Things mother forgot to tell you about the glycaemic index of your food - and how it influences your training
It gets confusing sometimes. You know you need carbohydrate to perform at your best (nutritionists tell us that if you're an endurance athlete you require about three to four grams of carbo per pound of body weight per day, depending on how strenuously you train), but there are so many different types of carbohydrate to think about. There are simple carbohydrates, complex carbos, monosaccharides, disaccharides, and polysaccharides. There are carbohydrate-containing foods with a high glycaemic index - and ones with a low index. The former are said to be great for you after a workout, while the latter are best before a training session, at least according to some 'experts'. How can you make sense of the complicated carbohydrate world, consume those carbohydrates which are best for your training and performance, and also time your carbohydrate intakes properly?
Well, some aspects of the carbohydrate scene are pretty straightforward. Take the difference between 'simple' and 'complex' carbohydrates, for example. Simple carbohydrates can be either monosaccharides or disaccharides. The mono-saccharides - glucose, fructose, and galactose - are small molecules with just six carbon atoms; the disaccharides - sucrose, lactose, and maltose - are still-simple compounds formed by linking two monosaccharides together to make a 12-carbon structure. Link three or more monosaccharides together and you have a polysaccharide, which literally means 'many sugars'. Some polysaccharides contain up to 3000 monosaccharides (glucose molecules) connected together. Since they tend to be long-chained structures, polysaccharides are known as complex carbohydrates, while - as mentioned - monosaccharides and disaccharides are simple.
Why should you care about carbohydrate classification? Well, if you're an athlete, the actual form a carbohydrate takes is important to you, because a key issue is how rapidly a carbohydrate moves from your small intestine into your blood, a process called absorption. The more quickly this process occurs, the more rapidly the carbohydrate can move through your blood to your muscles and make a difference to your training and racing.
You might think that simple carbohydrates would always be absorbed more quickly than complex carbohydrates. After all, think of all that extra work required to break down a poly-saccharide containing hundreds of glucose molecules, versus the ease with which a single molecule of glucose should slip across your intestinal wall and begin swimming through your blood. Unfortunately, it's not that simple.
Simple doesn't always mean quick
For example, fructose - the kind of carbohydrate often found in fruit - is a simple carbohydrate; it's a monosaccharide with just six atoms of carbon. However, unlike glucose, fructose moves rather lethargically across the wall of your small intestine, and when it finally enters the blood, it is 'trapped' quickly by the liver and processed before it can move onward to the muscles. Thus, a food item containing a lot of fructose, which is a simple carbohydrate, might not move carbohydrate into your blood and to your muscles as well or as quickly as a complex-carbohydrate food, if, for example, the complex carbohydrate were simply a straight line-up of glucose molecules.
So, to describe more accurately the rate at which various foods influence your blood's carbohydrate concentrations, scientists developed the glycaemic index, or GI. Don't be put off by this term; it simply describes the extent to which a particular food item causes the concentration of glucose - the key monosaccharide which your muscles and brain crave - to increase in your blood plasma after that food is consumed. The GI of various foods can be very useful to know. For example, if you need to recover very quickly after a tough workout or race, taking in high-GI foods can boost the recovery process by restoring the glycogen in your leg muscles more quickly, compared to low-GI comestibles.
How the GI is worked out
The test to determine the GI of a particular food is pretty simple: You eat 50 grams of carbohydrate from the food in question, and your blood glucose levels are then monitored for two hours. In fact, a graph is created, plotting your blood glucose concentrations versus time, and the area under your blood-glucose 'curve' is compared with the area obtained when 50 grams of pure glucose is consumed. The GI is just a percentage; it's the area under the curve after you've eaten 50 grams of - say - pickles, divided by the area under the curve associated with 50 grams of glucose, multiplied by 100. If the GI of grapes is 52, it simply means that ingesting grapes produces an increase in blood glucose which is 52 percent as great as that associated with eating an equivalent quantity of glucose.
