By the end of this lecture you will understand the importance of CHO to physical performance and be able to make practical recommendations for the type and amount of CHO needed for athletes in different sports.
You may already know or can guess that CHO is the primary energy source used during moderate to intense physical activity. And as you learned in metabolism, CHO is the only fuel source that can be used in the absence of oxygen. Although CHO is stored in the muscle as glycogen, stores are limited and used up quickly, requiring daily replenishment.
CHO plays several important roles in exercise including enhancing recovery, preventing fatigue due to depletion, and sparing muscle protein in the absence of adequate calories and prolonged exercise.
CHO is mainly absorbed in the small intestineThe concentration affects both gastric emptying & H2O absorptionHigh concentrations can slow emptying & draw H2O into the intestine leading to GI distress, namely diarrhea, during exercise
CHO leaves the blood stream to be distributed to a variety of body sites for use as energy or stored for later. Muscle will store about 300-400 g, while liver will store around 100 g. These are the two main sources of glucose exercising muscles depend on. How much gets stored is dependent on dietary intake.
In addition to dietary intake level of training can also influence CHO storage. Trained athletes will store more in the muscles being used for training than untrained athletes.
CHO’s main function is to supply energy. Two other key functions that we are typically interested in as sports RDs is the ability of CHO to spare protein and delay muscle and central nervous system fatigue.
Muscle glycogen can only be used by muscles. Liver glycogen can be used for energy or to supply glucose to the blood stream. When these 3 endogenous sources start to become depleted, glucose must come from somewhere else: fatty acid breakdown and amino acids from muscle tissue.
The amount of CHO needed by an athlete depends on multiple factors as indicated on the slide. Recommendations should be individualized for each athlete.
High endurance athletes and athletes doing strenuous workouts daily or two a day practices may need more that what is typically considered in the acceptable macronutrient distribution range for adequate recovery and energy.Recommendations can be calculated based on actual body weight and type of training as shown.
It is important to consider both calculations in making CHO recommendations. When basing CHO needs on g/kg, CHO will make up a greater proportion of the diet within the 2500 kcal recommendation shown in the example. You must use your clinical judgment to determine if this leaves adequate room for protein and fat for a balanced diet within calorie requirements and depending on the activity of the individual. The lower level based on 55% of calories may not be adequate to replenish glycogen stores depending on the sport, duration, and frequency of training as it only provides 4 g/kg versus 6 g/kg.This also allows you to design a plan with a range to allow more flexibility in the diet and still assure adequate CHO intake.The higher the intensity, duration, and frequency of training, the more CHO needed.Athletes in the Tour de France may be consuming up to 80% of their calories from CHO.
In regards to O2 consumption, CHO produces energy more efficiently than fat and protein (p. 81).Low intake of CHO is associated with more effort, poor mental focus, low energy, muscle fatigue. Trained athletes are better able to use fat for an energy source at higher intensities, thus sparing some CHO for late stages of endurance exercise.
CHO is heavily relied upon in the early stages of exercise.
The liver holds enough glycogen to maintain blood glucose levels for about 90 minutes of exercise. Then blood glucose levels start to drop below normal resting levels and fatigue sets in. If CHO is ingested during exercise, blood glucose levels can be maintained and prevent early fatigue.
Central nervous system fatigue is believed to occur with prolonged exercise. The theory is that the breakdown of fat as an energy source results in increased fatty acids floating around in the blood stream which competes with tryptophan for binding with albumin. This leaves more free tryptophan to cross the blood brain barrier that alters the levels of serotonin and dopamine, resulting in a feeling of fatigue. Consuming CHO during prolonged exercise prevents the breakdown of fat and boosts glucose levels enough to delay central nervous system fatigue.
Hopefully by now, you are starting to understand why CHO is such an efficient fuel. CHO can be stored in the muscle which makes it readily available as compared to fat which requires breakdown of stored energy in remote sites in to fatty acid chains and glycerol, then conversion of glycerol to glucose in the liver. Then release into blood stream and transportation to muscles.When oxygen is available, CHO uses less of it to produce energy than fat or protein.
Liquid meals can be a convenient and beneficial way to get calories and carbohydrate at times when eating solid food may not be feasible or may make them uncomfortable. Although I like to encourage real food most of the time, there is a need for liquid meals on occasion. I personally like a glass of skim milk as a pre-exercise snack. Not only does it provide a bit of CHO, it also aids in getting adequate fluid and is about 5% CHO solution.
