The blood sugar response to feeding as a critical factor in health, performance, recovery and body composition

In brief

• The glycemic index is a scale that ranks foods by speed at which the food is digested and absorbed, and thereby changes blood glucose levels
• Foods and diets that are lower in their glycemic index are generally associated with better health in terms of promoting lower body fat and reducing the likelihood of weight gain, while also being important for the prevention and management of heart disease, obesity and diabetes.
• Because blood sugar levels can influence metabolic processes before, during and after exercise performance, the glycemic index is a key concept in sports nutrition.
• In short, low glycemic index, high carbohydrate meals are optimal for pre-competition fuelling; high glycemic index, high carbohydrate meals are optimal for post-exercise refuelling when recovery time is short; and low glycemic index diets may be beneficial for athletes trying to maintain low levels of body fat.

Background

The glycemic index (GI), originally described in the early 1980s¹, is a ranking of foods, and in particular those containing carbohydrate (CHO), based on their effects on changing blood sugar (glucose) levels in the minutes and hours after being eaten. When any form of CHO is eaten, the process of digestion breaks down that food into individual sugar units, which are then absorbed into the bloodstream and appear as glucose (the most common form of blood sugar). The GI rating of a food (on a scale of 1-100 compared to the effect of eating white bread or glucose) reflects the speed at which the food or CHO is digested and absorbed, and changes blood glucose levels. Foods on the higher end of the scale are digested and absorbed more rapidly, producing a quick “spike” in blood sugar, whereas foods on the lower end of the scale are digested and absorbed slowly, resulting in a gradual, more sustained increase in blood sugar (Figure 1). Simply put, the faster the rise in blood sugar, the higher the rating on the index.

The GI of a food or mixed meal is dependent primarily on the amount and type of CHO, but several other factors also influence GI such as the macronutrient (CHO, protein, fat, fibre) content of the food, food particle size, cooking techniques, food processing, the presence of fructose (fruit sugar) or lactose (dairy sugar), the form of starch, presence of anti-nutrients such as phytates and lectins, and the metabolic health of the individual tested.

Because the international GI tables are based on measurements of individual foods, one area of discussion has been the ability to predict glycemic responses to mixed meals from the GI of the individuals foods within that meal. To this end, a related concept known as the glycemic load (GL) is proposed as a method to characterize the glycemic effect of meals or the overall diet. The GL is defined as the weighted mean of the dietary GI multiplied by the percentage of total energy from CHO. The GL therefore reflects the GI of a portion, serving size or on a meal-by-meal basis.

A common example used to illustrate the difference between GI and GL is watermelon. The GI of watermelon is high (~72), but on a per serving basis (120 g of watermelon), there is only 6 g of CHO per serving. So the GL of watermelon is low. In other words, you would have to eat a lot of watermelon before you would get any significant change in blood sugar levels, despite this being a high GI food. Examples of foods in each GI category are given in Figure 2.

Interest in the impact of the GI of a person’s diet on sports performance, health and well-being is emerging among sports nutritionists, healthcare professionals as well as consumers and athletes. One of the reasons for this interest is that by-and-large, the release of the hormone insulin is proportional to the relative increase in blood sugar levels after a meal. Insulin is an essential hormone for regulating blood sugar balance, but exerts many metabolic effects in the body including stimulating the storage of CHO in muscle in the form of glycogen (glycogen synthesis), stimulating muscle growth (protein synthesis), preventing the breakdown of fat (lipolysis) and stimulating the storage of fat (lipogenesis). In short, low GI and low insulinemic (insulin release) foods are generally considered more favourable choices because by preventing rapid fluctuations in blood sugar (sugar highs, hunger pangs and sugar crashes), they (i) promote higher rates of fat burning and fat loss, (ii) provide a feeling of fullness for longer, and (iii) contribute a steady, sustained release of energy that is positive for physical and mental performance. Some highlights from the recent scientific literature and details on how the concept of GI has been applied at ROS Nutrition are described below.

Role of GI in affecting body composition and metabolic health

In general terms, maintaining a relatively stable blood sugar concentration throughout the day is preferred. This is thought to prevent many of the consequences of sugar “spikes” and “crashes” such as mood swings, lethargy, hunger pangs and food cravings while at the same time reducing fat storage and promoting fat oxidation (burning) (Figure 3). Therefore, low GI foods are desirable to promote stable blood sugar levels and supply energy at a consistent rate over a number of hours. High GI foods on the other hand may cause rapid fluctuations in blood sugar and “reactive” hypoglycaemia (a sudden drop in blood sugar) in certain susceptible individuals.

The scientific literature on the role of dietary GI on health can be categorised under two main themes: (i) observational studies where the dietary GI of large populations are compared to the incidence of disease in that population, and (ii) intervention studies in which the GI of individuals diets are lowered using substitutions of high GI foods for healthier low GI options.

The evidence from observational studies links long-term consumption of a high GI diet to a higher risk of elevated triglycerides and lowered HDL (good) cholesterol, fatty liver, weight gain, obesity, and gestational and type 2 diabetes compared with a low GI diet^{2, 3, 4, 5}. On the other hand, when individuals switch to a low GI diet, meta-analyses (a type of review article that uses statistical modelling to calculate the effect of an intervention on a given outcome) of clinical trials support the use of low GI diets for weight loss, prevention of heart disease and the management of high blood lipids, and the prevention and treatment of diabetes^{6, 7} For example, low GI and GL diets result in average reductions of approximately 1 kg in weight, 1 kg in total fat mass, and 1.3 units in body mass index in comparison to either high GI or low fat diets⁷.

