Thursday, July 24, 2008

More Evidence That a High-Carbohydrate, Low-Fat Diet Is Effective For Glucose Regulation

The following is a small study; its benefit is that intake was highly controlled: "The [intervention] diet was prepared in the metabolic kitchen at MIT and consumed by the subjects under supervision."

High-Carbohydrate , High-Fiber Diets Increase Peripheral Insulin Sensitivity In Healthy Young And Old Adults, American Journal of Clinical Nutrition, 1990

Six young subjects (18-24 yrs)
Six older subjects (67-86 yrs)

All 12 subjects ate their usual diet (control), ad libitum, followed by a high-carbohydrate, high-fiber, low-fat diet (HCF) for 21-28 days.
  • Control diet consisted of 42% carbohydrate, 40% fat, 18% protein, 17g fiber.
  • HCF diet consisted of 68% carbohydrate, 14% fat, 18% protein, 78g fiber. (No sucrose, i.e. table sugar, was allowed.)
When compared to the control diet, the HCF diet:
  • Lowered fasting glucose (~5%)
  • Lowered fasting insulin (~24%)
  • Lowered cholesterol (27% and 19% in young and old respectively)
  • Improved insulin sensitivity (via increased glucose disposal rates)
All differences reached levels of significance, that is, they were unlikely to occur by chance.

Triglycerides were not different between the two diets. (High-carb diets are often accused of raising triglycerides.)

All of the above beneficial changes returned to prestudy values after subjects resumed their usual high-fat diets for 2-4 weeks. (That didn't take long!)

"The results of these studies reveal that an HCF diet increases significantly the sensitivity of peripheral tissues to physiologic concentrations of insulin in healthy adults."
Long-term Effect

There's an element of this study I want to highlight. All measurements were taken in subjects who were in a postabsorptive state. That is, they were fasting, for about 12 hours. Glucose and insulin were administered intravenously. This was done to factor out the immediate effects (intestinally, hepatically) of dietary components.

Why was this done? Because there is evidence that high-carb, high-fiber, low-fat diets cause changes in body cells over time, changes that lead to improved insulin sensitivity. (I touched on this long-term effect in my posts about resistant starch.) These changes have been shown to include a higher insulin receptor number and improved receptor binding.

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