In considering liquids during training, look for something that is within the 6-8% CHO concentration range and has sodium. One drawback of relying only on liquids only is that you will need to drink a lot to get enough CHO. I can tell you from personal experience that this get uncomfortable to say the least.
Adding easily digestible solids to the mix can help get adequate CHO with less fluid. The drawback here then becomes slower gastric emptying. This is fine when riding a bike but does not work so well for runners. Another thing you now may be questioning, is if you cut back on fluid to get CHO from solids, then you likely will not meet your fluid needs. Some experts argue that it is better to get the needed calories and carbohydrate for prolonged exercise, and worry about fully hydrating after the event.
The post exercise snack/meal should include easy to digest CHO either in the form of liquids, solids, or a combination of both. Something like a fruit smoothie or milk shake is a good choice for replenishing both CHO and fluid losses.
The post-training snack should provide at least 1.2-1.5 g of CHO/kg for those who have trained for 90 minutes or more. Follow this with a high CHO meal before the next training session.
Lowglycemic foods appear to be beneficial in enhancing endurance at least in cyclists. This is not particularly surprising since many low GI foods are also complex CHO foods which have been recommended and researched for years. Also, high GI foods which are typically simple CHO foods provide for a quick rise in blood glucose along with insulin to move it into muscle cells.
There are several limitations to classifying foods by their GI mainly because there are many variables that affect the GI of a food. It remains to be seen whether or not this is beneficial in providing sports nutrition recommendations. For now I will stick with finding out what the athlete can or cannot tolerate despite the GI.
Moving on, CHO loading is a strategy for supersaturating muscles with glycogen in preparation for an endurance type event. The better trained the athlete, the better able they are to store muscle glycogen.
Guidelines for CHO loading are to taper training starting 7 days prior to competition while increasing CHO intake. As training tapers off, less CHO is used up and the increased intake fills stores to the brim. Again, this only applies to endurance, intermittent, or tournament style competition. Sprinters or body builders will not benefit.
In summary, CHO is a pretty important fuel source for athletes and highly active individuals. It is particularly important for delaying fatigue, allowing the athlete to workout harder for a longer period of time. Recommendations for amount should be based on actual body weight. When designing a nutrition plan be sure to take into consideration, timing, type, and amount before, during, and after.
CH2O H OH C OHC H C OH HOH H C C H OH CARBOHYDRATES: FUELING THE FIRE Lona Sandon, M.Ed., R.D., L.D. Assistant Professor
OBJECTIVES Discuss the importance of CHO as a fuel source during exercise Explain the functions of CHO Calculate CHO needs for athletes Explain how CHO is stored & utilized during exercise Provide recommendations for type & quantity of foods & beverages before, during, & after exercise
IMPORTANCE OF CHO IN EXERCISE 10 energy source during moderate to intense exercise (65% VO2max) – typical training level Storesof CHOs in body are limited Adequate stores key to optimal performance Must be replenished daily Only macronutrient that can produce energy under anaerobic conditions Sole source of energy during very intense exercise
IMPORTANCE OF CHO IN EXERCISE Essential for adequate exercise recovery Prevents fatigue during exercise Prevent cumulative depletion of muscle glycogen from successive training sessions May prevent damaging effects of calorie restriction on muscles
ABSORPTION OF CHO Mainly absorbed in small intestine Too high or too low concentrations may slow gastric emptying Glucose empties from stomach & absorbs more rapidly than fructose Stored in muscle and liver as glycogen GLUT-4 receptors take up Glu in muscle & liver Uptake of glucose is stimulated by insulin & exercise
DISTRIBUTION OF CHOStored inmuscle (300 – Blood Glucose (25g) Stored in475 g) Liver (100 - 125 g) Excreted Converted Used by in urine to fat nervous system
