More research is needed on the potential benefits of a low-GI diet for weight control. The ideal design would be a long-term study where ad libitum food intake and fluctuations in bodyweight are permitted, and where the diets are similar in all aspects except GI.
Although it appears there may a beneficial effect of low-GI diets for weight control (Ludwig, 2000; Brouns et al., 2005), for consumers it is difficult to make the right choices. Information is often hidden in international tables containing only a limited number of products. Moreover, as explained earlier in this chapter, it is not always easy to measure the GI of a product accurately. To facilitate consumer choice, the food industry could provide information on the GI of their products, preferably directly on the package. An example of this approach is the GI Symbol Program in Australia (www. gisymbol.com). In this food labelling programme, producers can obtain certificates stating the GI for their products, provided the food has been properly tested for GI with a standard method. All products that contain a minimum amount of 10 g carbohydrates per serving and meet a set of nutritional criteria - including, for example, sodium, fat and fibre content - can be certified. So far this programme is only operating in Australia, but other countries may follow.
Many processed foods on the market have a high GI. For cereal products, in particular, the food industry has several different choices for processing, and thus may be able to produce lower-GI options. As mentioned in Section 3.3, the starch structure is important for the GI, and the less the structure of the grain is changed, the lower the GI. Therefore, 'whole-grain' products should be preferred to 'wholemeal' products (where the whole grain is included, but usually finely milled). Another important decision for the food industry is the choice of raw material. If cereals containing a higher amount of gel-forming dietary fibre, or a higher fructose: glucose ratio were used more frequently, the choice of low-GI foods for the consumer could be expanded.
Overall, the food industry could put greater effort into producing truly low-GI foods rather than foods that only appear to be low GI. Some examples of foods that might appear to be low GI but are not are several types of wholemeal, bran-flake breakfast cereals (GI = 74) and certain cereal bars (GI = 78). A labelling system such as the one in Australia would give additional impetus to producers, as they would be able to market their products as low GI with the support of a recognizable and trustworthy label.
Compared with glucose (GI = 100) or sucrose (GI = 68), the GI of fructose is very low (GI = 19). Therefore, it might be expected that to exchange fructose for glucose or sucrose, and thereby reduce the GI of a food product, might be beneficial in relation to weight control. However, this may not be true. Ingested disaccharides such as sucrose, maltose or lactose, are cleaved by disaccharidases as soon as they enter the small intestine. Released glucose then leads to an insulin response and enters the cells via an insulin-dependent mechanism (Glut-4). Once inside the cells, glucose is phosphorylated to glucose-6-phosphate, from which the intracellular metabolism of glucose begins. In contrast, fructose increases blood insulin levels only slightly and enters the cells via the Glut-5 transporter, which is not insulin dependent. This transporter, however, is absent from the brain, and therefore fructose may not send satiety signals to the brain as glucose does. Furthermore, the secretion of leptin, which is important in inhibiting food intake, is mediated by insulin. As fructose only increases insulin levels slightly, leptin levels may not rise much after fructose consumption. This could lead to decreased satiety and increased food intake (Mayes, 1993).
Another issue that argues against the use of large quantities of fructose in an effort to lower GI, is its effect on de novo lipogenesis. While only a small percentage (1-3%) of glucose carbon enters de novo lipogenesis and is incorporated into triglycerides in normal individuals, a proportionally much greater amount of carbon from fructose is metabolized to triglyceride. Thus, the positive effect of a lower GI with fructose-containing foods might be unfavourably balanced by the negative effects of lower satiety and a potential increase in de novo lipogenesis (Bray et al., 2004; Havel, 2005).
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