March 22nd, 2022

Carbohydrates and Exercise: the first third is all science the last two thirds is the to do.

We know that one of the keys to enhanced exercise performance is proper nutrition. Modern professional athletes are figuring this out as the era of sports doping has been blown wide open even as many still try to cheat the system. Many teams now employ nutritionists and chefs in order to meet the athletes needs and help their players excel.

What happens during exercise?

The Science says: when we exercise our muscles need to burn sugar or fat as fuel to work effectively. The choice of macronutrient depends on the intensity. At a high intensity state like sprinting or fast cycling, the muscles prefer rapid energy and that preferred fuel source is glucose which is rapidly burned to ATP in the muscles mitochondria by glycolysis ( video of glycolysis to ATP). In a slow burn state like a long run, the body prefers fatty acids because the muscles needs are less rapid leaving the preferred fuel source as fat which is burned to ATP in the muscles mitochondria by beta oxidation (video of beta oxidation to ATP). Fatty acid oxidation is more efficient and burns cleaner which is good for your mitochondria. Peter Attia uses this analogy which I really like: "When driving on flat roads, the car will use diesel gas (fatty acids for the muscles) because it’s more efficient and economical because you get more miles per gallon. If you’re driving through the mountains you’ll need more acceleration so you’ll use the regular gasoline (glucose for the muscles)".

KEY SCIENCE ASIDE: During exercise, an insulin independent release of a glucose transporter, GLUT4, occurs migrating to the muscle cell surface allowing glucose to enter the cell to be metabolized into ATP or adenosine triphosphate in the mitochondria via glycolysis and the Krebs cycle. ATP is energy and allows our muscles to contract and move us(Video). Our muscles can store a certain amount of ATP as itself as well as glucose in the form of glycogen which is akin to how a plant stores it as starch. The concentration of glycogen in the muscle pre exercise will dictate how much energy can be released during a work out. This storage form of sugar is depleted in intense work outs. Unfortunately, we cannot transport this sugar from one muscle to another. Therefore, when you exercise you can only get sugar from each individual muscle or to a lesser extent from the liver's glycogen or synthesized from fats and amino acids or exogenously consumed glucose. Thus, maximizing the muscles glycogen before exercise is one key to peak performance. Remember that the intensity dictates the fuel source used. Therefore, for a long run at moderate intensity, glycogen stores may not matter at all, whereas, a high intensity marathon will require these stores as well as consumed glucose during the run. This can be seen in training zones.

Training zones are dictated by your personal heart rate percentages of maximum which are further based on age and fitness level. For example, my max heart rate at 51 years old is around 186. Thus the training zones for me are a percentage heart rate of that number. Zone 1 would be 50-60% of 186. Zone 2 would be 60 to 75% of 186 which for me is 111 to 139. This is important as the new research shows that long training at zone 2 is very good for your heart and body in general. It also increases insulin sensitivity significantly. This is a win win. From a metabolic perspective, training zones 1 through 3 usually burn fat as the primary fuel source because of the higher energy yield per molecule and burn efficiency. Zones 4 to 6 burn carbohydrates primarily until it is an all out sprint which uses stored muscle ATP or rapid anaerobic ATP generating pathways like creatine phosphate system or lactate.

Our liver does store sugar as glycogen and can transfer this sugar anywhere, but it cannot store as much as a muscle can for exercise, making it ineffective as a serious source for serious athletes. When an athlete says that they hit "the wall" during intense exercise, they are telling you that they ran out of muscle glycogen for an energy source and start to burn lean tissue or fat to produce glucose which is very inefficient and counterproductive for muscle mass and cannot be a supply source during intense exercise leaving you exhausted.

As you go through the zones of training, you start to consume more oxygen for metabolic respiration and ATP production. This leads to muscle contraction and release activity. You switch from fat burning to glucose and lactate burning. The latter releases hydrogen ions into the muscle which causes the muscle aches at maximal workouts. High endurance and experienced athletes have many advantages. One of them is the ability to clear lactate and hydrogen from the muscle reducing exercise fatigue and cramps.

The fitter you are, the better you will be able to store glycogen in the muscle. You will also need it less as you remain more efficient at burning fat at higher power outputs. A win win. The science of sports is quite amazing now. We are truly maximizing the human machine's ability. Fit athletes have lowest volume of intramuscular fat and the most sensitive insulin and insulin independent glucose uptake.

Aside: I think of muscle fat in the context of animal meats. The least healthy animal meats for our consumption contain the most intramuscular fat. Think of a corn fed cow here. There is something to be said here about a natural diet and movement for the animal. This also brings the insulin resistance work full circle. Insulin resistance caused by excessively consumed free fatty acids, glucose and fructose from Americanized processed foods will drive intramuscular fat which is a hallmark of poor exercise fitness.

Carbohydrate loading pre exercise, post exercise and during exercise are all key.