Interestingly enough, the GI of a particular food is not a number that is necessarily 'set in stone'. For example, if you boiled up a batch of wheat kernels for breakfast, those kernels would have a GI of about 41. If you took the same kernels, ground them into wheat flour, and baked them in a loaf of bread, the GI of that same wheat would soar to about 70 or so. Mechanical processing of food - which usually breaks it into smaller particles which are easier prey for digestive enzymes - usually increases that food's GI.
Another interesting aspect of GI is that the glycaemic index of a food may vary according to what else is on your plate. For instance, if you eat your high-GI food with a fair amount of fat and protein, the food's GI may dip, since fat and protein tend to delay stomach emptying.
GI and your sports activity
Why is all this fuss about GI important to you? Well, one key concern for athletes is the nature of the pre-race meal: debate has raged about whether it should be low- or high-GI. As you're probably well aware, 'experts' have often recommended low-GI carbohydrate fare before races and strenuous workouts, based on the idea that high-GI foodstuffs might raise blood glucose and insulin levels too high, leading to a steep, fatigue-inducing plunge in blood glucose during the early stages of exercise. In contrast, low-GI foods are supposed to 'stay with you', allowing performance-enhancing glucose to seep steadily into your blood as you train or race.
To test that idea out, researchers recently compared pre-exercise meals containing lots of simple glucose (high GI), a carbohydrate called amylopectin (moderate GI), or another carbo-hydrate called amylose (lower GI). Amylopectin and amylose are both starches (polysaccharides found in plant foods), but amylose is constructed of a straight chain of connected, repeated glucose molecules, while amylopectin has a branched-chain glucose construction. Amylopectin has a higher GI than amylose because digestive enzymes seem to work more rapidly on its branched structure.
Athletes attempted to complete as much work as possible during 90 minutes of exercise following the various meals, and - despite the experts' predictions - they actually produced the most work after ingesting the higher-GI meals of pure glucose or amylopectin ('The Effects of Pre-Exercise Starch Ingestion on Endurance Performance,' International Journal of Sports Medicine, vol. 17, pp. 366-372, 1996). That's not so crazy when you think about it; the loftier GI meant that more carbohydrate was available to the athletes' leg muscles (exercise began only 30 minutes after the meals had ended).
Still, you should be cautious before wolfing down a high-GI meal just before you perform. Following such a meal, some athletes do experience a plunge in blood glucose after their exercise begins; not all of them have their performances harmed by such declines, but those that do may have real trouble completing their races at their hoped-for paces. To be safe, it's best to eat at least two hours before your exercise begins - and to eat food you are very comfortable with (that means foods which you have eaten on previous occasions - with absolutely no ill effects).
The rise and fall of lentils
One criticism of the above research is that real foods were not utilised; after all, athletes are pretty unlikely to ingest meals of pure glucose or amylopectin prior to competition. In a different study in which real foods were utilised, athletes cycled to exhaustion one hour after ingesting a glucose solution (GI = 100), baked potatoes (GI = 98), boiled lentils (GI = 29), or plain water (GI = 0). Of course, blood glucose levels spiked to the greatest extent before exercise and plummeted by the greatest amount after exercise began when the high-GI meals (baked potato or glucose beverage) were consumed. Blood-fat levels were highest after the plates of lentils, since the lentils spurred less production of insulin, a potent hormone which decreases concentrations of blood fats (of course, the upswing in blood fat is potentially a good thing - since fat can spare carbohydrate and thus keep your muscles from depleting their glycogen stores).
Not surprisingly, in this research total carbohydrate burning was greatest during exercise after the glucose or potatoes were eaten. And the cyclists actually were able to cycle for the longest amount of time after eating lentils, an effect which the researchers attributed to the unique qualities of the lentils meal. With lentils-only dining, there was less insulin and therefore more blood fat available for energy, plus a less-precipitous plunge in blood glucose after exercise commenced ('Carbohydrate Feeding before Exercise: Effect of Glycaemic Index,' International Journal of Sports Medicine, vol. 112, pp. 180-186, 1991).
