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Weight Loss Diet Study: Low Carb vs. Low Fat and Insulin Resistance

The weight loss diet traditionally recommended by health professionals, a low-fat, calorie-restricted diet, has been challenged by proponents of low-carbohydrate diets. Several weight loss studies have observed that insulin resistant adults have more success in losing weight with low carbohydrate diets, in contrast to insulin sensitive adults who have either more or comparable success with the low fat diets. The primary objective of this study was to determine if weight loss success can be increased if the dietary approach (Low Carb vs. Low Fat) is appropriately matched to an individual’s insulin resistance status.

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Eligible Participants

  • Generally healthy adults, 18-50 years of age
  • Overweight or obese

Study Design

Sixty-one participants were randomized into four groups—two classes of low-fat (LF) and two classes of low-carbohydrate (LC) diet, with approximately 50% insulin resistant and 50% insulin sensitive participants in each class for 6 months. Throughout they study, they attended 14 one-hour classes led by a registered dietitian. Participants were instructed to cut back their total fat or digestible carbohydrate intake to 20 g per day, with the goal to achieve the lowest level of fat or carbohydrate intake within the first 8 weeks. They then slowly added fats or carbohydrates in increments of 5 to 15 g per week until they reached the lowest level of intake they believed could be maintained indefinitely. There was an emphasis on the quality of the diet and to prioritize nutrient-dense, whole foods. Change in weight was assessed at the beginning and end of the study.

Conclusions

Participants, overall, experienced substantial weight loss after 8 weeks —an average of 20 pounds— which represented 9% of their baseline weight. However, there was no significant interaction between diet assignment and insulin status for weight loss. These findings suggest that matching the diet to the individual’s insulin resistance status did not lead to increased weight loss success. The opportunity to detect a significant difference could have been limited by the focus on high diet quality for both diet groups and sample size.

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