diet - health - nutrition

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Role of Carbohydrates

[NOTE: This article was originally found in the Iowa State University - Extension and Outreach - Human Sciences Web site]

The roles of carbohydrate in the body includes providing energy for working muscles, providing fuel for the central nervous system, enabling fat metabolism, and preventing protein from being used as energy. Carbohydrate is the preferred source of energy or fuel for muscle contraction and biologic work.
Foods containing carbohydrate are in the grains, fruit, and milk groups. Vegetables have a small amount of carbohydrate.

After carbohydrate is eaten, it is broken down into smaller units of sugar (including glucose, fructose and galactose) in the stomach and small intestine. These small units of sugar are absorbed in the small intestine and then enter the bloodstream where they travel to the liver. Fructose and galactose are converted to glucose by the liver. Glucose is the carbohydrate transported by the bloodstream to the various tissues and organs, including the muscles and the brain, where it will be used as energy.

If the body does not need glucose for energy, it stores glucose in the liver and the skeletal muscles in a form called glycogen. If glycogen stores are full, glucose is stored as fat. Glycogen stores are used as an energy source when the body needs more glucose than is readily available in the bloodstream (for example, during exercise). The body has limited storage capacity for glycogen (about 2000 calories), which is why carbohydrate is commonly referred to as the limiting fuel in physical performance.

carbohydratesCarbohydrate spares the use of protein as an energy source. When carbohydrate consumption is inadequate, protein is broken down to make glucose to maintain a constant blood glucose level. However, when proteins are broken down they lose their primary role as building blocks for muscles. In addition, protein breakdown may result in an increased stress on the kidneys, where protein byproducts are excreted into the urine.

Finally, glucose is essential for the central nervous system. The brain primarily uses glucose as its energy source, and a lack of glucose can result in weakness, dizziness, and low blood glucose (hypoglycemia). Reduced blood glucose during exercise decreases performance and could lead to mental as well as physical fatigue.

Carbohydrates in the Body

All living cells contain glucose. For glucose to enter the cells it needs help from a hormone called insulin. Insulin acts as gatekeeper and is released once carbohydrate is ingested. It signals the cells to absorb the glucose. The glucose is then used for energy, stored in the liver and the muscles as glycogen, or stored as fat.

glucoseGlycogen stores are essential for athletic performance, because they serve as an energy reservoir when blood glucose levels are decreased due to high intensity exercise or inadequate carbohydrate intake. Glycogen stores become depleted as the intensity and duration of the exercise increases. It is imperative for the athlete, whether a sprinter or endurance athlete, to restore glycogen by consuming carbohydrate on a regular basis.

A well-nourished adult can store approximately 500 grams or 2000 kcal of carbohydrates. Of this, approximately 400 grams are stored as muscle glycogen, 90-110 grams as liver glycogen, and 25 grams circulate in the blood as glucose. When the body needs more glucose than is available in bloodstream to support energy demands, glycogen stores are used to raise blood glucose levels. However, it is important to note that the glycogen stored in muscle is used directly by that muscle during exercise, it cannot borrow glycogen from other resting muscles.

Glycogen stores are a readily available source of energy to support the demands of physical activity and exercise. How quickly glycogen stores might be depleted depends on the duration and intensity of the exercise. For low intensity exercise (distance running etc.) glycogen stores can last as long as 90 minutes. For prolonged high intensity exercise, glycogen stores can provide energy for approximately 20 minutes.

Because glycogen is readily used as fuel source during exercise, it is important to optimize glycogen stores before exercise and replenish them after exercise. Glycogen stores are optimized by consuming a high carbohydrate diet (~60% of total kcal from carbohydrates). Eat more fruit, vegetables, and grains to include more carbohydrate in the diet. For diet recommendations see Pre/During/Post game meal.

The diagram below shows the impact of diet on muscle glycogen content throughout 3 days with 2 hour training bouts (indicated by the dotted lines) daily. Athletes were divided into two groups, one receiving a low carbohydrate diet and the other a high carbohydrate diet. With the high carbohydrate diet (red line), glycogen levels were almost completely replenished after each training bout. In contrast, the low carbohydrate diet (black line) did not replenish the glycogen and subsequent bouts of training decreased glycogen stores progressively. Athletes on the low carbohydrate diet most likely had very little energy in the final 2-hour exercise bout.

effects of diet on muscle glycogen

Carbohydrates in the Diet

orange juiceCarbohydrates serve as a primary energy source in the diet and provide 4 calories per gram of carbohydrate. Carbohydrates exists in many forms, which can be divided into primarily two groups; simple and complex carbohydrates.

