by Louise M Burke, Australian Institute of Sport, Canberra, Australia
Nutritionists like me have succeeded in convincing athletes to think of carbohydrates as their best fuel source. What they may not recognize is that carbohydrates cannot be lumped into one category. Most athletes have even heard that there are simple and complex carbohydrates. However, pardon the pun, carbohydrates are more complex than that. When it comes to planning their training meals, athletes need to understand and use the glycemic index.
For years, carbohydrate sources have been labeled as simple (containing sugars like glucose and sucrose) or complex (containing fiber and starch) based on the structure of the main carbohydrate. Simple carbohydrate foods have been charged with causing large and rapid changes in blood glucose. They are alleged to cause a rapid rise followed by a rapid and often greater fall - this is known as rebound hypoglycemia or the "sugar blues". Simple carbohydrates have also been considered to be lacking in nutrient value. On the other hand, it has been believed that the digestion and absorption of complex carbohydrate foods is slower, producing a flatter and more sustained blood glucose and insulin response. Complex carbohydrate foods have also been regarded as being more "healthy" or "nutritious".
While this classification system may have been developed as a quick education tool for the lay person, it has become a major headache for nutritionists. Because we now know that the effect of specific carbohydrate foods on the blood glucose response is neither simple nor predictable.
During the 1970s, diabetes specialists were amazed to find that simple carbohydrate foods did not always produce the high and short-lived blood glucose responses traditionally attributed to them. For example, fruit and sweetened dairy products produce a flattened blood glucose curve when they are eaten. The old no-no, sugar (sucrose), has a medium blood sugar profile. Curiously, some foods high in complex carbohydrates (e.g. bread and potatoes) produce a rapid blood glucose response, similar to that following the ingestion of glucose itself. Even the presence of dietary fiber in foods does not always delay absorption and flatten the after-meal blood glucose curve. For example, blood glucose responses to whole-grain breads are similar to those after eating white bread. The glycemic index (GI) was introduced in the early 1980s to classify the real effects of carbohydrate-rich foods on blood glucose levels. The GI is a ranking of foods based on their measured blood glucose response compared to that following a standard food. In some laboratories the standard food is glucose, while other scientists prefer to use white bread.
Tables of the glycemic index of a large number of carbohydrate-rich foods have now been published internationally. The numbers vary according to who measured them and the exact type of food. For example, there are a lot of different types of "white bread" in the world. And even things like potatoes and rice come in a variety of plant types. Each has a slightly different GI. Generally, nutritionists now divide foods into those that have a high GI (bread, potatoes, breakfast cereal, glucose-based sports drinks), a moderate GI (sugar, soft drinks, tropical fruit) or a low GI (dairy foods, lentils, legumes, oats, cold climate fruits such as apples). Some foods sit on the borderline, but this is not really a problem. The real interest is in foods that are extremely different in their GI. And the real message is that there is no way to predict blood glucose responses to eating specific foods without these actual measures.
Now that we know the effect of specific food items on blood glucose responses, we can advise people who want to control their blood glucose profiles during the day or after meals. In other words, you can eat the same amount of carbohydrate, but manipulate whether you want blood glucose spikes during the day (eat high and moderate GI foods), or a more even level (low GI foods). Diabetics are a classic example of a population that benefits from tight control of blood glucose and low GI foods. People with high blood lipid levels may also benefit from being able to achieve a more even blood glucose profile that has smaller rises and falls during the day. The glycemic index may also be a useful tool in weight control, since low GI foods have recently been shown to produce a longer-lasting "satisfaction" after meals - you don`t feel hungry quite so soon. A recently published book, The G.I. Factor, has made this information widely accessible.
Some people have quickly grabbed on to the idea that altering the GI of specific meals or the training diet may influence training and performance. The focus is on optimizing the muscle carbohydrate fuel sources, particularly for prolonged moderate-intensity exercise. Research at the Australian Institute of Sport, in conjunction with researchers at Deakin University and University of Melbourne, has examined the use of GI in sport. The following guidelines are drawn from this research.
1. The glycemic index may be useful in sport and deserves further attention. However, it is not intended to provide a single way to rank the virtues of carbohydrate foods. There are many other features of foods which may be of value to the athlete, such as nutritional value or practicality. Sometimes foods need to be chosen because they are tasty, portable, cheap, easy to prepare and unlikely to cause stomach upsets. These issue are specific to the individual and the exercise situation. In other words, foods must always be chosen to fit the "Big Picture" and not one single issue. In the case of food eaten before or during exercise, the athlete should practice any strategies in training so that they can be assessed and fine-tuned.
