Athletic performance is largely dependent upon stable blood sugar.

That’s it – one sentence. That is all you really need to know about nutrition for sports and exercise: eat the right foods at the right times to keep your blood sugar stable. Of course, the “right foods” at the “right times” is the tricky part, and requires a knowledge of the metabolism of energy in the human body. This introduction is an attempt to explain those right foods and right times and why, focusing on the science of digestion and energy use in the body, and to provide practical recommendations for athletes.

The term ‘blood sugar’ refers to the glucose concentration in the blood. A normal concentration is between 4 and 7 mmol (70-126 mg/dL). Insulin and glucagon are responsible for maintaining blood sugar at stable levels; they are released in response to changes in blood sugar. The pancreas secretes insulin when blood sugar rises (for instance, after a meal) and lowers blood sugar by stimulating its storage in the form of glycogen in the muscles and the liver, and in the form of triglycerides in fat cells. Glucagon, largely responsible for carbohydrate metabolism, is released when blood sugar is low. It stimulates the liver to convert stored glycogen to glucose and put it into the blood. Through these hormones, then, blood sugar is kept stable so that the muscles and brain have energy to do work.

Extremes in blood sugar are called hyperglycemia (too high blood sugar) and hypoglycemia. Neither are desirable states, and student athletes should know that eating small meals throughout the day are a way to avoid blood sugar extremes, thus keeping blood sugar stable.
Hyperglycemia is defined as blood sugar over 126mg/dL (7mmol). The danger of hyperglycemia is dehydration. Sugar is stored in the muscles and the liver, the kidneys can also work with sugar if liver and muscles are full. However, when the kidneys filter sugar, they require 2g of water per 1g sugar and the person may become dehydrated. Excessive thirst (polydipsia) and frequent urination (polyuria), then, are symptoms of hyperglycemia, as well as fatigue and blurred vision. Hyperglycemia is most often associated with diabetes. It is more likely that a student athlete experience hypoglycemia than hyperglycemia. That said, any person eating too much (particularly too many carbohydrate calories) at one time might experience hyperglycemia if so many calories are consumed that it overwhelms the body’s normal regulatory mechanisms (in this case, insulin, which would try to lower blood sugar). A specific instance of this might be a student athlete with bulimia nervosa (eating disorders are frighteningly common amon
g student athletes), as binge episodes can very quickly lead to rapid rises in blood sugar.

Hypoglycemia occurs when blood sugar drops below 70 mg/dL (4 mmol). A person experiencing hypoglycemia may feel dizzy, lightheaded, unable to concentrate, weak, and irritable. Additionally, hypoglycemia will cause a decrease in athletic performance. This is because Vo2 max is affected. The brain’s only source of energy is glucose, and if blood sugar drops too low, the brain feels threatened. If the muscles are stealing all the energy, the body will “slow down” the muscles by lowering Vo2 (oxygen consumption). Since Vo2 is a function of heart rate and stroke volume, the sympathetic nervous system slows down the heart rate by inhibiting epinephrine release. Epinephrine is the “fight or flight” hormone released from the adrenal glands and without it, heart rate decreases, oxygen consumption decreases, and the athlete slows down.

There are several ways for an athlete to become hypoglycemic. In the first, the athlete has done too much work (run too many miles, swum too many yards, and so on – any event in which the body has burned up too much energy) and liver glycogen stores are depleted. Distance runners will identify the feeling of “hitting the wall.” A poor diet in which the body is starved for glucose can cause hypoglycemia, and starting training when glycogen stores are low (such as early morning training prior to eating) can lead to early hypoglycemia. The other form of hypoglycemia is commonly called reactive hypoglycemia, and is characterized by an abnormal drop in blood sugar following the ingestion of carbohydrates. In this case, too much insulin is released and it takes all the ingested sugar and stores it, leaving the blood empty of sugar. An athlete is likely to experience this if she eats “simple” sugar and then immediately begins training. A working muscle has the same effect on blood sugar as does insulin – that is, both want to take the insulin and “use” it (the muscle will burn it, the insulin will store it). Since both start to act on blood sugar, too much is taken out of the blood and the brain, starved of glucose, becomes unhappy – and reacts by making the athlete feel dizzy, weak, and so on.

