The glycaemic index of potatoes - Nature

European Journal of Clinical Nutrition (1999) 53, 249±254 ß 1999 Stockton Press. All rights reserved 0954±3007/99 $12.00 http://www.stockton-press.co.uk/ejcn

The glycaemic index of potatoes: the effect of variety, cooking method and maturity NL Soh1 and J Brand-Miller1* 1

Human Nutrition Unit, Department of Biochemistry, the University of Sydney, NSW, 2006, Australia

Objective: The aim of this study was to determine the impact of variety, cooking method and maturity on the GI of potatoes, it was hypothesised that new potatoes may have a relatively lower GI. Design and subjects: Ten healthy volunteers were recruited as subjects through advertising on the campus of the University of Sydney. Equal (50 g) carbohydrate portions of eight potato meals (three varieties, four cooking methods, two states of maturity) and two reference white bread meals were fed in random order to each of the subjects over a period of 10 weeks. Capillary blood samples were taken in the fasting state and then at 15, 30, 45, 60, 90 and 120 min from the start of each meal. Samples were analysed for plasma glucose concentrations and incremental areas under plasma glucose curves were calculated. The GI of the potato was calculated as the AUC of the potato expressed as a percentage of the individual's average AUC of the white bread. This was then multiplied by 0.7 to index the GI to glucose as the reference food. Results: GI values (mean s.e.m.) ranged from 65 9 (canned new potatoes) to 101 15 (boiled Desiree potatoes), glucose 100. No signi®cant difference was found among the three varieties of potato tested (P 0.38) or among the four different cooking methods (P 0.55). The GI values of the canned new potato and boiled Desiree potato were signi®cantly different (P 0.047). The average size of the tuber was found to correlate with the GI (r 0.83, P < 0.05). Conclusions: Potatoes, regardless of variety, cooking method and maturity, have exceptionally high GI values. New potatoes have relatively lower GI values which is attributed to differences in starch structure. Sponsors: University of Sydney. Descriptors: glycaemic index; food; starch; digestion; potatoes

Introduction The glycaemic index (GI) is a method of ranking foods according to their postprandial blood glucose response with respect to that of an equicarbohydrate portion of a reference food (Wolever et al, 1994). The GI is relevant in both preventing and managing diabetes mellitus. Two six-year cohort studies, one in men (SalmeroÂn et al, 1997a) and one in women (SalmeroÂn et al, 1997b) have demonstrated that diets with high glycaemic load=low cereal ®bre content are linked with more than twice the risk of non-insulin dependent diabetes mellitus (NIDDM) when compared to diets with low glycaemic load=high cereal ®bre content. Hence, a diet with a low glycaemic index may help prevent NIDDM. Cross-over studies in NIDDM patients have shown that decreasing dietary GI leads to improved glycaemic control (Wolever et al, 1992; Brand et al, 1991), similar to that achieved by hypoglycaemic drugs. Studies have also shown improved lipidaemic control with low GI diets in NIDDM and hyperlipidaemic subjects (Wolever et al, 1992). Establishing and maintaining acceptable plasma lipid levels is important in diabetes management as the disease carries a greater risk of developing cardiovascular disease (Anderson & Geil, 1994). Cardiovascular disease has also been directly linked to serum insulin levels (DespreÂs et al, 1996; Salonen et al, 1998). *Correspondence: Prof J Brand-Miller, Human Nutrition Unit, Department of Biochemistry G08, University of Sydney NSW 2006, Australia. Received 30 June 1998; revised 27 October 1998; accepted 6 November 1998

