Meta-analysis and systematic review of the benefits expected when the glycaemic index is used in planning diets
Opperman, Anna Margaretha
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Motivation: The prevalence of non-communicable diseases such as diabetes mellitus (DM) and cardiovascular disease (CVD) is rapidly increasing in industrialized societies. Experts believe that lifestyle, and in particular its nutritional aspects, plays a decisive role in increasing the burden of these chronic conditions. Dietary habits would, therefore, be modified to exert a positive impact on the prevention and treatment of chronic diseases of lifestyle. It is believed that the state of hyperglycaemia that is observed following food intake under certain dietary regimes contributes to the development of various metabolic conditions. This is not only true for individuals with poor glycaemic control such as some diabetics, but could also be true for healthy individuals. It would, therefore, be helpful to be able to reduce the amplitude and duration of postprandial hyperglycaemia. Selecting the correct type of carbohydrate (CHO) foods may produce less postprandial hyperglycaemia, representing a possible strategy in the prevention and treatment of chronic metabolic diseases. At the same time, a key focus of sport nutrition is the optimal amount of CHO that an athlete should consume and the optimal timing of consumption. The most important nutritional goals of the athlete are to prepare body CHO stores pre-exercise, provide energy during prolonged exercise and restore glycogen stores during the recovery period. The ultimate aim of these strategies is to maintain CHO availability to the muscle and central nervous system during prolonged moderate to high intensity exercise, since these are important factors in exercise capacity and performance. However, the type of CHO has been studied less often and with less attention to practical concerns than the amount of CHO. The glycaemic index (GI) refers to the blood glucose raising potential of CHO foods and, therefore, influences secretion of insulin. In several metabolic disorders, secretion of insulin is inadequate or impossible, leading to poor glycaemic control. It has been suggested that low GI diets could potentially contribute to a significant improvement of the conditions associated with poor glycaemic control. Insulin secretion is also important to athletes since the rate of glycogen synthesis depends on insulin due to it stimulatory effect on the activity of glycogen synthase. Objectives: Three main objectives were identified for this study. The first was to conduct a meta-analysis of the effects of the GI on markers for CHO and lipid metabolism with the emphasis on randomised controlled trials (RCT's). Secondly, a systematic review was performed to determine the strength of the body of scientific evidence from epidemiological studies combined with RCT's to encourage dieticians to incorporate the GI concept in meal planning. Finally, a systematic review of the effect of the GI in sport performance was conducted on all available literature up to date to investigate whether the application of the GI in an athlete's diet can enhance physical performance. Methodology: For the meta-analysis, the search was for randomised controlled trials with a cross-over or parallel design published in English between 1981 and 2003, investigating the effect of low GI vs high GI diets on markers of carbohydrate and lipid metabolism. The main outcomes were serum fructosamine, glycosylated haemoglobin (HbA1c), high-density lipoprotein cholesterol (HDL-c), low-density lipoprotein cholesterol (LDL-c), total cholesterol (TC) and triacylglycerols (TG). For the systematic review, epidemiological studies as well as RCT's investigating the effect of LGI vs HGI diets on markers for carbohydrate and lipid metabolism were used. For the systematic review on the effect of the GI on sport performance, RCT's with either a cross-over or parallel design that were published in English between January 1981 and September 2004 were used. All relevant manuscripts for the systematic reviews as well as meta-analysis were obtained through a literature search on relevant databases such as the Cochrane Central Register of Controlled Trials, MEDLINE (1981 to present), EMBASE, LILACS, SPORTDiscus, ScienceDirect and PubMed. This thesis is presented in the article format. Results and conclusions of the individual manuscripts: For the meta-analysis, literature searches identified 16 studies that met the strict inclusion criteria. Low GI diets significantly reduced fructosamine (p<0.05), HbA1c, (p<0.03), TC(p<0.0001) and tended to reduce LDL-c (p=0.06) compared to high GI diets. No changes were observed in HDL-c and TG concentrations. Results from this meta analysis, therefore, support the use of the GI concept in choosing CHO-containing foods to reduce