Posted: October 17, 2006

Science of Sport: Making Sense Of The Index – The Full ‘Carbs’ Rundown

This article originally appeared in the sports science newsletter, Peak Performance Good carbohydrates have become the undoubted mainstay of any top athletes diet. As a result, the GI (glycaemic index) which tells them how good the carbs in different foods are has become somewhat of a bible. To determine the GI rating of a given carbohydrate, measured portions are fed to healthy people after an overnight fast, with blood samples collected at 15-30 minute intervals over the next two hours. These blood samples are used to construct a blood sugar response curve which determines the GI rating in relation to pure glucose. Pure glucose (one of the very quickest releasing carbohydrates) is assigned a value of 100 and all other foods are ranked by comparison. While GI is a very useful concept, it can’t be taken as the sole predictor of the effects of eating a particular type of carbohydrate. That is because blood glucose response is also determined by the amount of food eaten. A more reliable rating system is the ‘glycaemic load’ (GL), which takes account of both the quality (GI value) of a given carbohydrate and the amount consumed, so more accurately predicting its effects on blood sugar. The glycaemic load, in units, of a portion of carbohydrate is expressed as: GI rating x grams of carbohydrate in portion size/100. Note that each unit of GL produces the same effect on blood sugar as eating 1g of pure glucose. The glycaemic load rating makes sense of some of the surprising GI rankings. For example, a banana may have a GI rating of 58 compared with just 49 for chocolate, but comparing GL values paints the true picture. A typical 120g banana contains around 24g of carbohydrate, which has a GI value of 58. The GL is therefore 58 x 24/100, ie approximately 14 units. But 120g of chocolate provides 75g of carbohydrate, which has a GI value of 49, and so has a GL value of 75 x 49/100 = 32 units. In other words, gram for gram, chocolate has more than twice the impact on your blood sugar of bananas, despite its lower GI ranking. Table 1: GI rating for some common carbohydrates (Approximate values, varying according to brand/ variety/ripeness/ preparation etc) Glucose 100 Rice Crispies 83 Cornflakes 81 Puffed Wheat 80 Jelly beans 80 Dark rye bread 76 Doughnut 76 Potato (boiled or mashed) 74 Dates (dried) 72 Swede 72 Potato (jacket baked) 72 White bread 70 Shredded Wheat 70 Wholemeal bread 69 Croissant 69 French baguette 68 Parsnips 68 Pineapple 66 Rye bread 65 Mars bar 65 Table sugar 65 Apricot (tinned) 64 Raisins 64 Beetroot 64 Potato: new 62 Ice cream 61 Digestive biscuit 60 Pitta bread 58 Muesli 58 Banana (ripe) 58 Sourdough 57 Sultanas 57 Rich Tea biscuits 57 Mango 56 Sweet corn 55 Apricot (jam) 55 Popcorn 55 Orange juice 55 Special K 54 Potato crisps 54 Sweet potato 54 Kiwi fruit 53 Carrots 51 Oat bran 50 Mixed grain 49 Chocolate 49 Peas 48 Grapes 48 Baked beans (tinned) 46 Porridge 46 Pineapple juice 46 Fructose 46 Orange 44 Apple juice (clear) 44 All Bran 43 Spaghetti (white) 43 Peach 42 Pinto beans 40 Spaghetti (wholemeal) 39 Tomato juice 38 Apple 37 Pear 36 Chickpeas 33 Hazelnuts 33 Yoghurt (low-fat, sweetened) 33 Split peas 32 Strawberry 32 Milk (skimmed) 32 Plums 32 Butter beans 31 Apricot (dried) 30 Banana (unripe) 30 Peanut butter 29 Kidney beans 28 Lentils 28 Milk (full fat) 27 Grapefruit 25 Cherries 22 Cashews 22 Peanuts 22 Soya beans 20 Yoghurt (low-fat, unsweetened) 14 By totalling up the GL units for foods you eat throughout the day, you can arrive at an overall GL for the day. The average (processed) Western diet contains around 120 GL units per day, which is on the high side (see table 2 below). Table 2: GI and GL classified   Glycaemic index (GI) Glycaemic load (GL) Individual serving Glycaemic load Total daily intake Low 55 or below 10 or below Below 80 Medium 56-69 11-19 80-120 High 70-plus 20-plus 120-plus The glycaemic index and load of foods have important implications for training and recovery. The early research focused largely on the role of high GI carbohydrates and post-exercise recovery, and it soon became apparent that high GI foods accelerate and maximise glycogen resynthesis and recovery after training. One of the landmark studies looked at cyclists who undertook two exercise trials to deplete muscle glycogen and then consumed either high GI or low GI carbs. The high GI trial resulted in a bigger measured insulin response and increase in muscle glycogen during the 24-hour period after training. These findings were subsequently confirmed by other studies, which explains why high GI carbs are recommended for optimum recovery for 24 hours after training. Pre-training GI values Attention then turned to the issue of how different GI carbs affect performance when consumed before training, with Australian researchers noting that a low GI carbohydrate meal (lentils) eaten one hour before exercise increased cyclists’ time to exhaustion by comparison with an equal amount of carbohydrate eaten in the form of a high GI carbohydrate food (potatoes). Their explanation was that the lower glucose and insulin responses produced more stable levels of blood glucose throughout the cycling bout which, combined with a slower rate of muscle glycogen usage, would have enhanced endurance. This study lent credibility to the notion that consuming high GI carbs before training was probably not a good idea because they could impair performance by destabilising blood sugar levels. And it probably explains why endurance athletes are now advised to choose low glycaemic carbohydrate foods for their pre-event or pre-training meals. The problem is that much of the subsequent research has failed to support these findings. In a follow-up study, the same researchers fed cyclists either low GI or high GI meals one hour before cycling to exhaustion. They found that, although the low GI meals were associated with higher blood glucose levels after 90 minutes of exercise than their high GI counterparts, there were no differences in time to exhaustion. Another study compared the