Low carbohydrate does not mean low glycaemic index!

Dumesnil et al. (2001) reported the results of a study from Laval University, Québec, in which overweight men were fed under metabolic ward conditions for 6 d on each of three different test diets. They concluded that: ‘. . . a lowglycaemic index–low-fat–high-protein diet can produce a marked decrease in ad libitum energy intake without increasing hunger or decreasing satiety while having rapid and marked effects on metabolic risk variables’. However, the authors have provided no evidence that the effects they observed were in any way due to the use of low-glycaemic-index (GI) foods. The conclusions related to GI are unjustified because the paper fails to demonstrate that the GI of their low-GI–lowfat–high-protein diet is less than that of the control (American Heart Association; Krauss et al. 1996) diet. The GI of neither experimental diet is given. An appendix is provided giving sample menus from each dietary regimen and showing the GI values of the foods in each diet. However, the information is imprecise: there are no weights of foods given, in some cases the specific food is not indicated. Three foods, wholewheat bread, unsweetened jam and fresh fruits, provide most of the carbohydrate in the low-GI diet. The GI values of bread and jam are indicated as being: ‘Undetermined, but probably ,51’. The GI of ‘fresh fruits’ is listed as being 36–53. We have tested commercial varieties of wholewheat bread available in Canada, and wholewheat bread baked in our laboratory, on several occasions. The lowest value ever obtained was 65 (Wolever et al. 1994a), although the value obtained was usually closer to 71 (Jenkins et al. 1983a,b, 1986, 1988; Wolever et al. 1986, 1994a). Thus wholewheat bread is not a low-GI food. The published GI values of certain popular tropical fruits, readily available in Canada, are higher than the maximum value of 53 given by the authors; e.g. ripe banana (all yellow skin, GI 54), over-ripe banana (few brown spots on skin, GI 64) (Wolever et al. 1988), pineapple (GI 66) and watermelon (GI 72) (Foster-Powell & Brand Miller, 1995). Since the fruits used are not indicated, it is not possible to verify that the GI values cited are correct. To the best of my knowledge, there is no published GI value for jam, although Jenkins (DJA Jenkins and C Kendal, personal communication) found the GI of strawberry jam to be 84. The carbohydrates in regular jam consist of the sugars in the fruit, plus the sucrose or other carbohydrate added to sweeten and thicken the jam. Fruits contain varying proportions of sucrose, fructose and glucose with some, such as pear, peach and apricot, containing 100–160 g sorbitol/kg (Wolever et al. 1993). Dumesnil et al. (2001) do not indicate what kind of fruit was in ‘Unsweetened jam’. Presumably the jam contained no added sucrose, but could have contained extra fruit or fruit juice. It may or may not be correct to assume that it contained no other sweetener such as sorbitol or aspartame. These different kinds of jam would contain different amounts of carbohydrate, but the GI may not change. Removing sugar from jam, or replacing it with extra fruit or fruit juice would not be expected to change the GI appreciably, if the sugars in the fruit approximate the composition of sucrose. Sweetening jam with aspartame would not alter its GI since aspartame is not a carbohydrate. The effect on the GI of sweetening the jam with sorbitol is a contentious issue depending on whether sorbitol is included as a ‘glycaemic carbohydrate’. Dumesnil et al. (2001) indicate that cheese, salad and yellow waxed beans have a ‘Low GI’ and, in a footnote, further add: ‘. . .no data are available but GI probably ,30’. This represents a fundamental error about the definition of the GI. Foods which do not raise blood glucose do not necessarily have a low GI. The GI is an index of the blood glucose raising potential of the glycaemic carbohydrate in foods (Food and Agriculture Organization/ United Nations University/World Health Organization, 1998). Thus, the term ‘low-GI food’ is normally used to describe high-carbohydrate foods. Foods containing no carbohydrate do not have a GI. Foods containing only a small amount of carbohydrate theoretically could have a GI, but because they do not contain enough carbohydrate to raise blood glucose, it is difficult in practice to determine the GI. Since cheese, salad and yellow waxed beans contain very little carbohydrate, it is not meaningful to describe them as low GI. The available carbohydrate in cheese, lactose, has a low GI (Wolever et al. 1985), and so there is at least some justification in calling it a lowGI food. However, I do not know what glycaemic carbohydrates are contained in salad and yellow beans, though I would guess that it was glucose (high GI). Nevertheless, diet GI is not affected appreciably whether these foods are considered to have high or low GI, because they contain little glycaemic carbohydrate. I estimated the GI of the sample diets using previously described methods (Wolever et al. 1994b). For fresh fruits, I used apple. I took the GI value of unsweetened jam to be 61. The weights of foods was based on common portion sizes, adjusted so as to result in total food weight and intakes of energy, fat, protein, carbo-