DISTRIBUTION OF CHO Effected by: Dietary intake Level of training Trained athletes store more CHO than untrained athletes Higher dietary intakes = higher stores
FUNCTIONS OF CHO Main energy source during exercise >65% VO2max Only energy source for high intensity exercise Provides 4 kcal/g Needed for fat metabolism Provides O2 needed for fat breakdown Spares muscle tissue Prevents breakdown of amino acids for glucose production (gluconeogenesis) Primary energy source for nervous system Delays central nervous system fatigue
ENDOGENOUS SOURCES OF CHO1. Blood Glu2. Liver Gly3. Muscle Gly When muscle & liver Gly are low during exercise: Convert glycerol from fatty acid breakdown for energy Convert amino acids from muscle tissue for energy
HOW MUCH CHO IS NEEDED? Depends on: Other factors to Body weight consider: Total energy needs Environmental conditions Sport Personal preference Intermittent sports, Training/competition endurance, sprinting, schedule strength training Quantity of physical activity 3 d/wk or 6 d/wk
CHO NEEDS OF ATHLETES 45-65% of daily calories – acceptable macronutrient distribution range (AMDR) May need as high as 75% CHO kcal in high endurance sports Recommendations based on body weight: 3-5 g CHO/kg sedentary 5-7 g CHO/kg general & strength training 7-10 g CHO/kg endurance athletes >11 g CHO/kg ultra endurance athletes
COMPARE: CHO NEED CALCULATIONSCHO AS % OF KCAL CHO BASED ON G/KG 70 kg athlete 70 kg athlete, 2500 kcal 2500 kcal, 55% from CHO 6 g CHO/kg/d x 70 kg = 2500 x 55% CHO = 1375 420 g CHO/d kcal from CH0 67% of 2500 kcal 1375 CHO kcal/4 g (~6g/kg) CHO/kcal = 344 g of CHO (~4 g/kg) 344g vs. 420g – Both within AMDR, must consider other factors, heavier training days may go with higher range & low range for rest days, will it allow for adequate PRO & fat needs within kcal
IMPORTANCE OF CHO IN EXERCISE Stored CHO & maintenance of blood glucose can delay fatigue in prolonged exercise A high CHO diet is associated with greater time to exhaustion vs. high fat diet
CHO UTILIZATION IN EXERCISE Used in both aerobic & anaerobic conditions Major source of energy at > 60-75% VO2max Trained athletes use less CHO at higher intensities than untrained athletes
CHO UTILIZATION IN EXERCISE Highest utilization in early stages of exercise – mainly muscle glycogen As training progresses must use blood glucose & make glucose
CHO AND PERFORMANCE Depleted CHO leads to: Poor mental focus Muscle weakness In-effective training High CHO diets increase stores in both the liver & muscle High CHO diets lead to longer time to exhaustion Exercise longer Ingestion of CHO during activity delays fatigue
CENTRAL NERVOUS SYSTEM FATIGUE Occurs when: Muscles compete with brain for blood glucose Free fatty acids compete with Tryptophan for binding with albumin FFA increased due to breakdown for energy source Tryptophan crosses blood-brain barrier which impacts serotonin & dopamine levels CHO feeding during training prevents CNS fatigue
WHY IS CHO AN EFFICIENT FUEL? Stored in muscle making CHO easily available – no break down needed Quick energy w/o the need for O2 When O2 is available, uses less O2 than fat or protein for energy conversion More efficient Fat oxidation is not fast enough to meet ATP demands
WHAT TYPE OF CHO SHOULD ATHLETESEAT? Whole grains Vegetables and fruits Dairy and dairy alternative products Beans, lentils, nuts, seeds, and soy products Limited sweets
CONSIDERATIONS FOR CHOOSING CHO Type: Solid vs. liquid Complex vs. simple Low GI vs. high GI Quantity Consider rate of gastric emptying & absorption Timing Before, during, or after training
4-24 HOURS PRIOR TOTRAINING/COMPETITION Purpose of CHO: fill muscle & liver glycogen stores Balanced moderate to high-CHO meals/snacks Choose familiar foods Test tolerance of food during training, not on competition day
0 – 4 HOURS PRIOR TOTRAINING/COMPETITION Purpose of CHO: prevent hunger during training, “top off” liver & blood glucose Quantity: 1 to 4.5 grams CHO/kg depending on time until training Lower end of range closer to training period (1g/kg 1 hr before)
BENEFITS OF LIQUID CHO MEALS Easily digestible, short gastric emptying time Good for before morning training & as post training snack High CHO Low stool residue = minimal weight gain after meal Convenient for long competition days Provide energy & fluid Serve as a nutritional supplement