effects of low GI food (lentils) and high GI food (potatoes) in cyclists before 50 minutes of submaximal cycling followed by a 15-minute performance trial. As expected, the high GI meal led to an increase in blood glucose before exercise and a decline in blood glucose at the onset of exercise by comparison with the low GI meal. But again this made no difference to performance. However, not all the subsequent research has been negative. In a similar trial on cyclists, plasma insulin levels were lower for the low GI meal through the first 20 minutes of cycling, and the exercise time to exhaustion was longer. The low GI meal also maintained higher blood glucose levels at the end of two hours of exercise. There’s still some degree of uncertainty about the advantages of low GI carbs over high GI carbs as pre-race snacks/meals. And the fact that some individuals are known to be particularly sensitive to insulin-induced blood sugar falls may account for the somewhat mixed research results. Some research has also suggested that the GI of pre-exercise carbohydrate may affect the ratio of fat to carbohydrate used as fuel. In a study on runners, fed either low or high GI carbohydrate three hours before a treadmill run, researchers were intrigued to discover that, although performance times did not differ significantly, during the first 80 minutes of exercise, carbohydrate oxidation was 12% lower and fat oxidation 118% higher in the low GI trial than the high GI trial! This finding is supported by more recent research on runners, who took part in three treadmill runs three hours after being fed either high GI food, low GI food or no food at all. As expected, the researchers found that the fasting state produced the highest rate of fat oxidation during exercise. However, total fat oxidation was also significantly higher in the low GI trial than in the high GI trial, while the high GI meal caused a significant drop in blood glucose to below the fasting level – not a desirable effect! An increased rate of fat oxidation following a low GI meal could be important because it would conserve muscle glycogen, so prolonging endurance in longer events, while maintaining or reducing body fat. There is also some evidence that low GI pre- exercise meals may help endurance athletes by reducing blood lactate. Another study on trained cyclists involved an incremental exercise test to exhaustion 65 minutes after consuming either high GI, low GI or non-carbohydrate food. Although time to fatigue did not differ significantly between the groups, during exercise blood glucose levels were significantly lower in cyclists who’d eaten the high GI meal. Interestingly, blood lactate was also higher in the high GI group in the early part of the test (at submaximal intensities), suggesting that athletes engaging in prolonged low intensity exercise might benefit from a low GI pre-exercise meal. However, it may be that athletes who routinely use carbohydrate drinks during training have little to gain by manipulating the GI of pre-exercise meals. One study looked at trained cyclists who drank a carbohydrate solution during a two-hour submaximal workout followed by a high intensity ride two hours after consuming either a high GI food (potato), a low GI food (pasta) or a low energy jelly (control). Despite between-groups differences in blood glucose, insulin and fatty acids, the researchers found that the amount and proportion of carbohydrate used for energy was the same, regardless of the pre-exercise meal, with no differences in time taken to complete the high- intensity ride. The researchers concluded that when carbohydrate drinks are ingested in recommended amounts during exercise, the type of pre-exercise carbohydrate consumed has little effect on metabolism or subsequent performance. Working the Index How can this knowledge about GI and GL help you enhance your own training and nutrition? The following advice should help: • Be sure to include some moderate/high GI carbohydrates in your post-training snacks/meals to maximise glycogen repletion; • Despite the generally accepted advice, there is little evidence to suggest that higher GI pre-race snacks and meals adversely affect exercise performance during shorter events; • There is evidence to suggest that low GI carbs may be preferable before longer, lower intensity events (two hours-plus). However… • If you are susceptible to blood sugar swings (ie you often experience an energy dip 30-60 minutes after eating a carbohydrate-rich meal/snack), stick to low GI carbs for three hours before training, whatever the duration/intensity of your event, as these are less likely to disturb your blood sugar and adversely affect training; • If weight control is a priority, avoid high GI pre- exercise snacks, which reduce the proportion of energy derived from fat burning during subsequent training; • Away from training, try to emphasise low GI carbs in your diet, as these are less likely than high GI carbs to over-stimulate your insulin system; • Remember that the specific effect of a carbohydrate on your blood sugar results from both the quality (GI) and the quantity (GL) of that carbohydrate. Stick to low/medium GL food servings away from training and medium/high GL servings after training; • The GI and GL of carbohydrates will both be reduced by fat consumed with your meal. For optimum glycogen replenishment, consume your moderate/high GI carbs with only small amounts of fatty foods! Andrew Hamilton BSc, MRSC, trained as a chemist and is now a consultant to the fitness industry and an experienced science writer References 1. Journal of Applied Physiology 1993; 75:1019-1023 2. University of Sydney Glycaemic Index Research Service (SUGiRS) 2005 (www.glycemicindex.com) 3. International Journal of Sports Medicine 1991; 12:180-186 4. Int J Sport Nutr 1994; 4:361-373 5. Med Sci Sport Exerc 1998; 30:844-849 6. Med Sci Sport Exerc 1999; 31:164-170 7. Med Sci Sports Exerc 1999; 31(3):393-9 8. Br J Nutr 2003; 90(6):1049-56 9. Int J Sport Nutr Exerc Metab 2000; 10(1):51-61 10. J Appl Physiol 1998; 85(6):2220-6

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