weight back. There is a short term (5-6) month advantage for low-carb diets, but no long term advantage. Overall in such studies total weight loss is very modest. Weight loss comes from reduction of calories and increased exercise.
The media is reporting as surprising that the low-fat diet did not have an advantage to lipid profile over the low carb diet. This was surprising 20 years ago, but again this has already been established by previous trials. In this trial the ratio of LDL (bad cholesterol) to HDL (good cholesterol) improved by 20% for the low carb group, 17% for the Mediterranean, and 12% for the low-fat group. This is in rough keeping with the amount of weight loss.
This study does support prior research in establishing that a low-carb diet does not have a negative effect on lipid profile, as was feared by low-fat advocates. One interpretation of this is that weight loss is a dominant factor on lipid profile – trumping any differences in diet, at least as measured in these studies. Weight loss is a marker for level of exercise, which is known to increase HDL, and total caloric intake. A low total caloric intake may be a better predictor of total fat intake then the ratio of fat to carbs and protein.
This study, which appears to be a good study in terms of design and execution, and has the advantage of a 2 year follow up, generally supports the findings of prior studies. It reinforces my prior conclusions about different diet types and weight loss and overall health.
The primary lesson to take away, in my opinion, is that calorie control and exercise are of primary importance in losing weight, keeping it off, and maintaining a healthy lipid profile. Focusing on what kinds of food one eats makes little difference in the long run. Think about it – all the media hoopla is ultimately about 4 pounds difference over two years of dieting. That is nothing.
It’s even worse because the only difference really came from the induction phase of very low carbohydrates (20g per day) – after that there was no advantage to low-carb. This is a short-term, and ultimately failed strategy for weight control.
To be clear, the kinds of foods one eats does matter for things like overall nutrition. There is still evidence that it also mattes for lipid profile and diabetic health – but even for these issues overall weight and regular exercise are the dominant factors. They overwhelm the modest differences in diet types.
I agree with those who argue that studies like this that fuel the public’s obsession with diet strategies is partly to blame for the obesity epidemic. It is taking our eyes off the ball. What works long term is lifestyle changes, making good food choices easier for more people, increasing exercise, and portion control. Basic lifestyle factors.
What this study really shows is that tweaking the ratio of macronutrients in one’s diet is as effective as rearranging the deck furniture on the Titanic.
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