Nutrition Discussion Forum

The influence of the glycaemic index of breakfast and lunch on substrate utilisation during the postprandial periods and subsequent exercise – Comments by J. Galgani et al. Stevenson et al. (2005) recently assessed, in a group of athletes, the effect of highand low-glycaemic-index meals (breakfast and lunch) on fuel utilization under resting and exercise conditions using a crossover design. Their research was based on the hypothesis that, following the ingestion of high-glycaemic-indexmeals, augmented postprandial hyperglycaemia and hyperinsulinaemia would promote carbohydrate oxidation at the expense of fat oxidation, thereby leading to body fat gain. Conversely, low-glycaemic-index diets would promote fat oxidation and protect subjects from body fat gain. Their results seem to provide evidence in support of this hypothesis, but there are some aspects that justify further consideration. First, the authors did not find a difference in fuel oxidation after breakfast and during exercise. There was, however, significantly increased carbohydrate oxidation (þ22%, P,0·005) and decreased fat oxidation (228%, P,0·01) following the high-glycaemic-index lunch. Second, the accuracy and precision of the indirect calorimetry procedure was not reported – this information would allow an appreciation of the real meaning of the glycaemic index meal effects on fat oxidation. Third, the 3-h postprandial measurement period used after each meal is insufficient to describe the postprandial period appropriately. In addition to these considerations, a more relevant aspect is related to the long-term impact of contrasting glycaemic index diets. Thus, if Stevenson et al.’s results were extrapolated to the long term, high-glycaemic-index diets would generate a metabolic paradox whereby a positive fat balance would have to be accompained by a negative carbohydrate balance. Certainly, this does not happen under energy-balance conditions as it is known that macronutrient balance is needed to achieve energy balance (Flatt, 1987; Schrauwen & Westerterp, 2000). This means that the dietary macronutrient composition, particularly the dietary fat:carbohydrate ratio, determines the amount of macronutrient oxidized. This condition is represented by the equation FQ 1⁄4 RQ in 24-h, where FQ (food quotient) is the proportion ofmacronutrient present in the diet, andRQ in 24-h is the mean RQ during a day, which represents the proportion of macronutrient oxidized in 24 h (Black et al. 1986). This implies that dietary macronutrient intake and oxidative metabolism are perfectly matched. Hence, if energy balance is present, macronutrient balance will occur. This condition has been repeatedly demonstrated (Bobbioni-Harsch et al. 1997; Whitley et al. 1997; Schrauwen & Westerterp, 2000; Smith et al. 2000). Considering the aforementioned, the changes in fuel partitioning reported by Stevenson et al. would be expected to be transient and compensated throughout the 24-h period. Given the short measurement period (3 h for each meal), it may have been that they were unable to observe this situation. Thus, after a longer time period, no differences in substrate oxidation would have been found, as recently reported (Dı́az et al. 2005). In our research, involving twelve obese women, we determined the influence of highand low-glycaemic-index meals (breakfast and lunch) on fuel oxidation measured in a respiratory chamber for 10 h using a crossover design. No differences in fuel partitioning after breakfast and lunch were found even when significant differences in serum glucose and insulin response were induced. This evidence is supported by several studies in which no differences in fuel partitioning were observed (Würsch et al. 1988; Ritz et al. 1991; Howe et al. 1996; Kiens & Richter 1996; Sparti et al. 2000; Korach-André et al. 2004; Sloth et al. 2004), although an exception can be quoted (Bouche et al. 2002). The specific role of the glycaemic index on fuel partitioning have been recently reviewed (Dı́az et al. 2006).