2. Despite early speculation, there is insufficient evidence to support the statement that all athletes will benefit from eating low GI carbohydrate meals prior to prolonged exercise. The idea is that a more sustained glucose response might sustain fuel and performance. In fact, in sports events where carbohydrate stores can become depleted, the typical way to sustain the carbohydrate supply during exercise is to consume carbohydrate during the event. The athlete should let practical issues and individual experience guide the choice of a pre-event meal. You may happen to like a carbohydrate food that is low GI (e.g. pasta), or you may find that your choices tend to foods with a high glycemic index such as rice, breakfast cereal, toast. Both choices can work.
3. For specific individuals or during unique training situations, a low GI pre-event meal may be of particular benefit. Some athletes show an exaggerated and negative response when they eat carbohydrate foods in the hour before exercise. About 5% of the population experience a rebound hypoglycemia or blood sugar drop - and they feel terrible. Why this response occurs in some people is unknown. During unusual endurance sessions such as open water swimming where practical difficulties prevent the athlete from consuming carbohydrate during the session, the pre-event meal may have greater bearing on metabolism and fuel availability during the event, and a low GI carbohydrate meal may sustain blood glucose, and performance.
4. Athletes performing prolonged exercise should consume carbohydrate during the event to supply additional fuel and thereby enhance their performance. Which carbohydrate drink or food to consume depends generally on their previous experience, the logistics of the event, gastrointestinal comfort and the need for fluid replacement. A carbohydrate source of moderate to high GI appears to be sensible - such as a glucose-based sports drink. However, practical issues and individual tastes are more important than GI when choosing a carbohydrate source for prolonged exercise situations.
5. Moderate and high GI carbohydrate foods appear to enhance glycogen recovery after exercise compared with low GI foods. The reason for this is not clear. The most important point, however, in post exercise refueling is to eat enough total carbohydrate. We give recommendations to athletes about how much carbohydrate they should consume immediately after exercise and throughout the day to meet their refueling needs. Foods must be available and appetizing to the athlete so that these recommendations can be met. It is OK to let some favorite low GI carbohydrate foods contribute to total fuel intake - especially if these are foods that are handy and easy to eat. However, it makes sense to focus on carbohydrate foods and drinks with a moderate to high GI for glycogen recovery. The overall message: choose what is practical.
Brand Miller, J., Foster-Powell, K., & Colagiuri, S.(1996). The G.I. Factor: The Glycaemic Index Solution. Sydney, Australia: Hodder and Stoughton.
Here are some thoughts concerning carbohydrate intake prior to endurance exercise.
The studies we`ve looked at recently suggest that horses taking in high caloric diets prior to two hours of moderate exercise demonstrated lower blood glucose post-ex than those which either fasted or took hay only. Why would that happen?
Insulin regulates blood glucose--without insulin the blood would carry high levels of glucose and clog up every small capillary in the body. Insulin does not cause the excretion of sugar--lack of insulin does that. What insulin does is move glucose from blood and puts it into muscle cells as glycogen. Chromium speed up this process.
So, if we end up with a lower blood glucose at the end of two hours of exercise, because insulin has sequestered most of the intake into muscle cells, are we better or worse off as far as sustained athletic performance is concerned? People from this group have tried glycogen loading and have seen good results (some saw no results), so my vicarious experience tells me that there is something wrong with the idea that lower blood glucose is always a bad thing. We know that going into a high speed race we need an elevated blood glucose in order for the CNS to allow maximum effort. But after 2 hours of work, are we seeing a decrease in performance of those animals fed grain? If not, then whose to say that a lower blood glucose at the point doesn`t indicate a beneficial response?
Let me give you a parallel example: It has always been assumed that lactic acid was Public Enemy Number One when it comes to racehorse performance. The higher the lactate, the more paralyzed the muscles become--that`s the theory, with significant support from the literature. A typical post-race lactate for a Thoroughbred running 6 furlongs to 1 mile is 22mmol/L. Let`s say this effort is a poor one--the horse races last or next to last, fading dramatically at the end of the race, as most Thoroughbreds do.
Let`s say we have another race, and post race lactic acid is 40mmol/L. Where does the horse finish this time--in the grandstand? No. He wins, after a 4-day glycogen loading protocol. I was astonished by this lactate number because it predicts a horse that can hardly move--but here we have a winning horse, bright-eyed and bushy-tailed, who went right on winning several more times. This process has been repeated enough thousands of times that we know it works. Liz Quesnel took one Standardbred from having $3000 in winnings on her card to over $100,000 in four months with glycogen loading.
So, we have a case here where the result you`d expect from a scientific measurement does not play out in the real world. And the results we`ve had from this endurance group--those with the courage and curiosity to experiment--also shows benefit, or no result, but no negative result, from glycogen loading. Many of the people I`ve talked to in this group feed grain to horses competing in endurance events within hours of the beginning of the ride and report performance benefits from that.