While many athletes and coaches concluded from this study that low-GI foods were better than high-GI victuals before exercise, it was inappropriate for them to do so. The problem with the lentils work was that it was quite unrealistic. In a real competition, athletes would utilise sports drinks as they competed, and the carbohydrate in the sports drinks would prevent plunges in blood glucose and would also provide muscles with fuel as they began to run low on their own intrinsic supplies of glycogen. Give athletes sports drinks and it's doubtful that lentils would be better; the sports drinks could certainly provide a greater flow of carbos toward the muscles, compared with a single, pre-exercise lentils meal, and the sports drinks would spare athletes all the intestinal gas which lentils can create (some experts have suggested that lentils are of greatest value to swimmers, because of the potential jet-propulsion provided).
Remember also that the lentils' coaxing of fat metabolism is not necessarily a good thing, since a higher intensity of exercise can be supported by carbohydrate burning. In addition, the same laboratory which carried out this lentils investigation was unable to confirm its earlier findings when it followed up with a study comparing lentils with bran cereal (GI = 30), rice cereal (GI = 73), and potato flakes (GI = 100). In this follow-up, athletes used the most fat for fuel after eating bran flakes and the most carbohydrate after taking in potato flakes, but all four meals led to similar levels of performance ('Plasma Glucose Levels after Prolonged Strenuous Exercise Correlate Inversely with Glycaemic Response to Food Consumed before Exercise,' International Journal of Sport Nutrition, vol. 4, pp. 361-373, 1994). In other words, it didn't really matter whether the pre-exertion meal was low- or high-GI.
What about during and after exercise?
Some experts contend that pure fructose is the ideal carbohydrate source during athletic activity, since it doesn't stimulate overproduction of the glucose-decreasing hormone insulin and tends to enhance fat burning. However, the truth is that you should avoid fructose at all costs. The damned stuff actually increases your risk of stomach upsets as you exercise and may also magnify 'perceived exertion,' which is simply how difficult your exercise FEELS to you. You should look for commercial sports drinks with a high glucose - rather than fructose - content. Sucrose, which is a combination of glucose and fructose, is also preferable to pure fructose. Don't forget that you can make your own cheap and very effective sports drink simply by combining five tablespoons of table sugar (sucrose) and 1/3 teaspoon of salt with a quart of water.
After your workout or race is over, the idea is to elevate your blood glucose levels as rapidly as possible, so that your muscles can begin the important task of replacing their lost glycogen. That would seem to mean that high-GI carbohydrates are pre-eminent, especially during the two-hour 'window' after your exercise ends, the time period when your leg muscles are most receptive to taking new carbohydrate on board.
However, the issue is a little more complicated than that. In one particularly interesting study, Danish researchers glycogen-depleted the leg muscles of cyclists and then put them on either high-GI or low-GI diets, both of which provided 70 per cent of total calories from carbohydrate. The rate of muscle glycogen construction was twice as fast during the six hours after exercise with the high-GI diet, indicating that if a subsequent workout is going to be performed on the same day, high-GI foods are absolutely essential. It's no accident that the elite Kenyan runners, many of whom train three times a day during serious blocks of training, have chronically high daily intakes of a number of high-GI foodstuffs, including corn meal, white bread, potatoes, and highly sugared tea. It's also no accident that the Kenyans go straight to their breakfast/lunch/dinner tables immediately after their workouts are over.
Interestingly enough, the Danes found that after 24 hours, glycogen building was about the same with the high- and low-GI diets. In other words, high-GI foods got the glycogen-loading process off to a quick start, but low-GI fare eventually caught up. That suggests that if you train only once a day, the difference between high- and low-GI foods might be negligible ('Benefit of Dietary Simple Carbohydrates on the Early Postexercise Muscle Glycogen Repletion in Male Athletes,' Medicine and Science in Sports and Exercise, vol. 22, p. S88, 1990).
