Glycemic Index, Carbs & Weight Control
Glycemic Index & Carbohydrates
The glycemic index of each dietary carbohydrate provides a measure of
its ability to raise post-prandial blood glucose values. High glycemic
index foods give higher post-prandial blood glucose levels than low glycemic
index foods when compared to a standard of white bread or glucose. It
has been demonstrated that the glycemic index of a carbohydrate provides
a good predictor of the insulin response to that food. The nutritional
branch of the WHO endorses the use of the glycemic index as a method of
categorising carbohydrates as this provides information on the likely
metabolic effects of that carbohydrate. Low glycemic index diets have
been shown by ourselves and others to reduce fasting and post-prandial
insulin, glucose, tryglyceride and non-esterified fatty acid concentrations.
In addition these diets increase HDL-cholesterol and decrease fasting
total cholesterol, while improving in-vivo and in-vitro insulin mediated
glucose uptake. Prospective studies have demonstrated that low glycemic
carbohydrates improve insulin sensitivity in subjects with diabetes, obesity
and CHD, as well as those at risk of CHD. Intervention studies using low
glycemic index diets have shown VLDL concentrations are lowered and one
recent study has reported an increase in HDL concentrations in a small
cohort of type 2 diabetic subjects. From the above studies we conclude
that low glycemic index diets have been associated with a wide range of
benefits on the established metabolic risk factors for CHD.
Low Glycemic Diets
Our most recent work has provided insight into the mechanisms by which
low glycemic diets improve insulin sensitivity. We have shown that low
glycemic index diets improve both adipocyte insulin-mediated glucose uptake
in-vitro and insulin sensitivity in-vivo as assessed by the post-prandial
fall in non-esterified fatty acids (NEFA) levels. The literature suggests
that a 10 percent fall in the glycemic index of a diet will result in
a 30 percent increase in insulin sensitivity. These observations support
the commonly held hypothesis that reducing post-prandial NEFA levels optimises
insulin stimulated glucose uptake in muscle, thereby increasing insulin
sensitivity. Reducing post-prandial NEFA levels is important as their
concentration has a rate-limiting effect on hepatic VLDL synthesis. High
levels of VLDL production result in reduced HDL-cholesterol and increases
in the formation of atherogenic small dense LDL. In a large cross-sectional
study (2200 healthy adults) we have demonstrated that the glycemic index
of the diet is a greater determinant of HDL cholesterol than any other
aspect of the diet, be it fat or dietary fibre. The Framingham study found
that a 3 percent decrease in female and a 2 percent decrease in male cardiovascular
morbidity was associated with a 0.026mmol/l increase in HDL-cholesterol.
In our study, the HDL-cholesterol of the women in the lowest quintile
for glycemic index was 0.25mmol/l higher than for the women in the highest
quintile. Extrapolating from published data this difference would translate
to a 29 percent reduction in CHD morbidity. The corresponding potential
decrease in male CHD morbidity would be 7 percent reflecting the 0.09
mmol/l difference in HDL-cholesterol between the lowest and the highest
glycemic index quintiles, this has been confirmed recently by Willitts
team. Also prospective evidence suggests that low glycemic diets will
lower total cholesterol, affect clotting through suppression of plasminogen
activator inhibitor compound 1, reduce blood pressure and induce weight
loss.
Source: Dr Gary Frost, Head of Nutritional
and Dietetics/Senior Lecturer, Hammersmith Hospital, London. (2001)
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