SPORTS NUTRITION SERIES PART II: CARBS

Updated: Apr 30


CARBS ARE NOT YOUR ENEMY

Despite years of research confirming the importance of carbohydrate consumption, many athletes still believe protein is the single most critical nutrient for improving athletic performance. Although the importance of protein should not be diminished, delivering the right amount of carbohydrates at the right times optimizes the limited energy stores, ensures better carbohydrate delivery to the brain, reduces the possibility of depleting the limited stores and sustains athletic performance at a high level. And while protein is critically important for health and plays a role in sustaining and building muscle mass, reducing muscle soreness and improving recovery, consuming too much protein does little to improve performance when it replaces carbohydrate.


Carb! What is it good for? Absolutely everything..

1. Carbohydrates provide the fuel for working muscles;

2. Dietary fibers control cholesterol and assist digestion;

3. They help the body absorb nutrients and water;

4. They maintain blood sugar levels.


All the different types of carbohydrates

There are various types of carbohydrates and our bodies react differently to each type. Because of these differences, athletes should carefully consider the best type of carbohydrate to consume before, during and after exercise.


The basic unit of all carbohydrates is the monosaccharide (single-molecule carbohydrate) and the three most common dietary monosaccharides are: glucose, fructose and galactose. Glucose is the basic metabolic unit for human cells (it is the one that provides energy). All other monosaccharides get converted into glucose in order to be used as fuel.

Two-molecule carbohydrates are called disaccharides, and the three most common are sucrose (consisting of one glucose and one fructose molecule), lactose (glucose and galactose) and maltose (two glucose molecules).

Together the monosaccharides and disaccharides are referred to as simple carbohydrates, or sugars.

Larger carbohydrate molecules are commonly referred to as complex carbohydrates. These are further divided into:

1. Oligosaccharides - partially digestible carbohydrates, commonly found in legumes. Although they may cause gas or bloating, they are generally considered healthy;

2. Starches (digestible polisaccharides) - these should provide the main source of carbohydrate energy;

3. Dietary fiber (indigestible polisaccharides) - these are important for GI tract health and disease resistance.

What about glycemic index?


The glycemic index is a measure of how quickly consumed carbohydrates get converted into blood glucose. The higher the glycemic index number, the greater the blood sugar response to eating that food. Glucose has a glycemic index score of 100 which is the basis of comparison for other foods. A glycemic index over 70 is considered high, between 56 and 69 medium and lower than 55 is considered low.


Because the volume of glucose and the speed at which it enters the blood can influence the amount of insulin produced, it is generally desirable for people to consume foods with a medium to low glycemic index. However, there are times, such as during and immediately after exercise when high glycemic foods can be better for athletes.


Generally speaking, carbohydrate foods higher in fiber have a lower glycemic index, making them a good choice for athletes. However, dietary fiber may be a source of gas and distention, making them poor choices for consumption just before or during competition. Athletes should experiment to determine which foods are most easily tolerated. Starchy carbohydrates with low fiber content such as pasta are usually easily tolerated and deliver a high volume of carbohydrate athletes require.

Ideally, athletes should consume complex carbohydrates when possible, but may consume simple carbohydrates during and immediately after exercise. Other energy substrates (protein and fat) should also be consumed to fulfil total nutrient requirements, but carbohydrate should remain the predominant energy source.



Where do we keep the carbs?


Unlike protein or fat, we have a limited storage capacity for carbohydrate. Our bodies can store approximately 350 grams (1.400 kcal) of carbs in the form of muscle glycogen, an additional 90 grams (360 kcal) as glycogen in the liver and a small amount of circulating glucose in the blood (~5 grams, or 20 kcal). The larger the muscle mass, the greater the potential glycogen storage, but also the potential need for glycogen.


Greater glycogen storage is also associated with greater fluid storage. For some sports this can be an advantage (like for a marathon runner on a hot day, who needs the extra water to sustain the excess sweat rate), but for other sports it can be considered a disadvantage (for a gymnast who needs the highest strength-to-weight ratio, the excess water weight could be a problem). This means that different athletes need to optimize glycogen storage in the best possible way for their specific sport.


Glucose is the main fuel for the creation of muscular energy, in the form of ATP (adenosine triphosphate). A failure to sustain glucose delivery to working muscles results in cessation of high intensity activity. Therefore, understanding how to avoid glucose depletion should be a major focus of an athlete's nutrition practice.


Carbohydrate adequacy is most critical at higher levels of exercise intensity, because there is a greater reliance on carbohydrate as a source of fuel. Higher intensity training regimens, therefore, require more frequent intakes of carbohydrate to satisfy needs.

