One thing is clear: there is no single diet that works best for everyone. I think that a major goal of the next phase of diet research is to figure out which diets will work best for which people. This small, but extremely well-controlled study, published in Obesity Research earlier in 2005, is a great contribution to that effort.
Bottom line: Participants who were insulin resistant lost more weight on a reduced carbohydrate diet, whereas people who were insulin sensitive lost more weight on a high carb/low fat diet.
Details: The researchers hypothesized that people who are insulin resistant might respond better to a diet that is less glycemic (due to fewer carbs and more fat) than people who have normal insulin sensitivity. So they took a group of mildly-moderately obese women and put the ones who were most insulin resistant in one group, and the ones who were most insulin sensitive (least resistant) in another. They didnt use people who were in the middle, to make the results clearer.
Next, they figured out everyones metabolic rate, body composition, and how many calories their bodies were using. Then, for 16 weeks, each subject was given food to eat that would be 400 calories less than they needed to maintain their body weight. Half of each group received a diet that was low in fat and high in carbohydrates (60% of the diet), and the other half ate a diet that was higher in fat and lower in carbs (40% of the diet).
The results were nothing short of astonishing. Despite being on diets where the calorie levels were tailor-made for each individual, the results varied wildly depending both upon diet content AND insulin sensitivity level:
- The insulin resistant people lost 13.4% of their body weight average almost 25 lbs - on a low-carb diet, but only 8.5% (average 16 lbs) on a high carb/low fat diet
- The results for the insulin sensitive folks were precisely the reverse 13.5% body weight lost (average 25 lb) on the high carb diet and 6.8% (average 13.5 lbs) on the low-carb diet.
- Fasting insulin levels dropped in all groups, but more in both insulin resistant groups.
- Insulin sensitivity improved in the insulin-resistant groups, the low-carb more than the high-carb (possibly because more weight was lost).
- Triglycerides improved in all groups except for the insulin resistant group on the high carb diet. That group increased their triglycerides significantly.
- The two mismatched groups (losing the least amount of weight) lost the amount of weight that would be expected, given their calorie levels. The big mystery still to be solved is why the matched groups lost so much more that would have been expected.
- People did not report changing their activity levels through the study (and had been instructed not to).