So, for me, something`s not right here. We`re concluding that feeding a horse grain on the day of the ride is going to result in poor performance. Does it? Does any study show performance decrements from feeding grain prior to the activity? If so, they would directly disagree with the human science and with equine experience.
Here`s another way to think of this problem: Insulin regulates blood glucose, which is central to the survival of the central nervous system. When glucose gets low, the brain and spinal column limit glucose uptake by the muscle -- pulling back insulin production while limiting muscle firings (called central fatigue). If, on the other hand, the body is loaded with glucose and glycogen (insulin pushes glucose into CNS cells as was as into all the other organs), maybe the brain is saying, "Hey, we`re loaded with fuel--don`t worry about shunting it out to working muscles."
This happens with ketone levels. As ketones become higher in the blood, the CNS limits the number of firings of a muscle cell to a certain level of depletion -- say 50-60%. At that point the muscle cell shuts down and performance begins to be compromised. Louisiana State, though, showed that four days of corn oil loading prior to a race resulted in zero ketones and better performance--because the muscle cells could burn most or all of their fuel. The trouble with constant corn oil is that muscle cells become dependent on fat, and it`s not a racing fuel--but that`s another story.
I don`t know if the above hypothesis has any validity, but I would like to know if any of these studies post-grain/exercise showed a decrement in performance coinciding with the drop in blood glucose after 2 hours. Or has that extra glucose been stashed away into places where it will do a lot of good?
Recently, a veterinarian consultant for Equine Athlete suggested in a Q&A that feeding a racehorse grain on raceday would not help because the effect of the feeding would not be seen for as long as 18 hours. That was the old theory--now we know that digestion of carbohydrate (grain) has an impact on blood sugar within 15 minutes and peaks at about two hours.
Everybody has the opportunity to be wrong, but we still have some things to learn here. I`d like more discussion, particularly about performance with grain fed, or glycogen-loaded endurance horses.
I asked Dr. Eleanor Kellon what she thought of this discussion. Eleanor is the major contributor to Michael Plum`s newsletter and has been a great source of applied science to me. Here`s what she said:
I think what you are referring to is lower blood glucose during work with grain feedings 2 to 3 hours before exercise. This goes along with the higher insulin levels present at the time of exercise and there is also an associated decrease in the amount of glycogen that is burned - all of which points to higher utilization of blood glucose and glycogen sparing with the grain feeding. Nothing wrong with that!
I replied privately to ti but, in the light of his recent posts, feel I have to once again remind everyone that the benefits he is claiming for grain and glycogen loading pertain to Standardbred and thoroughbred race horses, which perform anaerobically at maximum speed for less than 3 minutes. Their muscle metabolism requires the glucose/glycogen stores to be available. HOWEVER, our endurance race horses are performing for hours AEROBICALLY-a type of work in which while glucose/glycogen can be used, the most efficient energy source (lowest heat generation, highest calories of energy available for muscle contraction) are fatty acids-either from moblization of fat stores or the by-products of fermentation of hay or forage (yes, hay has a lower calorie per pound as fed, but once those fatty acids are made and used by the muscles they are efficient). High glucose stimulates high insulin which inhibits the mobilization and utilization of FAT while enhancing use of glucose. Great for that final race to the finish line but not in the best interest of the horse during the hours it takes to get there! Yes, many horses are fed grain before and during endurance races without obvious ill effects. There is a lot of leeway here and individual variation. Unfortunately equine nutrition is not an exact science as yet, and probably never will be due to the extreme variability of performance type, effects of weather, terrain and individual variation! My best advice is to keep it simple, avoid extremes, learn as much as you can about digestion and metabolism in horses from scientific sources (much as I like Michael Plumb`s Journal, he openly admits it is not scientific-great for comparing rump rugs but sometimes a bit inaccurate when discussing biological things like feeding and textbooks written before 1989 or so can be woefully out of date). Above all else, if it ain`t broke, don`t fix it-if your horse is performing well, bright eyes, shiny coat good weight, don`t go trying all sorts of additives or drastic changes in what you are doing! I highly recommend Lon Lewis`s Feeding and Care of the Horse, 2nd edition, published in 1995 by Williams and Wilkins (A paper back-relatively inexpensive) for those who want the most up to date scientific info on horse nutrition.
Sarah Ralston, VMD, PhD
The purpose of feeding carbohydrates is not to raise blood glucose, but to supply energy to working muscles. The potential energy doesn`t go away just because the body regulates blood sugar back to normal. Those carbs are moved into muscle, as glycogen, through the action of insulin. As far as blood glucose is concerned, the only thing you have to avoid is very low levels at the beginning of a high intensity exercise--the horse will be too sleepy to blast out of the starting gate--not a problem with endurance horses.
Drawing significant fat from body stores represents an emergency situation for the body. It`s best not to be dependent on fat at any time during virtually every athletic event.