Simple carbohydrates are also known as simple sugars and have a smaller structure.

  • Some common simple carbohydrates include sucrose (sugar found in candy, soda, juice etc.), and lactose (sugar found in milk)
  • Simple carbohydrates are readily broken down and absorbed in the intestine into the blood stream. This rapid increase in blood glucose leads to a rapid release of insulin that is needed to help transport the sugar into the cells.
  • Energy is rapidly produced but only lasts for a short period of time.

Complex carbohydrates are also known as starch and fiber and have a larger structure.

  • Some common foods containing complex carbohydrate include bread, pasta, and whole grains.
  • Complex carbohydrates take a longer time to digest and absorb. This creates a slow and steady increase in blood glucose and a slow and steady increase in insulin levels.
  • Energy is slowly produced, not as quickly as simple carbohydrates; however, the energy lasts for a longer period of time.
  • Fiber is a complex carbohydrate, and can be classified as soluble or insoluble. Soluble fiber can be broken down and provides energy. Soluble fiber is found in many fruits and vegetables including green leafy vegetables, celery, carrots, apples, pears, among others. Insoluble fiber cannot be digested in our digestive system, and does not provide energy. Insoluble fiber is commonly found in whole grain cereal, bread, and rice. Insoluble fiber has an important role in health by promoting gut motility and satiety.

grainsThe most important carbohydrate for an athlete is the complex carbohydrate, starch. Starch is the major energy source in the diet and is broken down into glucose in the body that can be stored in the liver and in the muscles as glycogen. Glycogen can then be used to maintain blood sugar at a constant level for optimal energy supply. Common starches include bread, cereal, pasta, and other various grains, but also corn and peas. Make Half your Grains Whole Grains in order to consume more complex carbohydrates.

For more information, review the carbohydrate content of foods chart.

Glycemic Index

The body uses glucose for energy and blood glucose levels reflect the readily available energy supply for exercise. If glucose levels drop due to depleting glycogen stores or inadequate carbohydrate consumption, the athlete is said to "hit the wall". The rate at which the glucose levels drop depends in part on the type of carbohydrate consumed before, during, and after exercise.

Different foods increase the blood glucose levels to different degrees. The degree of increase in blood glucose levels after consuming a food containing carbohydrate is called its Glycemic Index (GI). There are three categories of GI, high, moderate, and low; the higher the GI the faster the glucose is absorbed, used, and depleted in the body.

There are many factors that can influence the GI of a food. For example,

  • the structure of the carbohydrate
  • the size of the food particle
  • the degree of thermal processing to the carbohydrate (i.e. flour has higher GI than the whole wheat kernels due to smaller particles and higher susceptibility to heat, etc.),
  • the contents and timing of the previous meal
  • consumption of other nutrients (such as fat, fiber, or protein) with the carbohydrate

In general, the more refined the carbohydrate in the food, the higher the GI. Examples of refined food include, processed white flour foods, crackers, rice, noodles, many ready-to-eat cereals, etc. In contrast, high fiber, high protein and high fat containing foods have low GIs.

Complex carbohydrates with low to moderate GI should be consumed before exercise to slowly raise and maintain blood glucose levels during the exercise. In contrast, carbohydrates with high GI can be consumed during and immediately after exercise to maintain blood glucose levels or quickly restore the glycogen stores.

CAUTION: Consumption of high GI foods should be limited immediately before exercise to avoid hypoglycemia. Hypoglycemia (low blood glucose) occurs when high insulin levels are released due to high blood glucose levels, which leads to a rapid absorption of glucose into the cells and a rapid reduction in blood glucose levels.

For more information, review the glycemic indexes of common foods. To learn more about the glycemic index visit the Gatorade® Sports Science Institute.

Dietary Reference Intake (DRI)

The minimum recommended intake of carbohydrates necessary for survival is 130 grams or 520 kcal per day. However, this level is recommended only to support the central nervous system, red blood cell production, and tissues dependent on glucose; it does not support any physical activity.

A minimum intake of carbohydrate is about 250 g or 1000 kcal per day for athletes is recommended. Carbohydrate recommendations vary with the sport as well as individual metabolic needs. Most athletes should consume 6-10 grams of carbohydrate per kilogram of body weight per day (3-5 gm/lb). Athletes participating in heavy exercise, specifically endurance athletes (bikers, marathon runners, triathlon athletes etc.), may need to increase their carbohydrate consumption up to 70% of the daily caloric intake.

How Much Carbohydrate to Eat and When for Exercise

estimate your carbohydrate needsThe amount, timing, and kind of carbohydrate consumed can determine the physical performance of an athlete. Although specific sports have different energy needs, carbohydrate is the most important source of energy to support the demand of all physical activity. Estimate your carbohydrate needs with our calculator.