Post workout pre match loading:

Data shows that 2 days prior to an endurance event like a marathon, soccer game, long swim, etc.., if an athlete works out really hard and depletes the muscles sugar stores(glycogen) while withholding sugars from the diet, they can enhance the storage of glycogen post exercise by carb/sugar loading. Therefore, 2 days later during an event they have maximal muscle glycogen stores to compete with.

In other words: 2 days before big events an athlete should have a hard workout and not eat flour or sugar based foods. Eat nuts, seeds, vegetables, protein like organic soy or meat. Then post workout eat whole grain pasta, brown rice, and other rapidly digesting carbohydrates. The key is to get the meal within the first 2 hours post workout.

Having a healthy life style bar immediately after the workout makes good sense to get the process started before a big meal. The data shows that high end sports supplements like Recoverite or other post workout drinks are as effective as a home cooked meal. However, I tend to think that a natural food meal is still preferable to a processed variety, because it also provides fiber and micro biome enhancing molecules that a drink cannot over a longer period of use.

If you do not get the meal in the first 1-2 hours post exercise, you have lost the ability to fill your muscles up with optimal sugar stores.

The day in between the carb load and the race or game should be a rest or light workout day with a diet that is whole foods based. Remember that inadequate carbohydrate stores will lead your body to break down lean mass which would be your muscles. Not good.

Remember that this is primarily for endurance athletes and not the weight lifter or golfer.

Now that endurance athletes have preloaded their muscles with glucose/glycogen days before the event, what do they do dietarily the day of and during the event?

This is where we need to understand nature a bit. When you run a race or run away from a bull, you body releases chemicals called catecholamines which alters the physiology of your metabolism to reduce insulin use and maximize sugar availability and fatty acid breakdown for energy. In this way your body makes as much sugar available as possible to feed your muscles in order to evade the bull.

Based on this principle, it is clear from the research that during a long intense exercise event like a bike ride, soccer game or marathon, you can consume high glycemic foods like honey, dark chocolate or sports goo/drink to enhance muscle work while you are exercising since the sympathetic nervous system is in control and driving all sugar toward the skeletal muscle system which has insulin independent GLUT4 transporters in place. Doing this same action prior to the beginning of exercise will cause an insulin spike and be counter productive to the goal of maximal effectiveness as insulin will want to store the sugar as fat instead of keeping it available for muscle use.

During long endurance exercise events, the goal is to avoid breaking down muscle in order to make sugar via a pathway called gluconeogenesis. Thus, it is imperative that you have this strong supply of muscle gycogen, fat and sugar to burn during the event at the zone stages of macronutrient use.

When do we consume carbohydrates before a race or game event? Depending on the time of the event, I usually recommend 3 hours to digest a small to medium meal on race day. Therefore, if you have an 8 am race, skip the meal and grab some nuts and then consume a fast acting sugar during the endurance event. If the race is at noon, then have a full breakfast of high quality food like eggs and bacon with fresh fruit and yogurt. I would still consume the fast acting sugar during the race based on the above principles.

The carbohydrate dilemma - when, how much and why? Non endurance training or muscle growth work outs.

A study by Greg Haffe in 2000 showed that with weight lifting or resistance training, the carbohydrate that you ingest prior to and during the event has a beneficial effect on muscle glycogen storage but no effect on muscle work output. In other words it has a good effect on recovery but no effect on the current muscle building. Protein is the key to muscle building. However, being carbohydrate depleted, i.e. low carb diet, may make you feel washed out unless you are keto adapted which is to say that you burn fat as a primary fuel source and do it well.

Anabolic cellular growth occurs when we stimulate certain pathways like mTOR which occur during the feeding of all three macronutrients: carb, fat and protein. Study after study does show that protein is the key and 1.4 grams per kilogram per day for endurance athletes and 1.8 to 2 grams per day for weight lifters who want maximal muscle growth.

There is evidence that if you eat carbohydrates within one hour of weight lifting you break down less protein for energy and thereby increase muscle production and mass when protein is provided. This is a good thing for a trained athlete. Increased muscle mass is useful for strength during sport activity.

What types of carbs to consume? Pre workout, I recommend complex carbs 1 hour prior to lifting. Choose trail mix, green banana with peanut butter, fresh fruit and nuts, hummus and red peppers. Post workout, I recommend more complex carbs in higher volume and the protein load. For those that are not dairy sensitive, this is where dairy and whey protein come in handy.

Remember to look at the glycemic index and load for what types of food to eat. Aim for a load less than 10 for best results. See the link below.

Load up,

Dr. M


Glycemic Load
Low Glycemic Load Foods
Men's Fitness Article
Nutrient Timing Article
Nutrient Timing Article 2
Endurance Meal Timing
Marathon Metabolism
Metabolism and Endurance
Bacha Naturę Education
Inigo San Milan Peter Attia