Low GI diets have been shown to decrease fasting and postprandial insulin responses (Wolever et al, 1991; DAA, 1997) and improve insulin sensitivity in patients with advanced coronary heart disease (Frost et al, 1996). The potato (Solanum tuberosum) generally has one of the highest GI values of any food, although some types such as `new' potatoes appear to be lower than others. The published GI values for potatoes vary from as low as 56 to as high as 85 (on a scale where glucose 100) for reasons which are not clear and when centres of study are compared, the potato is the food which yields the most variable glycaemic response (Wolever, 1990). Furthermore, the variety of the potato investigated is often not speci®ed, leading to some confusion over the impact of variety vs preparation method. The GI of potatoes needs to be clari®ed because they make a major contribution to total starch intake. In industrialised countries, potatoes contribute about 15 ± 20% of total starch intake compared with bread, another staple, which contributes about 35% (English et al, 1987). Given the popularity of the potato, a low GI variety and=or cooking method could signi®cantly lower the overall glycaemic load of the western diet, this in turn may decrease the risk of NIDDM. The aim of this present study was to determine the differences in GI between the more popular varieties of potatoes commonly sold on international markets and the effects of different cooking methods on GI. Emphasis was placed on determining the true available carbohydrate content of the different potatoes studied, as opposed to

The glycaemic index of potatoes NL Soh and J Brand±Miller

250

reliance on tables of food composition. It was hypothesised that new potatoes may have a relatively lower GI due to differences in starch structure affected by maturity. Materials and methods Subjects Ten healthy volunteers (eight female and two male) were recruited through advertisements placed around The University of Sydney campus. The study was approved by the Medical Ethics Committee of The University of Sydney and all volunteers gave written informed consent. Mean age s.d. was 25 7 y and mean body mass index s.d. was 22.8 2.8. Before each test, subjects fasted for 10 h overnight, during which time consumption of water was permitted. To avoid the `second meal effect' (Wolever et al, 1988), they were instructed not to eat legumes in the meal preceding the fast. Alcohol was limited to two drinks the day before each test and a similar meal was eaten before each fast. On the morning of each test, two ®nger-prick capillary blood samples were collected ®ve minutes apart to determine baseline glucose levels. Foods were eaten at 0 min time and over the 2 h following the start of each test meal, 1 ml capillary blood samples were collected at 15, 30, 45, 60, 90 and 120 min. To enhance peripheral blood circulation to the ®ngers, subjects warmed their hands with hot water bottles for approximately 5 min before each blood sampling. Blood samples were taken using an Autoclix1 device (Boehringer Mannheim, Australia), collected into 1.5 ml microcentrifuge tubes coated with heparin (10 IU heparin sodium salt, Sigma Chemical Co., St Louis, USA) and immediately centrifuged at 12 000 g for 30 s. The plasma was removed and stored at 720 C for later analysis. Test foods Eight test meals and two reference food meals (white bread) were given to each subject in randomised order over a 10 week period. White bread (Tip Top1 Sunblest1, Sydney, Australia) was chosen as the reference food over the original glucose as it was considered to be a more physiological standard (Wolever, 1990). Fresh potatoes were purchased from Sydney retail supermarkets, in bulk quantities suf®cient to conduct all tests; canned new potatoes (EdgellTM Mint Tiny TatersTM, Edgell-Birdseye, Melbourne, Australia) were donated by Cowra Export Packers Ltd. The three varieties of potato chosen for testing were the major varieties grown for the Australian fresh market (Kirkham, 1995) and also featured in international markets in the United Kingdom and United States, namely, Sebago, Desiree and Pontiac. Fresh new potatoes and canned new potatoes were tested because immature potatoes are known to contain starch with different amylose to amylopectin ratios (Brunt and Zinsmeester, unpublished data). Four methods of cooking were assessed using one variety, that is Pontiac: boiled, boiled and mashed, oven-baked and microwaved. All tests were conducted on the peeled potato, except in the case of fresh boiled new potatoes which are generally eaten with the skin intact. Therefore, in total, eight different potato meals, each containing 50 g available carbohydrate (starch and sugars) were fed to all 10 subjects. Water was used to make up the meal volume to 750 ml in each case.

