This study showed a significant effect of low-GL diet on cardiovascular risk factors including total cholesterol, TG, LDL, HDL, FBS, and HbA1c. In our study, as we hypothesized, the administered low-GL diet suppressed the HbA1c of the patients to 7.8 ± 0.3%, which is not considered as poorly controlled level[1
] and was our target in the present study.
While there is widespread concern about increasing diabetes and obesity and related health care costs, development of an appropriate diet for cardiovascular risk factor reduction and weight management is a public health issue. The reduction in cardiovascular risk factors in poorly controlled diabetic patients in our study was due to weight loss and also low GL of diet.
Although the poorly controlled diabetes patients had similar isocaloric diet before and during intervention, the low-GI, low-GL diet caused significant weight reduction after 10 weeks of intervention. Several studies have examined the effect of GI on human appetite, and most of them demonstrated increased satiety, delayed return of hunger, or decreased ad libitum
food intake after consumption of low compared to high-GI foods.[14
] In contrast, hyperinsulinemia resulting from high-GI food intake may cause weight gain by directing nutrients away from oxidation in muscle and toward storage in fat. In animal study it was shown that hyperinsulinemia elevates glucose utilization in fatty tissue, but decreases utilization in muscles, a process that results in increased food intake and weight gain.[14
] In epidemiological studies, it has been reported that Pima Indian children with increased fasting insulin levels gain more weight than those children having normal insulin concentration.[15
] Energy-restricted diet based on low-GI foods produced greater weight loss than did an equivalent diet based on high-GI foods, and among healthy pregnant women, high-GI diet resulted in greater weight gain at term than isocaloric low-GI diet.[16
] The weight changes found in adult rats fed isoenergic, nutrient-balanced diets based on high-GI or low-GI diet for 32 weeks were significantly different. The low-GI group had reduced weight, while the high-GI group demonstrated increased weight.[16
] These diets have been reported to be beneficial as they control diabetes, increase HDL-C, lower serum TG,[17
] and reduce glycated proteins.[18
] In contrast, consuming high-GI diet and consequently high-GL diet was 4 times greater among women with a higher BMI,[19
] which may lead to diabetes and cardiovascular disease (CVD). Also, the epidemiological studies such as the Nurses Health Study and Health Professional Follow-Up Study,[20
] and also Framingham Offspring Study[21
] have demonstrated the association between GL and type 2 diabetes, CVD, and metabolic syndrome.
All the above studies confirm weight reduction in diabetes subjects of our study, following low-GL diet. The present study also gains support from a study in which consumption of an ad libitum
low-GL diet by obese adults during 6 months resulted in significant body weight reduction which was comparable with conventional restricted energy (250–500 kcal/day deficit) diet group (–7.8% and –8.4% weight reduction, respectively).[22
] In our study, diabetes subjects with low GL and sufficient energy intake had 4.4% weight reduction during 10 weeks intervention. The low-GL diet in our study may have increased oxidation of nutrients in muscles rather than storing them in white tissue. In addition, the low-GL diet may have elevated satiety and reduced the intake of foods.
In epidemiologic studies, both GI and the GL of the overall diet were associated with a greater risk of type 2 diabetes in whole adult population and low-GI diet had significant effect on reducing glycosylated proteins.[19
] The low-GI and low-GL diets independent of weight loss have significant effect on improving cardiovascular risk factors. In a study, ad libitum
intake of the low-GI diet resulted in a 10% decrease in LDL-C compared with isocaloric high-GI diet after 10 weeks intervention,[23
] and also ad libitum
intake of low-GI diet showed a significantly greater mean decline in plasma triacylglycerols than did the conventional restricted diet.[22
] In our present study, the LDL-C reduced by 4%, while the HDL-C increased by 8%. Beneficial effect of low-GI diet in the management of diabetes is well documented. A meta-analysis showed that after average duration of 10 weeks, subjects with type 1 and 2 diabetes who were consuming low-GI diets had HBA1c concentration of 0.4% points lower than those who were following a high-GI diet.[18
] Comparing low-GI versus high-GI diet, the low-GI diet significantly improved fasting blood glucose and HbA1c of type 2 diabetes. The patients who followed low-GI diet demonstrated a reduced HbA1c level and it was 0.39% points lower than the HbA1c level of those who followed high-GI diet.[4
] In our study, after 10 weeks intervention, the fasting blood glucose reduced by 28.1 ± 12.5 mg/dl (16.6%), and HbA1c by 1.1 ± 0.3%.
The mechanism underlying improvement of fasting blood glucose and HbA1c in the present study probably is the elevated whole-body glucose disposal.[4
] The low-fat, high-carbohydrate diet, which causes postprandial hyperglycemia and hyperinsulinemia, has a significantly less favorable effect on circulating triacylglycerol and PAI-1 (plasminogen activator inhibitor-1; a marker of fibrinolytic capacity) concentration than does low-GL diet.[24
] In turn, these episodes may enhance hepatic triacylglycerol production or reduce peripheral clearance.[24
] Higher concentrations of triacylglycerol and PAI-1 have direct association with cardiovascular events.[26
In our study, the moderate carbohydrate diet with GL = 67–77 g/day, including 42% carbohydrate as energy intake, and 15% of fat derived from olive oil and nuts sources was almost similar to ADA's recommendation which is more appropriate and compelling for glycemic control for long period. The GL <80 g/day is considered low-GL diet.[27
] The higher the GL, the greater the glycemic effect[28
] and insulinogenic effect.[10
] The GL of diet in our study was even lower than maximum g/day recommendation for low-GL diet.