Keep reading to get a deeper look into our body's need for macronutruents in exercise.
Carbohydrates are the simple and complex sugars of the food world. They include sugars like glucose and lactose, starches, and fibers. They serve four primary functions in the body:
- Provide energy for the body in the form of glucose delivered through the blood and glycogen stored in muscles.
- Protein sparers. If there are insufficient carbs for the body's energy needs then protein will be used.
- Primers for breaking down fats. Too few carbs lead to poorly broken down fats and a potentially harmful condition called ketosis.
- Fuel for the Central Nervous System. This system requires a continuous flow of carbs for proper functioning, and with insufficient carbs central nervous fatigue and hypoglycemia can result. Glucose is the main fuel, except in starvation or super low carb diets where adaptations to use fat as an energy source happen after about 8 days.
Carbs in Exercise
Carbohydrates are stored in three primary places in the body. For a typical 80 kilogram well-nourished man, about 400 grams (1600 calories) are stored as muscle glycogen, 100 grams (400 calories) as liver glycogen, and 3 grams as plasma glucose (12 calories). Totaling about 2000 calories, this is enough to power a 20 mile run, and helps explain why runners typically hit “the wall” at that point in a marathon. Studies have shown a direct correlation with the glycogen storage levels and one's endurance in cycling and other aerobic sports.
The upper limit of this storage capacity is about double that (or 15 grams per kilogram of body mass), which takes about three days to load to. This means a pasta dinner the night before the race isn't really going to load a body with carbs- greater planning is required. Also, a 24 hour fast or low carb diet will nearly deplete these glycogen stores. For physically active people, carbs should be about 60% of total calories consumed, and be upped to 70% for intense training. For reference, the typical American eats about 50% of their diet in carbs, and much of this is in simple sugars rather than fruits, vegetables, and grains.
During exercise, glycogen is broken down in the liver and muscles to glucose and sent into the blood for uptake by working muscles to meet energy demands. This is done through hormones that regulate liver and muscle glycogen stores by controlling blood sugar levels As blood sugar levels go down due to use as fuel, more glycogen is broken down and sent where it is most needed through the circulatory system. Carbs are the preferred fuel during intense aerobic exercise because the body can get energy from it twice as fast as from fat or protein.
As exercise intensity increases, liver glycogen is broken down more while muscle glycogen remains the primary fuel source throughout. In high intensity exercise, blood glucose can supply up to 30% of energy needs. By 40 minutes in, glucose uptake can be 7 to 20 times the uptake at rest. Liver glucagon stores go down by about 55% after one hour, and by two hours nearly all liver and muscle stores are finished. At this point, the exerciser can only maintain about 50% of the initial intensity.
In moderate intensity and prolonged exercise, the use and rate of carbs is a little different. During the first 20 minutes of moderate exercise, 40% to 50% is supplied by liver and muscle stores. The rest is supplied by the breakdown of fats and a little bit by protein. During low intensity exercise, carbs use goes down even more and fat plays a larger role in energy supply for muscles.
An odd and somewhat unknown point in exercise physiology is why fatigue happens once glycogen stores are depleted. At this point there is still plenty of oxygen and fat stores, but that seems to be insufficient. It may be due to central nervous system fatigue (too little glucose for its functioning), too little muscle glycogen to help with fat breakdown, and the overall slower rate of energy release from fat.
Fats are a category of macronutritent that includes saturated and unsaturated fats, trans-fatty acids, and cholesterol. It serves as an energy source and reserve, protecting the vital organs, providing thermal insulation, and acting as a vitamin carrier and hunger suppressor. Its great as cellular fuel because it carries so much energy per unit weight, transports and stores easily, and provides a ready source of energy.
During rest, fat provides as much as 80% to 90% of energy requirements. A gram of fat contains twice as much energy as a gram of carbohydrates or protein. We store an amazing amount of energy in the form of fat. Assuming an average 80-kg man with 15% body fat, we are already at 108,000 calories of stored energy! In addition to the stored fat in subcutaneous flesh, another 2700 calories is stored within muscles for the same theoretical man. A small amount is also stored as free fatty acids and neutral fats in blood plasma.
During exercise, fat provides 30% to 80% of the overall energy supply. As exercise intensity increases, energy from fat fails to rise above a certain point and the body turns to carbohydrates for its energy needs. In low intensity exercise fat supplies as much as 80%, decreasing to about 50% during moderate intensity exercise (the other half being from carbs). After an hour of moderate exercise fat begins providing a higher percentage of energy- increasing to about 70% in the third hour. During intense exercise, carbohydrates are the preferred fuel, but keep in mind that prolonged exercise results in higher fat use.
Consistent aerobic training has a very positive effect on fat use. For trained men during two hours of moderate intensity exercise, intramuscular fat use can rise from 25% to 40% with a concurrent decrease in carb use from 58% to 38%. Well trained athletes can exercise at a higher intensity for longer in part due to more efficient use of fat as energy, although they will still become fatigued once glycogen stores become depleted. Physiologically, they have more capillaries and mitochondria (the powerhouses of the cell); breakdown and transfer fat faster and more efficiently; and are less dependent on glycogen reserves to maintain cellular integrity and function.
Proteins are the third type of macronutrient. They are amino acids and combinations of amino acids used for an enormous array of purposes and functions. Unlike carbs and fats, there are no major reserves of protein available for energy. Its all being used by the body as important tissue structures (like our muscles) or to help maintain body functioning.
Protein does serve as a more major energy fuel than previously thought, but maintaining good carb reserves can prevent much of the protein energy use. The argument for more protein is mostly based on the twin arguments of how exercise damages or tears muscles and increases protein breakdown for energy use as well.
While athletes do require a higher protein intake, this has tended to be overemphasized in the media and by many coaches and trainers. Studies have shown that bodybuilders typically consume far more protein than the body can use, while endurance athletes could use more protein. One well researched recommendation is to increase protein intake from .83 grams per kg of body weight to 1.2-1.8 for those who are training intensely.
So there you have it! Thanks for reading and happy running!