Available carbohydrate contents of the fresh potatoes were analysed using a previously described method (Wills et al, 1980). Brie¯y, a homogenised food sample is extracted with methanol and ®ltered, the extract is analysed for sugar content by HPLC while the starch in the residue is hydrolysed with amyloglucosidase before also being analysed for sugar content by HPLC. The available carbohydrate content of the food is provided by the sum of the sugars in the extract and the residue. This method excludes most resistant starch. Carbohydrate contents for the reference white bread and the canned new potatoes were taken from manufacturers' information. Table 1 shows the carbohydrate contents of the foods and the weight of 50 g carbohydrate portions. The average weights per tuber s.d. (unpeeled) for Sebago, Desiree, Pontiac and new potatoes were 196 66 g, 188 64 g, 91 28 g and 73 20 g. The average weight in the canned product was 33 g, the range being 22 ± 45 g. Fresh potatoes were peeled, weighed out, dipped for 30 s in a 0.1% sodium metabisulphite solution to prevent browning and stored covered in the refrigerator overnight. To avoid starch retrogradation, potatoes were freshly cooked each morning. Potatoes were cooked until soft and large potatoes were halved before cooking. Potatoes for boiling were heated to 100 C in an excess of boiling water and then boiled for 35 min. New potatoes were left unpeeled, the skins pricked with a fork and added to an excess of already boiling water and boiled for 20 min. Potatoes for mashing were cut into 1 ± 2 cm cubes, heated to 100 C in an excess of boiling water and boiled for 15 min. The potato was then drained and mashed with a fork. In all the potato meals subjected to the boiling process, the cooking liquid was included in the water balance of the meal. Potatoes for baking were prepared by wrapping the peeled potatoes individually in aluminium foil and baking them at 190 C in a gas oven for 25 min. Potatoes for microwaving were placed in a covered ceramic dish and microwaved at full power (650 W) for 6 ± 7.5 min. The canned new potatoes were drained, weighed out on the morning of each test and microwave-heated at full power for 3 min before serving. Plasma glucose analysis Plasma samples, stored at 720 C, were thawed at room temperature, vortexed and centrifuged before analysis for glucose concentration. Photometric analyses were conducted in duplicate on a Cobas Fara centrifugal analyser (Roche Diagnostica, Basle, Switzerland) using a hexokinase=glucose-6-phosphate dehydrogenase method (Unimate 5 Gluc HKTM, Roche Diagnostic Systems, Frenchs Forest, Australia). All plasma samples pertaining to the one food Table 1 Carbohydrate (CHO) content of the foods tested based on the raw product with the exception of canned potatoes Food CHO (g=100 g) 1

1

Tip Top ; Sunblest White bread Sebago, peeled Desiree, peeled New, unpeeled Pontiac, peeled Canned new, drained

Available portion (g)

Weight of 50 g CHO balance (ml)

Water

43.8

114

620

11.0 11.2 14.0 12.1 12.0

454 446 358 414 416

315 325 415 355 360


251

were analysed at the same time. Inter- and intra-assay coef®cients of variance were 1.16% and 1.45% respectively. Calculation of GI The incremental areas under the plasma glucose curves (AUC) were determined for each food according to standardised criteria (Wolever et al, 1991), ignoring any area below the baseline. The average AUC for the two white bread tests was used as the reference value and each subject's individual GI for each food was calculated. The GI for each food was taken as the average of all 10 individual values. This was then multiplied by 0.7 to convert the value to one indexed to glucose, GI 100 (Foster-Powell & Brand Miller, 1995). Statistical analyses The glycaemic responses of the two reference white bread tests were compared using a paired t-test. Two-way analysis of variance was used to compare GI values between potato varieties (boiled Sebago, Pontiac and Desiree), between cooking methods (baked, boiled, mashed and microwaved Pontiac potatoes) and between the new potatoes and the potato variety with the highest GI. The correlation coef®cient between GI value and tuber weight was determined, the level of signi®cance was taken as P < 0.05. Results The GI of the products tested is shown in Table 2, no statistically signi®cant difference was found among the GI of the different varieties of potato (Sebago, Pontiac and Desiree) (P 0.38, Figure 1) nor among the various cooking methods (boiled, baked, microwaved and mashed Pontiac potato) (P 0.55, Figure 2). The only signi®cant difference was between the canned new potato and boiled Desiree potato (P 0.047, Figure 3). The GI of the three mature varieties and two `new' potato products correlated with their average tuber weight (r 0.83, P < 0.05, Figure 4). Discussion This study con®rms that potatoes, irrespective of variety, cooking method or maturity, have exceptionally high GI Table 2 GI values of test foods s.e.m. (n 10, except n 9 for new potatoes, canned and microwave-heated) Food Variety Sebago, peeled and boiled Desiree, peeled and boiled Pontiac, peeled and boiled Cooking method Pontiac, peeled and boiled Pontiac, peeled, boiled and mashed Pontiac, peeled and microwaved Pontiac, peeled and baked Maturity New, unpeeled and boiled New, canned and microwave heated