It is awfully hard to train (or race!) if one feels lightheaded and weak. It is thus important for an athlete to know how to prevent hypoglycemia. The first is to start training with enough stored glycogen, and this can be accomplished by eating enough calories (particularly adequate carbohydrate) prior to training. For athletes with a high caloric requirement, it is important to eat frequently so that the body gets enough fuel but not so much at one time that it will overwhelm the metabolic hormones (insulin and glucagon) and lead to hyperglycemia. In other words – eat small meals throughout the day (300-400 calories every couple of hours). Secondly, it is important that athletes eat something prior to early morning training. Even during sleep, the body is working and consuming energy. Thus, a person wakes up with slightly depleted energy stores. A small snack can top off liver glycogen stores, providing the athlete with energy to complete a morning workout. Lastly, athletes should consume calories during training, particularly in sessions of over 90 minutes (the time it takes to deplete glycogen stores). Gatorade and gels are good choices, though don’t ignore solid food (ask your favorite long distance triathlete what she’ll eat on a long bike ride!) – it’s easy to eat a banana between swim races, or at halftime of a soccer game. Avoiding reactive hypoglycemia is pretty simple: don’t eat something high in sugar right before you train, and in general (meaning during the day), eat meals balanced in carbohydrate, fat, and protein so as not to spike blood sugar.

In order to measure how different foods effect blood sugar, nutrition scientists (Dr. David Jenkins, 1981) devised the Glycemic Index. This measures the insulin response to different foods. Pure glucose is assigned a GI of 100 (meaning that if one ingests 50g glucose, the blood sugar rise is 100. This is an arbitrary number – it does not mean that blood sugar rises 100mg/dL. It merely uses glucose as a reference food. In fact, white bread can also be a reference food, and if white bread is used as a base at 100 then glucose is assigned a GI of 140). All other foods GI values are calculated as relative to the reference food. It should be noted that glucose – pure sugar – is 100. Thus, high glycemic is defined as foods with a GI of 70 or more; examples include watermelon, candy, white rice, and baked potato. These foods will cause a rapid rise in blood sugar. Medium glycemic foods are those with an index of 59-69; examples include brown rice, sucrose, and croissants. Low glycemic foods are those with an index below 55, which includes most fruits and vegetables, and whole grains. These will cause a slow rise in blood sugar. A low glycemic load means slower digestion and absorption, a lower insulin demand, and possibly a reduction in blood lipids.

The glycemic index has a lot to do with how macronutrients (carbohydrate, fat, and protein) are metabolized. Sugar raises blood sugar quickly because it is absorbed in the jejunum (upper intestine, after going through the stomach). Fat and protein are absorbed lower in the digestive tract (ileum). It takes longer after eating for them to digest, thus it takes blood sugar longer to react. Alcohol is absorbed immediately in the stomach. This is an important consideration, as it goes right from the stomach to the liver and then into metabolic reactions. Thus, if you have a couple drinks before dinner, by the time you get around to the actual food, your body won’t need the calories and will store them as fat. Alcohol is not good training fuel – it lowers heart contractility and thus the ability to use oxygen, as well as causing liver damage.

Given what we know about blood sugar and its response to exercise, the glycemic index may be helpful in selecting food choices around training sessions. To avoid blood sugar spikes, it’s best not to eat high GI foods alone (more on that later). Low and medium GI foods are best prior to training sessions. High GI foods should be reserved for immediately following training, when glucose receptors in the muscle cell are near the membrane and will suck up the sugar and store it. In fact, one of the best things an athlete can do to make sure she recovers from her training is to eat immediately following training – the cell receptors are nearer the membrane for about 15 minutes, and this window is the peak of glycogen resynthesis. In other words, if you want to maximize energy of your next training session, eat as soon as you finish the first. Practically, this can be accomplished by keeping easily-transported foods in a backpack (bananas, energy bars, Gatorade – whatever might be palatable after a hard workout when appetite is likely suppressed). It is most important that the food contain carbohydrate, though studies have shown that protein is important for muscle repair (a 4:1 ratio of carb to protein is optimal). Fat, which is slow to digest, should be avoided immediately after training.