However, that conclusion wasn't supported by separate work carried out with elite cyclists, who consumed 4.5 grams of either high- or low-GI carbohydrate per pound of body weight during the 24-hour period after glycogen-depleting exercise. In this study, muscle glycogen increased nearly twice as much after 24 hours with the high-GI diet, compared to low-GI eating ('Muscle Glycogen Storage after Prolonged Exercise: Effect of the Glycaemic Index of Carbohydrate Feedings,' Journal of Applied Physiology, vol. 75, pp. 1019-1023, 1993).
It's unclear whether this effect would persist over several days of training and eating, but the study suggests that high-GI foods can augment the glycogen-stockpiling process during periods of very heavy training.
Carbos plus protein?
Should you add some protein to the carbohydrate you take in following exercise? That notion is supported by a recent study in which a protein-carbohydrate combo led to significantly greater glycogen storage, compared to pure carbohydrate eating, even though the quantity of carbohydrate in each case was the same ('Carbohydrate-Protein Complex Increases the Rate of Muscle Glycogen Storage after Exercise,' Journal of Applied Physiology, vol. 72, pp. 1854-1859, 1992). Adding protein to the carbohydrate produced a larger increase in insulin and blood glucose, compared to eating the carbohydrate by itself. Although this study suggests that including protein with your carbohydrate is a good idea, the unexpectedly fine results might have been the result of the added CALORIES supplied by the protein, rather than the outcome of a unique blood-glucose-boosting property of the carbo-protein mix. In other words, the athletes might have gotten the same effect simply by eating a little more carbohydrate, instead of putting some protein on their plates. We'll wait for further studies to straighten this one out.
Although high-GI eating may be great for storing glycogen in your muscles, don't be surprised if such dining also makes you hungrier than you would be with low-GI fare. That's because there are more steep ascents and downturns of blood glucose with high-GI eating, with the downswings sometimes producing intense feelings of hunger. As mentioned, low-GI food tends to release carbos into your blood more slowly and steadily, helping you feel okay for longer periods of time. Low-GI diets also tend to be associated with lower total and low-density-lipoprotein cholesterol readings, according to recent research.
The bottom line?
So, here are our conclusions. If you absolutely must eat during the hour before your race or workout begins (because you're so damned hungry that not eating would make it impossible to exercise), you need to be careful. High-GI foods are more likely to make your blood glucose crash and burn after you start exercising, and these crashes might hurt your performance. On the other hand, if you can tolerate the glucose dips, the high-GI stuff will probably quicken the rate at which carbohydrate reaches your muscles - and thus might actually help you exercise longer or faster. It's your call. If you're going to be sipping a sports drink as you run, you're probably safer with the low-GI foods. The wisest strategy is to plan ahead and take your meal at least two hours before exercise commences.
If you're really training heavily, or training more than once a day, make sure you ingest plenty of high-GI foods, especially during the two hours after a workout. Potatoes and most breads are particularly good, as are waffles, bagels, cornmeal, most cereals (but not All Bran), watermelon, pineapples, raisins, carrots, rice cakes, and honey. If you know you won't be able to take a meal shortly after a workout, it's a smart idea to include a box of raisins, some rice cakes, bread or bagels, and a carton of pineapple juice in your workout bag, all of which can kick-start the glycogen-re-synthesis process after your training session is over.
If you don't train more than once a day and your workouts usually last for less than 60 minutes, you can probably store adequate leg-muscle glycogen between workouts by relying on low-GI foods such as most fruits, pasta, rice, and legumes (beans, lentils, and chick peas). Bonuses associated with such intakes, compared to a reliance on high-GI stuff, include a potential lowering of cholesterol levels and decreased hunger between meals.
To help you make your choices, we've listed GI values below for some commonly eaten foods. Some of the readings may surprise you (note, for example, the exalted GI of baked potatoes and the rather low figures for rice and spaghetti). Unfortunately, GI readings for the various commercial sports drinks, which would be quite useful to know, have not been published.
Jim Bledsoe