GI s.e.m. GI s.e.m. (white bread 100) (glucose 100) 124 10 144 22 125 13

87 7 101 15 88 9

125 13 130 13

88 9 91 9

112 13 133 15

79 9 93 11

112 17 93 13

78 12 65 9

Figure 1 Top: Incremental glycaemic response to potatoes by variety. Bottom: Glycaemic index of potatoes by variety s.e.m. (glucose 100), none of the differences is statistically signi®cant.

values, with one case equivalent to that of a 50 g glucose load (Desiree, boiled: GI 101 15). Current nutritional advice to increase the intake of starchy foods such as potatoes may therefore lead to a diet with increased glycaemic load and hence greater risk of NIDDM (SalmeroÂn et al, 1997a, 1997b). In the literature, there is a wide range of GI values for boiled potatoes, from as low as 56 to as high as 85. Because the variety is usually not speci®ed, it is not possible to determine whether this is the source of the variation. In this present study, the GI of boiled Pontiac potatoes was 88, which is 32 units higher than a previously published value of 56 (Brand et al, 1985). Such discrepancies may be largely attributed to the use of different food composition data when calculating the serving size of the food (Jenkins et al, 1988). For example, published carbohydrate content for peeled, boiled new potatoes varies from 12.8 g=100 g (English & Lewis, 1991) to 17.8 g=100 g (Holland et al, 1991). This results in 50 g carbohydrate portion sizes of 391 g and 281 g respectively, a difference of nearly 30%. In this present study we used 414 g of Pontiac potato, fresh weight while the previous study speci®ed 294 g of boiled Pontiac potato based on data from food tables (Brand et al, 1985), this difference alone can account for the discrepancies in GI values.


252

Figure 2 Top: Incremental glycaemic response to Pontiac potato by cooking method. Bottom: Glycaemic index of Pontiac potato by cooking method s.e.m. (glucose 100), none of the differences is statistically signi®cant.

Authorities differ in their conclusions regarding the impact of cooking methods on the GI of potatoes. In this present study, there were no signi®cant differences in GI of Pontiac potatoes whether they were boiled, oven-baked, microwaved or mashed. In contrast, Lunetta et al (1995) found that baked potatoes produced a signi®cantly lower incremental glycaemic response compared with boiled potatoes, even when the same amounts of fat were added to each. They concluded that the baking process led to less cooking of the internal part of the potato and reduced the potato's digestibility. Wolever et al (1994), like us, found no signi®cant differences between baked, boiled and canned potatoes. However, they found that mashing signi®cantly increased the glycaemic response (by 15 ± 20%). On the other hand, Englyst & Cummings (1987) found that starch digestibility of cooled potato was identical when eaten as large lumps or ®nely sieved twice, implying that the glycaemic response is unlikely to be affected. Our study showed no effect of cooking method on the GI of just one variety of potato (Pontiac). This does not necessarily imply that other potato varieties will behave in the same way. However, we can think of no scienti®c reason why there might be differences. Cooking methods

Figure 3 Top: Incremental glycaemic response to canned new potato, boiled new potato and Desiree potato. Bottom: Glycaemic index for canned new potato, boiled new potato and Desiree potato s.e.m. (glucose 100).