TC and improve blood glucose control in diabetics. The systematic review combined the results of the preceding meta-analysis and results from epidemiological studies. Prospective epidemiological studies showed improvements in HDL-c concentrations over longer time periods with low GI diets vs. high GI diets, while the RCT's failed to show an improvement in HDL-c over the short-term. This could be attributed to the short intervention period during which the RCT's were conducted. Furthermore, epidemiological studies failed to show positive relationships between LDL-c and TC and low GI diets, while RCT's reported positive results on both these lipids with low GI diets. However, the epidemiological studies, as well as the RCT's showed positive results with low GI diets on markers of CHO metabolism. Taken together, convincing evidence from RCT's as well as epidemiological studies exists to recommend the use of low GI diets to improve markers of CHO as well as of lipid metabolism. 3 From the systematic review regarding the GI and sport performance it does not seem that low GI pre-exercise meals provide any advantages over high GI pre-exercise meals. Although low GI pre-exercise meals may better maintain CHO availability during exercise, low GI pre-exercise meals offer no added advantage over high GI meals regarding performance. Furthermore, the exaggerated metabolic responses from high GI compared to low GI CHO seems not be detrimental to exercise performance. However, athletes who experience hypoglycaemia when consuming CHO-rich feedings in the hour prior to exercise are advised to rather consume low GI pre-exercise meals. No studies have been reported on the GI during exercise. Current evidence suggests a combination of CHO with differing Gl's such as glucose (high GI), sucrose (moderate GI) and fructose (low GI) will deliver the best results in terms of exogenous CHO oxidation due to different transport mechanisms. Although no studies are conducted on the effect of the GI on short-term recovery it is speculated that high GI CHO is most effective when the recovery period is between 0-8 hours, however, evidence suggests that when the recovery period is longer (20-24 hours), the total amount of CHO is more important than the type of CHO. Conclusion: There is an important body of evidence in support of a therapeutic and preventative potential of low GI diets to improve markers for CHO and lipid metabolism. By substituting high GI CHO-rich with low GI CHO-rich foods improved overall metabolic control. In addition, these diets reduced TC, tended to improve LDL-c and might have a positive effect over the long term on HDL-c. This confirms the place for low GI diets in disease prevention and management, particularly in populations characterised by already high incidences of insulin resistance, glucose intolerance and abnormal lipid levels. For athletes it seems that low GI pre-exercise meals do not provide any advantage regarding performance over high GI pre-exercise meals. However, low GI meals can be recommended to athletes who are prone to develop hypoglycaemia after a CHO-rich meal in the hour prior to exercise. No studies have been reported on the effect of the GI during exercise. However, it has been speculated that a combination of CHO with varying Gl's deliver the best results in terms of exogenous CHO oxidation. No studies exist investigating the effect of the GI on short-term recovery, however, it is speculated that high GI CHO-rich foods are suitable when the recovery period is short (0-8 h), while the total amount rather than the type of CHO is important when the recovery period is longer (20-24 h). Therefore, the GI is a scientifically based tool to enable the selection of CHO-containing foods to improve markers for CHO and lipid metabolism as well as to help athletes to prepare optimally for competitions. Recommendations: Although a step nearer has been taken to confirm a place for the GI in human health, additional randomised, controlled, medium and long-term studies as well as more epidemiological studies are needed to investigate further the effect of low GI diets on LDL-c. HDL-c and TG. These studies are essential to investigate the effect of low GI diets on endpoints such as CVD and DM. This will also show whether low GI diets can reduce the risk of diabetic complications such as neuropathy and nephropathy. Furthermore, the public at large must be educated about the usefulness and application of the GI in meal planning. For sport nutrition, randomised controlled trials should be performed to investigate the role of the GI during exercise as well as in sports of longer duration such as cricket and tennis. More studies are needed to elucidate the short-term effect of the GI post-exercise as well as to determine the mechanism of lower glycogen storage with LGI meals post-exercise.
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