It should be noted here that most athletes don’t eat right after training. They talk, shower, go back home and then eat. If this happens, most of the ingested energy will be stored as fat (lipogenesis). Where energy goes is determined by what needs it – which may be working muscles, the liver, or the brain. Where the energy goes is determined by relative intensity: working muscles will take sugar. The brain always gets a little sugar because sugar is its only source of fuel. If liver stores are depleted, insulin will transport it there, but quite possibly store it as fat. You have to eat after training at some point, so eat when most if it will be stored as glycogen… right after you finish a workout.

The problem with the glycemic index is that it measures the effect on blood sugar of 50g of a single food item. First, most athletes will eat more than 50g of whatever food they’re consuming. Second, most athletes won’t eat just one food – they will eat foods in combination (a peanut butter and jelly sandwich, for instance). If foods are eaten together, the blood sugar effect is different than if the same foods were eaten separately. For instance, in a peanut butter and jelly sandwich, the high GI jelly won’t spike blood sugar because the fat in the peanut butter and the whole grain bread will slow digestion, and thus the blood sugar response. An athlete should know about and pay attention to the glycemic index, but should pay more attention to how certain foods seem to affect their body and figure out what works best for sustained energy.

It takes roughly 90 minutes to burn through stored energy and after a two-hour practice, glycogen is most definitely depleted. At this point, protein catabolizes (in other words, you’re breaking down muscle tissue). A couple days of really long workouts lead to chronic glycogen depletion – even if the athlete is taking in enough calories, glycogen stores will not be fully replenished, and the athlete may wind up underrecovered and lacking energy for subsequent sessions. The athlete may not feel that badly for a couple weeks but by the end of the season, a lot of fatigue can be blamed on chronic glycogen depletion. Exercise can act as an appetite suppressant, which further complicates the problem of eating enough at the right times.

In conclusion, sports and exercise create high energy demands and there is a relatively simple interplay in the body of energy absorption, storage, and use. The athlete’s goal should be to stabilize blood sugar by eating both during the day, prior to, and right after training, and eating foods that will have the most positive effects on blood sugar at those times. As a final note, hydration is also critical in sports performance, and along with eating, an athlete should ensure proper fluid intake at all times.

In part I, we discussed how important carbohydrate and a stable blood sugar are. However, there are two more macronutrients of importance to athletes: protein and fat. Carbohydrate should make up 60% of an athlete’s calories, as they provide the fuel for working muscle. The other 40% should be split between protein and fat. Protein’s major role in the body is rebuilding tissue; fat helps with many things, including hormone health and forming cell membranes.

While athletes are commonly pressured to lose body fat and be lean and mean, a certain amount of body fat is essential. Beyond essential fat, it is true that fat mass will likely slow an athlete down (this is particularly true in running and cycling, though not so much in swimming where body weight is supported by water, and a little fat may help with buoyancy). Females in particular need fat, as it is a precursor to estrogen. Too little body fat for a female may result in amenorrhea, though it is not exactly known what causes this condition (interaction of other hormones, a prolonged energy deficit – which might lead to that low body fat in the first place), it is known that it is unhealthy (a lack of menses means sub-optimal estrogen levels, and as estrogen is necessary for bone health, amenhorreic athletes are at risk for osteoporosis). The take-away message is this: a little body fat is not necessarily a bad thing and it is better to eat and focus on performance than fixate on weight and body fat percentage. That essential fat is there to protect organs, keep a person warm, and provide some cushioning for bones and joints. Adipose tissue (the technical name for body fat) is also potentially the largest reserve of energy in the body.

Certain vitamins are fat-soluble, meaning that the body needs fat to be able to absorb them, and that they are stored in fat (as opposed to water-soluble vitamins, excesses of which are excreted). Vitamins A, D, E, and K are fat-soluble, as are caroteinoids. It is also important to eat fat to get the essential fatty acids linoleic acid and alpha-linoleic acid.

Fat’s other role is in cell membranes, which have a phospholipid bilayer. “Phospho” means phosphorus and the “lipid” part refers to fat. The membrane has two layers and is symmetrical such that the ‘heads’ of the molecules are on the outside (touching both the exterior fluid around the cell and the cytoplasm inside) and the ‘tails’ are on the inside. They can move a bit, and have channels, so that nutrients can be brought into the cell and waste can get out. Dietary fat, then, is needed to construct body cells.