Figure 4 Correlation between average weight of potato tuber and GI ( s.e.m.).


involving `moist heat' (mashing, baking, microwaving) are likely to affect different varieties in similar ways. Potato starch gelatinises at 55 ± 66 C (Crapo et al, 1981) which is well below the temperatures involved in baking and boiling potatoes. Furthermore, fresh potato tubers contain suf®cient water to fully gelatinise their starch contents when heattreated (Kingman & Englyst, 1994) and resistant starch granules are unlikely to remain in potatoes cooked using conventional domestic methods. To avoid differences in resistant starch content as a result of cooling, we served all the products immediately after cooking (or heating in the case of the canned product). Both boiled new potatoes and canned new potatoes had the lowest GI values of the potatoes tested, the difference reaching statistical signi®cance when compared with the highest GI potato. In absolute terms, the average GI of canned potatoes was almost 36% less, a difference that is probably biologically important. The relatively lower values of the new potatoes may be due to differences in starch structure. As potatoes mature, the quantity of amylose increases but the difference is small and not likely to affect the glycaemic response (Brunt & Zinsmeester, unpublished data). On the other hand, the degree of amylopectin branching signi®cantly increased with the maturity of the potato. Amylopectin has an irregular, branching structure and is more readily gelatinised than the linear amylose molecule leading to a higher glycaemic effect (Wolever, 1990). We speculate that the lower GI of new potatoes may be due to reduced amylopectin branching and hence greater resistance to gelatinisation. Canned new potatoes may be more immature again as the typical size is smaller: the average weight of the fresh new potato was 73 g while that of the canned was only 33 g. To test the hypothesis that the size of the potato in¯uenced the GI (via differences in maturity and starch structure), we determined the correlation between GI and the average size of the potato tuber in our study (Figure 4). The r value of 0.83 (P < 0.05) suggests that the starch of more mature potatoes is easier to digest, leading to a higher GI. The unintentional inclusion of some resistant starch in the 50 g available carbohydrate portion of canned new potatoes might also have contributed to its low GI. The carbohydrate content of this product was provided by the manufacturer who used the `carbohydrate by difference' method (calculated by substracting the percentages of water, protein, fat, ash and dietary ®bre from 100). The manufacturer used the AOAC method (Prosky et al, 1988) of measuring dietary ®bre which includes some but not all resistant starch (RS). In the published literature, the RS of cold canned new potatoes is 6.8% of total starch but reheating reduces this to 1.4% (Kingman & Englyst, 1994). The canned new potatoes in our study had a total starch content of 11.6 g=100 g drained weight. Hence, of the 50 g carbohydrate portion served, 48.3 g was starch of which 1.4% may have been RS 0.7 g. This small amount would not account for all of the difference between the GI of the canned new potatoes and the mature potatoes. The very high GI values of the potatoes tested begs the question: is the potato an `undesirable' food? Foods containing glucose are not recommended to people with diabetes because of their assumed potential to cause hyperglycaemia. Potatoes are still recommended in diabetes due to their low fat and high micronutrient content. Their high satiety (Holt et al, 1995) may also bene®t NIDDM patients as weight control is often part of their treatment. Despite its