A continual supply of carbohydrate in the diet each and every day diet is necessary to ensure an adequate supply of energy to support physical activity. For example, one carbohydrate rich meal before exercise will not greatly impact the performance if all other meals eaten in the days and weeks before are inadequate in carbohydrates.

Before exercise: Before exercise it is important to consume carbohydrates to optimize glycogen stores.

2 to 4 hours before exercise

  • Consume a meal rich in carbohydrates 2 - 4 hours before exercise. This time is needed to allow for digestion and prevent heartburn.
  • Consume about 400 - 800 kcal, of which 250 - 500 kcal (65 - 125 grams) are carbohydrate. This provides energy to prevent hunger during the physical activity without feeling overly full.
  • Choose foods high in carbohydrate food and low in protein and fat. For example, a whole grain bagel with tomato slices and low-fat cheese, one cup of banana slices in vanilla yogurt, a small portion of spaghetti and meat sauce, or an energy bar high in carbohydrate plus a fruit are combinations of high carbohydrate and low protein/fat foods.

2 to 3 hours before exercise

  • Liquid meals are recommended if the athlete has a sensitive stomach and solid foods tend to be poorly tolerated. Liquid meals are calorie rich drinks that include carbohydrate, protein and fat. They can be found in many grocery stores, pharmacies and convenience stores.
  • Some athletes may tolerate a light meal, such as a sandwich (low in protein and fat), salad, or soup. Others prefer a light snack including granola bars, fruit, smoothie etc.

1 to 2 hours before exercise

  • At this time the key is to prevent hunger and indigestion before exercise. To prevent hunger, consume a carbohydrate replacement beverage, a low-fat smoothie, or fruit and a few low fat crackers.

Less than 1 hour before exercise

  • If only 1 hour remains before exercise only water and fluid replacement beverages are recommended such as Powerade┬«, Gatorade┬«, etc.
  • To avoid potential hypoglycemic effects (sudden drop in blood glucose levels), minimize high GI foods, such as candy, pop, concentrated fruit juices, and simple sugar containing foods 30 minutes before exercise.

5 to 10 minutes before exercise

  • If hunger sets in a few minutes before exercise a simple carbohydrate COULD be consumed if the duration of the exercise is short.
  • If the duration of the exercise is long, simple sugars should be avoided, instead a sports drink might be suitable.

During exercise:
For exercise lasting longer than 90 minutes, consuming carbohydrate 30 minutes into the workout will allow for longer and more intense exercise. The goal is to consume 15-30 g of carbohydrates every half-hour depending on type of exercise and intensity. Do not wait until you get tired to start consuming carbohydrate.

Recommended sources of carbohydrate to eat or drink during exercise include:

  • fluid replacement drinks
  • energy gels
  • other sources of easily digested carbohydrate such as fruits
  • energy bars with <5g of fiber, <4g of fat, and <5g of protein Note: some energy bars contain over 200 kcal and are high in protein (>20% of DV), which are digested slowly, thus are not ideal for consumption DURING exercise.

carbs on blood glucoseThe figure on the left shows the effect of consuming carbohydrate on blood glucose levels during prolonged aerobic exercise. Consuming carbohydrates during exercise increases plasma glucose levels and provides an available source of energy for the exercising muscle.

After exercise:
Consuming carbohydrate within the first 30 minutes after exercise optimizes replenishment of glycogen stores. Although, many athletes have difficulty eating a meal this soon after a workout, a snack of 25-100 g of high to moderate glycemic carbohydrate-rich foods is sufficient. Consider a fluid replacement drink, 16 oz of milk, 8 oz of fruit juice, or a smoothie. Ultimately, the goal is to consume .5-.7 gm of carbohydrate per ounce of body weight within 30 minutes after exercise and at two-hour intervals up to six hours post-exercise.

Eating some protein in addition to carbohydrate within 30 minutes of exercise will provide amino acids for building, maintenance, and repair of muscle. Research shows just 7-10 g of protein along with the carbohydrate is enough to start muscle synthesis and repair. Consuming 20 g of protein is the maximum amount needed to stimulate synthesis and repair of muscle.

To find out if you are meeting your carbohydrate need, calculate your recommended intake per day. Although the Dietary Reference Intake for adults is 130 grams of carbohydrates, athletes have higher energy demand. The recommended carbohydrate intake for athletes is 50-60% of total calories or 3-5 grams of carbohydrates per lb of body weight.

Female athletes typically fall at the lower end of the range and male athletes at the higher end of the range.

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