Most people are aware that olive oil is a “good fat” and McDonalds foods are filled with “bad fat.” The “good fats” are unsaturated fats (including monounsaturated and polyunsaturated) and are commonly found in olive oil and nuts. Avocado is also a decent source. Saturated, or “bad” fats, are bad because they raise LDL cholesterol. Saturated fats are found primarily in foods as solids at room temp – butter and cheese, for instance, or on meat. Athletes do not have to worry too much about cholesterol, but should still lean toward consuming unsaturated fats.

Another issue of concern that has been in the news are “trans fats.” The difference between all types of fats lies in their chemical structure, and the body can’t really absorb trans fats. They wind up in the colon as waste. Partially-hydrogenated oils should also be avoided. These are made in the process of hydrogenation, or turning liquid corn oil into margarine. Partially hydrogenated fats increase the shelf life of things like crackers and cookies. Current data show that these type of fats too raise LDL cholesterol.

From a practical perspective, fat is good. It is, in many instances, what makes foods taste good (peanut butter, anyone?) and for athlete needing lots of calories, it is an energy-dense source (fat contains 9 kcal/gram, whereas carbohydrate and protein only each have 4kcal/gram). Fat is a large molecule and thus takes awhile to break down, so it provides a feeling of fullness. A lot of athletes fear that eating fat will make them fat, but the fat-phobia is just that: irrational. Fat is an essential (and often yummy) nutrient.

Like fat, protein has a variety of roles in the body. It is important for the growth, repair, and maintenance of body tissue, muscle in particular. Hormones, enzymes, neurotransmitters, and some components of the immune system are made from protein, as is hemoglobin. An athlete is most concerned with the fact that muscle fibers are made from proteins and in training, those fibers tear. Eating protein helps repair those tears so an athlete can get stronger.

Athletes need roughly 1.5g of protein per kg of body weight (90g for a 60 kg (132 lb)) athlete. It is also fine to look at it as 20% of total calories. Eating more than 1.5g/kg will likely not do anything, as the body can’t break it down and resynthesize more than that. It is also quite easy to eat enough protein, particularly in the US where meat and dairy are so abundant. A vegetarian will find no trouble eating enough protein, though may be more aware of getting an adequate amount than an omnivore. Good sources include meat, dairy, whole grains, nuts, legumes, seeds, and soy. Protein is graded by quality (the number of amino acids) and egg is the best source of protein, containing all the amino acids.

Proteins are made of amino acids and there are 20 different – some essential, others nonessential. The body can make some of the amino acids, others you need to get from food sources. All protein foods have different amounts of each of the 20 amino acids, making it necessary to eat a variety of protein-rich foods to get all the amino acids (especially for vegetarians). Meat and egg have all of the amino acids; the only plant food with all the amino acids is the grain quinoa. It used to be thought that foods would need to be “matched” to make a complete protein (beans and rice, for instance, or milk and a peanut butter sandwich) but we now know that as long as all the amino acids are eaten at some point, the body will find and use them.

Protein foods are thus broken down into the amino acids, then, and the amino acids circulate in the blood. They are used when there is a need for them. In other words, if you do a lifting workout, your muscle will pull amino acids out of blood to repair muscle tissue. However, those amino acids need to be in the blood to begin with. Therefore, it makes more sense to eat protein BEFORE a lifting workout so that the amino acids will be broken down and in the body, circulating when they are needed. Eating protein after a workout will take too long to digest, and the athlete will miss the peak resynthesis (testosterone, necessary for building muscle, peaks during the workout so for maximal strength gains, eat the protein before the workout so that the amino acids are bioavailable).

Like fat, protein adds to feeling full, and often tastes good as well. The Colby Swimming and Diving Handbook recommends that athletes focus on filling their plate 2/3 with carbohydrate foods and 1/3 with protein foods, and the fat will take care of itself. In conclusion, while carbohydrate is the main fuel for exercise, protein and fat are important for rebuilding and maintaining body tissue, keeping hormones intact, and making foods taste good and contributing to a feeling of fullness.

Nicely done, Alyie