status as a high carbohydrate food, the potato is relatively low when compared with most cereal foods: close 0.4 kg was needed to achieve the 50 g carbohydrate portions used in this study. It is unlikely that such an amount would normally be consumed in one sitting and this would affect the glycaemic response in the usual setting. Nonetheless, to achieve a prescribed goal in carbohydrate (200 g) or number of exchanges (12), potatoes could well contribute to an exceptionally high glycaemic load, this, in turn, may increase insulin demand and exacerbate insulin resistance. It is possible that `wild' or less domesticated varieties of potato have lower GI values than current ones. Starchy vegetables which were important foods in the diets of some Australian Aborigines (for example, cheeky yam (Discorea bulbifera), pencil yam (Vigna lanceolata), bush potato (Ipomoea costata)) and Paci®c Islanders (for example, sweet potato (Ipomoea batatas)) have been shown to be digested more slowly than Western starchy staples (Thorburn et al, 1987a). and to produce signi®cantly lower glycaemic responses when compared to Sebago potatoes (Thorburn et al, 1987a, 1987b). Given the popularity of the potato and its extraordinarily high GI, it may be valuable to manipulate the genotype of wild or cultivated varieties of potato to yield a commercially viable low GI potato. Conclusions Neither variety nor cooking method appears to have marked effects on the GI of potatoes tested in this current study. Apparent differences among varieties in the literature may be methodological artifacts resulting from differences in assumed carbohydrate content. Apart from the canned variety, all the potatoes tested in the present study had high GI values (GI > 78). The lower GI of canned new potato (GI 65) may result from differences in starch structure in immature potatoes but may also be due to unintentional inclusion of some resistant starch in the 50 g carbohydrate portion. The lower GI of canned new potatoes is probably of suf®cient magnitude to be clinically important in the dietary management of diabetes. The correlation between the average size of the potatoes and their GI needs to be con®rmed in further studies. Acknowledgements ÐWe are grateful to Cowra Export Packers Ltd who donated the canned new potatoes, Edgell-Birdseye, Australia who provided nutritional information and the volunteers who undertook the ten weeks of GI testing.

References Anderson JW & Geil PB (1994): Nutritional management of diabetes mellitus. In Modern Nutrition in Health and Disease. In Vol 2, 8th edn, ME Shils, JA Olson & M Shike (eds.) Philadelphia: Lea and Febiger. pp 1259 ± 1286. Brand JC, Colagiuri S, Crossman S, Allen A, Roberts DCK & Truswell AS (1991): Low-glycemic index foods improve long-term glycemic control in NIDDM. Diabetes Care 14, 95 ± 101. Brand JC, Nicholson PL, Thorburn, AW & Truswell AS (1985): Food processing and the glycemic index. Am. J. Clin. Nutr. 42, 1192 ± 1196. Crapo PA, Insel J, Sperling M & Kolterman OG (1981): Comparison of serum glucose, insulin and glucagon responses to different types of complex carbohydrate in noninsulin-dependent diabetic patients. Am. J. Clin. Nutr. 34, 184 ± 190. DespreÂs JP, Lamarche B, MaurieÁge P, Cantin B, Dagenais GR, Moorjani S & Lupien PJ (1996): Hyperinsulinemia as an independent risk factor for ischemic heart disease. N. Engl. J. Med. 334, 952 ± 957. Dietitians Association of Australia (1997): Glycaemic index in diabetes management. Aust. J. Nutr. Diet. 54, 57 ± 63.

