Observations and Questions
After studying this research paper and the data presented, I have a few observations and questions:- The low-fat diet could not be considered very low in fat, as 30% of the calories were from fat. In fact, the fat intake of subjects on the diet was about the same as before the diet. Although it is the generally accepted standard for a "healthy low-fat diet," advocates of more restrictive low-fat diets correctly point out that this is not a test of the very low-fat diets they advocate. Dr. Dean Ornish is one of these advocates, and the diet he advocates -- for example -- includes, at most, 10% of calories from fat.
- Likewise, although the low-carb group started out eating a diet very low in carbs, by the six-month mark, they were consuming 40% of their calories from carbohydrates. This is surprising considering they were supposed to be following Dr. Atkins' plan, which instructs the dieter to slowly raise the amount of carbohydrate until weight loss slows to about 1 to 2 lbs. per week. It would appear that they kept raising the amount of carbohydrate until 120 grams was reached, as dictated by the study. It's not surprising, then, that the participants mostly stopped losing weight at about the sixth month. What is more suprising is that they still lost more weight than the calorie-restricted diets at all points along the way, including the 2-year mark. On the other hand, they continued to eat only half of the amount carbohydrate that they ate before started the diet.
- The other two groups were also eating less carbohydrate than before the study began -- about 50 grams less for the Mediterranean group and 85 grams less for the low-fat group. This was because these groups were consuming fewer calories than before. So, even though the percentage of carbohydrate was the same, they were eating less total carbohydrate. Were any of the changes these participants experienced due to the slight decrease in carbohydrate? We can't say.
- In the diabetics, the fasting blood glucose was, on average, down more in the Mediterranean diet group, but the Hemoglobin A-1C was down more in the low-carb group. Since both are a measure of blood glucose, why the discrepancy? Well, Hemoglobin A-1C is an indicator of average blood glucose, including after meals. It would seem that, overall, the blood glucose in diabetics was reduced more by the low-carb diet.
- The low-carb group ate less fiber than any group. Compared to before they started the diet, at 6 months, they were eating 7 grams less. At 24 months, they were eating 10 grams less. Why were they eating less fiber as time went on? It would seem that they were adding low-quality (refined) carbohydrate to their diets. Interestingly, the Mediterranean group only minimally increased their fiber intake, while the low-fat group ate about 5 grams less than before starting the diet (baseline).
- In all of these changes from baseline, we aren't given the starting amounts, which would have been very helpful in analyzing the study.
- We are not told why the low-carb group was instructed to seek vegetarian sources of protein and fat, as Atkins was neutral on this point except to seek a balance of fats and avoid trans fats.
- This study is more well-controlled than most, as participants were provided with intensive education and diet-appropriate lunch food, and measures were taken to check on adherence to the assigned diet.
- At the very least, it would seem that this two-year study should "tip the balance" as to the question of whether a low-carb diet is healthy and as effective as other options for weight loss. I think that low-carb diets should at least be offered to patients alongside other options.
- The researchers speculated that monounsaturated fat may have contributed to the positive outcomes in the Mediterranean diet. Others have speculated that the higher fiber level may also be a partial explanation. As a side note, there's no reason why fiber and monounsaturated fat can't be part of a low-carbohydrate diet. Could this be the best of both worlds? Possibly, but more research is indeed necessary to say for certain.
Source:
Shai, Iris, et al. "Weight Loss with a Low-Carbohydrate, Mediterranean, or Low-Fat Diet." The New England Journal of Medicine. 359:229-241 (July 17, 2008)