CHO DURING TRAINING & COMPETITION Purpose: to delay fatigue, maintain blood glucose Considerations: Nature of the sport – is it feasible to eat solids, liquids, bars, gels, etc? Availability of food/beverage during training or competition Individual tolerance Convenience/Portability Food safety
CHO DURING TRAINING/COMPETITION Quantity: 30 - 60 grams per hour during prolonged exercise Timing of intake Eat early in the training session Continue at regular intervals 15 g every 15 minutes for exercise lasting more than 90 minutes Do not wait until hunger sets in
CHO DURING TRAINING & COMPETITION Type: Easily digestible Orange slices, banana, crackers, pretzels, cookies Rapid absorption Glucose, glucose polymers, starches Sport nutrition products Gels Bars Sports drinksNote: Fructose has slower absorption rate,must be converted to glucose in the liver, &draws water into the gut leading to risk ofdiarrhea, cramping, & bloating when taken inhigh amounts
CHO DURING TRAINING & COMPETITION Liquid or Solid? Athlete’s preference. Liquids – i.e. sports drinks Formulated for rapid absorption – 6-8% CHO solution (~15g/cup); sodium Encourage fluid intake – sodium & sweet tastes encourages fluid intake Drawback – high fluid volume to meet CHO needs, bloating
CHO DURING TRAINING & COMPETITION Solids – i.e. energy/sports bars, gels, gummies, snack crackers, cookies More calories & satiating, high CHO Easy to carry Less flavor fatigue Salty snacks encourage fluid intake Drawback – slow gastric emptying
CHO AFTER TRAINING/COMPETITION Purpose: replenish muscle & liver glycogen stores for next training session Timing: eat within 30 – 60 min of finishing exercise Full glycogen replenishment may take 20+ hours Most important for athletes who train daily or do 2-a-day workouts There is a higher rate of muscle glycogen synthesis within 2-hours of exercise Greater blood flow to muscles, muscle cell more sensitive to insulin
CHO AFTER TRAINING/COMPETITION Type: Easily digested & absorbed CHO rich Glucose & sucrose more effective than fructose High glycemic index foods Liquids or solids depending on athletes preference Liquids (i.e. chocolate milk) help replace fluids & are good for athletes with a blunted appetite post training
CHO AFTER TRAINING/COMPETITION Quantity: 1.2-1.5 g CHO/kg immediately & at 2 hrs post-exercise for those training > 90min/d Those exercising less than 90 min with rest days do not need a special post training snack; may lead to weight gain if exceeds kcal needs
DOES GI OR GL MAKE A DIFFERENCE?Glycemic Index (GI) Glycemic Load (GL) Glycemic effect of Considers total CHO 50 g CHO of an consumed isolated food (GI x CHO content Measure rise in per serving) 100 blood glucose after 2 hours
ATHLETES & THE GLYCEMIC INDEX Optimize CHO intake before, during, & after exercise Pre-exercise meal DeMarco et al found – low GI foods were associated with longer duration & higher glucose levels in cyclists During exercise Moderate to high GI foods provide rapid release of CHO for immediate energy Post-exercise meal Medium to high GI food for rapid glycogen replacement w/in 2 hours of exercise
LIMITATIONS OF GI Highly variable among CHO containing foods Affected by fiber, protein, & fat in foods – lower GI effect due to decreased rate of gastric emptying Liquids usually higher, absorbed more quickly Timing of intake to last meal alters GI Foods eaten together changes GI of individual food Trained may respond differently than untrained
CHO LOADING Purpose: supersaturate muscles with glycogen Trained athletes store more glycogen Higher glycogen storage is specific to the muscles trained Must be endurance trained for effectiveness Benefits: endurance trained athletes Athletes competing in intermittent tournament sports (volley ball, tennis, swim meets) Guidelines – decrease training while increasing CHO intake 7 days prior to event
SUMMARY CHO is essential for performance Glycogen is the storage form of CHO in the body CHO key energy source in high intensity exercise Lack of CHO leads to impaired mental focus & fatigue Timing, type, & amount of CHO before, during or after exercise is important CHO intake should be customized to the athletes tolerance & preference For maximal muscle glycogen synthesis, consume CHO within 2 hrs of exercise
REFERENCESFink HH, Burgoon LA, Mikesky AE, eds. Practical Applications in Sports Nutrition. Sudbury, MA: Jones and Bartlett; 2006Coleman EJ. Carbohydrate and exercise. In M. Dunford, ed., Sports Nutrition: A Practice Manual for Professionals, 4th ed. Chicago, IL: American Dietetic Association; 2006.