A little science

The amount of glucose in the blood is maintained within a relatively narrow range, by recruiting two pancreatic hormones: insulin and glucagon. High levels of blood glucose stimulate the secretion of insulin. Insulin affects the cell membranes of muscle and fat cells, allowing glucose from the blood to enter the cell and thus lowering blood glucose levels and enabling the cells to receive a needed source of energy.


If blood glucose levels are low (between meals or during exercise), glucagon helps break down liver glycogen and move free glucose into the blood stream. Glucagon can also stimulate gluconeogenesis (the production of glucose from non-glucose substances, like fat or skeletal protein (muscle breakdown).


Besides insulin and glucagon, two other hormones also influence blood glucose: epinephrine and cortisol. Epinephrine, a stress hormone, initiates an extremely rapid breakdown of liver glycogen to quickly increase blood glucose levels. Cortisol, also a stress hormone, promotes muscle breakdown, making certain amino acids available for gloconeogenesis, ultimately resulting in an increased blood glucose.



Why is this important?


Both epinephrine and cortisol are released as a result of exercise related stress. Consuming carbohydrates during exercise helps control the release of these hormones. This results in preserving liver glycogen (by controlling epinephrine production) and preventing muscle protein breakdown (by controlling cortisol production). That's a pretty strong argument for getting some carbs in during exercise.

What happens to the carbs we eat?

The glucose in the blood is derived mainly from dietary carbohydrate, with starches constituting the major food source, which are then digested into monosaccharides for absorption into the blood. Excess glucose is stored as glycogen in the liver and muscles, but only up to the glycogen saturation point (87-100 grams in the liver and approximately 350 grams on average in the muscles). Liver glycogen is primarily responsible for stabilizing blood glucose, while muscle glycogen is mainly responsible for providing an energy source to working muscles. All additional glucose above the saturation level will be stored as fat.


Blood sugar is not easily maintained when liver glycogen is depleted, even when muscle glycogen stores are full. Blood glucose is the primary fuel for the central nervous system (aka your brain). Low blood sugar may be related to mental fatigue, which results in muscle fatigue. Because liver glycogen and blood glucose stores are easily depleted during even short duration activities, the intake of carbohydrate during exercise is critical for maintaining mental function and, ultimately, muscle function. In simple terms, athletes who allow blood sugar to drop below normal experience a drop in athletic performance because of compromised CNS function, even if the muscles are full of fuel.

Carbs and exercise


The higher the exercise intensity, the more athletes rely on carbohydrate for energy. However, even low intensity exercise that derives most of its fuel from fat still requires some level of carbohydrate for the complete combustion of fat and maintaining blood glucose.


Another reason why carbohydrate is so critical for athletes is because we can more efficiently create energy per unit of oxygen from carbohydrate than from any other fuel. In addition, aerobic glycolysis can produce ATP in a larger quantity and at a faster rate than the oxidation of fat can produce it.

Athletes need carbohydrates, regardless of whether their sport is mainly aerobic or anaerobic in nature, for several reasons:


1. to provide energy to satisfy the majority of caloric needs

2. to optimize glycogen stores

3. to allow for muscle recovery after physical activity

4. to provide a well-tolerated source of energy during practice and competition

5. to provide a quick and easy source of energy between meals to maintain blood sugar

6. to maintain blood sugar during physical activity.

So how much do we need?

A lot..


The recommendation for the general population is that carbohydrate should supply 50-55% of total daily calories and the dietary reference intake is 130 grams per day (this is the average minimal usage of glucose by the brain). These recommendations also advise that no more that 25% of carbohydrate intake be derived from sugars, and dietary fiber consumption should be 38 grams per day for adult men and 25 grams for adult women.


The amount typically recommended for athletes is between 55 and 65% of total calories, assuming an adequate total caloric intake, or 7-8 (sometimes up to 12) grams of carbohydrate per kg body weight. These recommendations should be adjusted for individual athlete requirements which are partially based on the state of conditioning and the duration and intensity of the activity, with higher requirement for greater duration and greater intensity. For daily recovery from moderate duration, low intensity training: 5-7 grams per kg body mass; for moderate to heavy endurance training: 7-10 grams; and for extremely intense exercise with a duration of 4-6 (or more) hours per day: 10-12 grams per kg body mass per day. That is a buttload of carbs..


Consuming sufficient carbohydrate will ensure that athletes meet the majority of fuel requirements for their training programme and optimize the restoration of muscle glycogen stores between bouts of physical activity. Athletes who have less than 8 hours between exercise bouts should optimize available eating time by snacking on high carbohydrate, nutrient-rich foods immediately after the first workout.


It is also important to choose recovery foods that contain a wide variety of nutrients besides carbohydrate. Protein is an important component of recovery intake because it aids glycogen recovery.

First make sure you eat enough, and then make a plan to optimize your intake