Interestingly from a weight management point of view, some of the reasons that low GI meals have favourable effects on body composition is likely because they promote feelings of fullness for longer (i.e. reduce appetite) than high GI meals⁸, and when eaten as the last meal prior to exercise result in a greater amount of fat being burned within that exercise session⁹. Managing macronutrient amount and type to optimise fat “burning” at rest and during exercise now seen as a realistic strategy in weight management¹⁰. In short, by switching to eating lower GI foods and supplements, it is easier to manage and even lose body fat as appetite is better controlled, hunger is delayed, and fat is less likely to be stored and more likely to be burned at rest and during exercise.

Role of GI in sports performance and recovery

Because blood sugar levels can influence metabolic processes before, during and after exercise performance, the glycemic index is a key concept in sports nutrition^{11, 12}. Modulation of dietary GI affects many processes involved in sport nutrition, including pre-match meal preparation and exercise metabolism, glycogen resynthesis after the ingestion of recovery meals, and regulation of body composition. In short, low GI, high CHO meals are optimal for pre-competition fuelling; high GI, high CHO meals are optimal for post-exercise refuelling when recovery time is short; and low GI/GL diets may be beneficial for athletes trying to maintain low levels of body fat. The general guidelines are as follows:

Prior to exercise, a low GI meal is associated with a more stable blood sugar response to exercise, reduces the likelihood of reactive hypoglycemia that can impair performance, tends to produce better endurance performance and allows for greater fat burning during exercise. A high CHO, low GI pre-exercise meal tops up liver and muscle CHO stores, provides a steady supply of blood sugar and controls appetite in the lead-up to performance.

During exercise, a high GI CHO source that is rapidly digested and absorbed is recommended i.e. a CHO-electrolyte sports drink such as CHO CHARGE^®. In addition, combining such a CHO source with a fructose-based sugar is the best approach as this takes advantage of the multiple CHO transporters present in the small intestine, and allows for the highest rates of CHO use to support performance.

During recovery, if recovery is short (<24 h; e.g. two-a-day training, or multi-race days), a high GI recovery meal followed by high GI snacks at regular intervals is recommended. If recovery is longer (>24 h), low to moderate GI meals are acceptable. The choice may also depend on body composition goals as low GI diets are generally associated with more favourable effects on body composition with respect to maintaining low percentage body fat (see above). As this is likely to be a concern for many athletes involved in endurance and team sports, those athletes wishing to CHO load prior to competition are advised to use a low GI CHO source such as FUEL LOAD^®.

How do ROS Nutrition products reflect current guidelines on glycemic index?

At ROS Nutrition, we have taken the approach of combining the current best practice guidelines in terms of GI in sports nutrition, and sourcing the best available ingredients for modulating the GI of our formulations. This is achieved by strategically incorporating the designer sugars Vitargo (high GI, >100), and isomaltulose (low GI, ~32) into our formulations on a goal-orientated basis. The table below summarises this approach. Please click on the individual product links to read more about these advanced formulations.

Further reading

• Jenkins DJ, Wolever TM, Taylor RH, Barker H, Fielden H, Baldwin JM, Bowling AC, Newman HC, Jenkins AL, Goff DV (1981) Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr. 34 (3), 362-366.
• Salmeron J, Ascherio A, Rimm EB, Colditz GA, Spiegelman D, Jenkins DJ, Stampfer MJ, Wing AL, Willett WC (1997) Dietary fiber, glycemic load, and risk of NIDDM in men. Diabetes Care. 20 (4), 545-550.
• Salmeron J, Manson JE, Stampfer MJ, Colditz GA, Wing AL, Willett WC (1997) Dietary fiber, glycemic load, and risk of non-insulin-dependent diabetes mellitus in women. JAMA. 277 (6), 472-477.
• McKeown NM, Meigs JB, Liu S, Saltzman E, Wilson PW, Jacques PF (2004) Carbohydrate nutrition, insulin resistance, and the prevalence of the metabolic syndrome in the Framingham Offspring Cohort. Diabetes Care. 27 (2), 538-546.
• Liu S, Manson JE, Stampfer MJ, Holmes MD, Hu FB, Hankinson SE, Willett WC (2001) Dietary glycemic load assessed by food-frequency questionnaire in relation to plasma high-density-lipoprotein cholesterol and fasting plasma triacylglycerols in postmenopausal women. Am J Clin Nutr. 73 (3), 560-566.
• Thomas D, Elliott EJ (2009) Low glycaemic index, or low glycaemic load, diets for diabetes mellitus. Cochrane Database Syst Rev. (1), CD006296.
• Thomas DE, Elliott EJ, Baur L (2007) Low glycaemic index or low glycaemic load diets for overweight and obesity. Cochrane Database Syst Rev. (3), CD005105.
• Ludwig DS (2000) Dietary glycemic index and obesity. J Nutr. 130 (2S Suppl), 280S-283S.
• Stevenson EJ, Williams C, Mash LE, Phillips B, Nute ML (2006) Influence of high-carbohydrate mixed meals with different glycemic indexes on substrate utilization during subsequent exercise in women. Am J Clin Nutr. 84 (2), 354-360.
• Volek JS, Vanheest JL, Forsythe CE (2005) Diet and exercise for weight loss: a review of current issues. Sports Med. 35 (1), 1-9.
• O'Reilly J, Wong SH, Chen Y (2010) Glycaemic index, glycaemic load and exercise performance. Sports Med. 40 (1), 27-39.
• Mondazzi L, Arcelli E (2009) Glycemic index in sport nutrition. J Am Coll Nutr. 28 Suppl, 455S-463S.