Carbohydrate type glycaemic response and weight control

It has been debated whether excess dietary carbohydrate can increase adipose stores. Although test animals are able to convert significant amounts of ingested carbohydrate into body fat, in humans, de novo lipogenesis from carbohydrate appears to be limited (Strawford et al., 2004). Despite this, excess dietary carbohydrate may indirectly increase body fat stores. Dietary carbohydrate, in the form of starch or sucrose, increases blood insulin levels, which in turn increase activity of the enzyme lipoprotein lipase. Lipoprotein lipase mediates storage of dietary fat in adipose cells. At the same time, insulin decreases the activity of hormone-sensitive triglyceride lipase, an enzyme that regulates the release of fatty acids from stored fat. Thus, excess dietary carbohydrate increases the amount of dietary fat that is stored, and decrease fat turnover (Allred, 1995).

Short- and long-term studies in humans and animals indicate that high-GI diets affect appetite and nutrient partitioning to promote fat storage. However, human studies showing reduced bodyweight after consumption of low-GI diets need to be interpreted with caution. The outcome can rarely be attributed solely to the GI, because interventions designed to modify the GI of a diet usually also modify other variables that influence bodyweight (e.g. fibre content, palatability, energy density). Pawlak et al. (2004) assigned rats and mice either to a low- or a high-GI diet. The carbohydrate portion of the low-GI diet consisted of 60% amylose : 40% amylopectin starch, whereas the carbohydrate in the high-GI diet was 100% amylopectin starch. Other than this, the two diets were similar in nutrient and energy content. In both mice and rats, animals consuming the high-GI diet had more body fat and less lean body mass than those on the low-GI diet. The rats on the high-GI diet required less food to gain the same amount of weight than the low-GI group. The high-GI group also showed a greater increase over time in the area-under-the-curve of blood glucose and insulin after an oral glucose load. The authors suggested hyperinsulinaemia resulting from the high-GI diet altered nutrient partitioning in favour of fat deposition, shifting metabolic fuels from oxidation in muscle to storage in fat. Overall, the findings indicate the consumption of a high-GI diet per se adversely affects body composition in rodents (Pawlak et al., 2004).

Ludwig et al. (1999) examined the effect of isoenergetic low-, medium-and high-GI breakfast and lunch meals on ad libitum food intake during the 5 hours after lunch. Compared with the low- and medium-GI groups, ratings of hunger were higher in the high-GI group during the postprandial period. In addition, voluntary energy intake after the high-GI lunch was 53% and 81% higher than after the medium-GI and the low-GI lunches, respectively. In addition, compared with the other two meals, the high-GI meal induced hormonal changes, including higher serum insulin and lower plasma glucagon levels. The combination of hyperinsulinaemia and hypoglucagonaemia tends to promote glucose uptake in muscle and liver, restrain hepatic glucose production and suppress lipolysis (Ludwig et al., 1999).

In order to analyze the effect of low-GI or low-GL diets on weight loss, two different kinds of studies need to be distinguished: studies of isoener-getic low- versus high-GI diets, and ad libitum low- versus high-GI diets. Livesey (2005) reviewed 14 isoenergetic diets and 7 ad libitum studies and found that only in ad libitum studies could an effect of GI or GL on body-weight be determined. A 12-week pilot study in children, which reduced the

GI of the diet by giving brief instructions and a handout about dietary changes to the parents, resulted in a reduction of body mass index (BMI) Z-scores of the children (Young et al., 2004). A 5-week study in healthy men allocated to a high- or a low-GI diet reported both groups experienced an increase in lean body mass but no changes in BMI after the low-GI period (Bouche et al., 2002). A more marked effect was reported in a study of men with abdominal obesity given an ad libitum low glycaemic load or low-fat diet for 6 days; there was a reduction in energy intake, bodyweight, and waist and hip circumference with the low glycemic load diet, but not with the low-fat diet (Dumesnil et al., 2001). Although these findings suggest a potential advantage of low-GI or low-GL diets, the definitive long-term study, where ad libitum intake is permitted but diets are similar in all aspects except the GI, has not yet been done.

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