253


254

English R, Cashel K, Bennett S, Berzins J, Waters A & Magnus P (1987): National Dietary Survey of Adults: 1983ÐNo. 2 Nutrient Intakes. 29. Canberra: Australian Government Publishing Service. English R & Lewis J (1991): Nutritional Values of Australian Foods. Canberra: Australian Government Publishing Service, pp 31 ± 32. Englyst HN & Cummings JH (1987): Digestion of polysaccharides of potato in the small intestine of man. Am. J. Clin. Nutr. 45, 423 ± 431. Foster-Powell K & Brand Miller J (1995): International tables of glycemic index. Am. J. Clin. Nutr. (suppl) 62, S871 ± S890. Frost G, Keogh B, Smith D, Akinsanya K & Leeds A (1996): The effect of low-glycemic carbohydrate on insulin and glucose response in vivo and in vitro in patients with coronary heart disease. Metabolism 45, 669 ± 672. Holland B, Unwin ID & Buss DH (1991): Vegetables, Herbs and Spices. 5th Supplement to McCance and Widdowson's The Composition of Foods 4th Edition, 10 ± 15. Cambridge: Royal Society of Chemistry and Ministry of Agriculture, Fisheries and Food. Holt SHA, Brand Miller JC, Petocz P & Farmakalidis E (1995): A satiety index of common foods. Eur. J. Clin. Nutr. 49, 675 ± 690. Jenkins DJA, Wolever TMS & Jenkins AL (1988): Starchy foods and glycemic index. Diabetes Care 11, 149 ± 159. Kingman SM & Englyst HN (1994): The in¯uence of food preparation methods on the in-vitro digestibility of starch in potatoes. Food Chemistry 49, 181 ± 186. Kirkham R (1995): Potatoes. In: Horticulture Australia: The Complete Reference of the Australian Horticultural Industry. B Coombs (ed.) Hawthorn East, Victoria: Morescope Publishing, pp.250 ± 256. Lunetta M, Di Mauro M, Crimi S & Mughini L (1995): In¯uence of different cooking processes on the glycaemic response to potatoes in non-insulin dependent diabetic patients. Diab. Nutr. Metab. 8, 49 ± 53. Prosky L, Asp N-G, Schweizer TF, DeVries JW & Furda I. (1988): Determination of insoluble, soluble and total dietary ®ber in foods and food products interlaboratory study. J. Assoc. Off. Anal. Chem. 71, 1017 ± 1023.

SalmeroÂn J, Ascherio A, Rimm, EB, Colditz GA, Spiegelman D, Jenkins DJ, Stampfer MJ, Wing AL & Willett WC (1997a): Dietary ®ber, glycemic load, and risk of NIDDM in men. Diabetes Care 20, 545 ± 550. SalmeroÂn J, Manson JE, Stampfer MJ, Colditz GA, Wing AL & Willett WC (1997b): Dietary ®ber, glycemic load, and risk of non-insulindependent diabetes mellitus in women. JAMA 277, 472 ± 477. Salonen JT, Lakka TA, Lakka HM, Valkonen VP, Everson SA & Kaplan GA (1998): Hyperinsulinemia is associated with the incidence of hypertension and dyslipidemia in middle-aged men. Diabetes 47, 270 ± 275. Thorburn AW, Brand JC & Truswell AS (1987a): Slowly digested and absorbed carbohydrate in traditional bushfoods: a protective factor against diabetes? Am. J. Clin. Nutr. 45, 98 ± 106. Thorburn AW, Brand JC, O'Dea K, Spargo RM & Truswell AS (1987b): Plasma glucose and insulin responses to starchy foods in Australian Aborigines: a population now at high risk of diabetes. Am. J. Clin. Nutr. 46, 282 ± 285. Wills RBH, Balmer N & Green®eld H (1980): Composition of Australian foods. 2. Methods of analysis. Food Technol. Australia 32, 198 ± 204. Wolever TMS (1990): The glycemic index. World Rev. Nutr. Diet. 62, 120 ± 185. Wolever TMS, Jenkins DJA, Jenkins AL & Josse RG (1991): The glycemic index: methodology and clinical implications. Am. J. Clin. Nutr. 54, 846 ± 854. Wolever TMS, Jenkins DJA, Ocana AM, Rao VA & Collier GR (1988): Second-meal effect: low-glycemic-index foods eaten at dinner improve subsequent breakfast glycemic response. Am. J. Clin. Nutr. 48, 1041 ± 1047. Wolever TMS, Jenkins DJA, Vuksan V, Jenkins AL, Wong GS & Josse RG (1992): Bene®cial effects of low-glycemic index diet in overweight NIDDM subjects. Diabetes Care 15, 562 ± 564. Wolever TMS, Katzman-Relle L, Jenkins AL, Vuksan V, Josse RG & Jenkins DJA (1994): Glycaemic index of 102 complex carbohydrate foods in patients with diabetes. Nutr. Res. 14, 651 ± 669.