Why Is Nutrient Timing Important?
Researchers have observed that the timed ingestion of carbohydrates, proteins, and fats may significantly influence adaptations to exercise, enhance recovery, prevents injury and supports rehabilitation. It should be noted that individual differences should always be accounted for, and that there is no recommendation that would apply to every individual. The following recommendations are appropriate for researchers, practitioners, coaches and athletes who may use nutrient timing as a means to achieve optimum health and performance goals. The scientific literature associated with nutrient timing is an extremely popular, and thus ever-changing, area of research. At this point in time, research suggests that dietary focus should center on adequate availability and delivery of carbohydrates (CHO), protein (PRO) and water (H2O). However, consuming small amounts of fat is not harmful, and may help to control glycemic responses during exercise. The recommendations are divided into three primary sections: pre-exercise, during exercise and post-exercise
Pre-exercise: 3 – 4 hours before exercise
Carbohydrate intake requirements, and thus glycogen stores, will vary based on the nutritional status, exercise intensity and training level of the athlete. Endogenous glycogen, stored within skeletal muscles, during moderate to high intensity levels (65 – 85% VO2max) of exercise may only last from 90 min (beginner) to 3 hours (advanced athlete). Exercise intensity is directly related to glycogen level stores; that is, exercise intensity, rate and work output will decrease as glycogen levels diminish. Depletion of glycogen stores is correlated with increased levels of muscle tissue breakdown and suppression of the immune system. Some evidence suggests that maximal endogenous glycogen stores are developed by following a high-glycemic, high-CHO diet (~8 – 10 g/kg/d or 600 – 1000 grams). It is not advised to consume high glycemic index foods 30-60 minutes prior to high intensity exercise due to a rapid blood glucose spike, which results in the stimulation of insulin allowing glucose into the cells which ultimately results in a drop in blood glucose; a phenomena known as reactive hypoglycemia that results is a sudden onset of fatigue. While the most advantageous nutrient (CHO + PRO) content of a pre-exercise meal is dependent upon a number of factors including exercise duration and fitness level, general guidelines recommend ingestion of:
CHO: 1 - 2 grams per kilogram of body weight
PRO: 0.15 - 0.25 grams per kilogram of body weight H2O: 500 mL (17 ounces) 2 hours before exercise 16 ounces 30 minutes before exercise 8 - 16 ounces 15 minutes before exercise For a 175-pound male this is a meal consisting of 80 -160 grams of CHO and 11-19 grams of PRO For a 130-pound female, this meals consists of 60 - 120 grams of CHO and 9-15 grams of PRO |
Pre-exercise consumption of essential amino acids or PRO alone increases muscle protein synthesis. However, ingesting PRO in combination with CHO before exercise has been shown to produce significantly greater levels of muscle protein synthesis, strength, and a favorable impact on body composition, when compared to CHO alone.
During Exercise
Endurance performance (>60 min) is largely determined by the availability of CHO during exercise and intramuscular glycogen levels. As glycogen levels become depleted upon beginning exercise, the availability of CHO becomes more important. CHO is required to maintain blood glucose and muscle glycogen levels. Adding PRO to CHO, at a ratio of CHO: PRO as 3:1 or 4:1 has been observed to increase exercise endurance performance, depending on the individual, exercise intensity and duration. Consuming CHO with or without PRO during resistance training (every 10 minutes) increases muscle glycogen stores, counterbalances muscle damage, and augments greater training adaptations after acute and prolonged periods. Researchers recommend:
CHO: 30 - 60 grams per hour of exercise
PRO: 10 - 20 grams per hour of exercise H2O: 4 - 8 ounces every 15 - 20 minutes This can be obtained by drinking 1 – 2 cups of a 6 – 8% CHO solution (8 – 16 fluid ounces) every 10 – 15 minutes. |
The rate at which the digestive system absorbs CHO is limited and can become saturated. The rate at which muscles "burn" CHO increases, from 1.0 gram of CHO/minute to 1.2-1.75 grams of CHO/minute, when mixing different varieties of CHO; a phenomena associated with improved time trial performance. However, consuming large amounts of fructose is not recommended due to the greater likelihood of gastrointestinal problems.
Post-exercise: 30-60 minutes
It is important to consume 1.5 grams of carbohydrates per kilogram of body weight 30-60 minutes after exercise to stimulate the muscles to re-synthesize glycogen levels. Consuming PRO, in addition to CHO, at a ratio of CHO: PRO as 3: 1 has been observed to stimulate glycogen re-synthesis to a greater extent. Ingesting essential amino acids (EAA), anywhere from immediately after through 3 hours after exercise, has been observed to stimulate strong increases in muscle PRO synthesis. The addition of CHO may increase PRO synthesis even further. Although, pre-exercise consumption may result in the best response of all. Evidence suggests that consuming CHO + PRO after prolonged resistance training has been shown to stimulate strength gains and reduce body fat percentage when compared to control, placebo, or CHO-only. Adding creatine, at 0.1 grams of creatine per kilogram per day, to a post exercise meal containing CHO + PRO may facilitate even greater adaptations to resistance training, when compared to not creatine at all. Based on this information, immediately following exercise, it is recommended to consume, at a minimum:
CHO: 50 - 60 grams
PRO: 20 grams EAA: 6 - 20 grams H2O: 24 ounces for every pound lost during exercise (within 6 hours after exercise) |
Fat Requirements
Fat is an energy source used during low-intensity exercise that lasts for long durations. Fat supplies a concentrated source of calories (9 calories per gram of fat), when compared to CHO or PRO (4 calories per gram of CHO and PRO). Diets low in fat may limit performance by inhibiting the storage of triglycerides within the muscle, resulting in earlier fatigue during exercise. Inadequate fat intake can decrease circulating levels of testosterone, therefore decreasing muscle mass. Excess fat intake can increase fat storage and may cause gastrointestinal discomfort before exercise (DeLee, Drez & Miller, 2003).
References
DeLee, J., Drez, D. and Miller, M. (2003). DeLee & Drez's Orthopaedic Sports Medicine. Philadelphia, PA: Saunders.
Kerksick, C., Harvey, T., Stout, J., Campbell, B., Wilborn, C., & Kreider, R. et al. (2008). International Society of Sports Nutrition position stand: Nutrient timing. Journal Of The International Society Of Sports Nutrition, 5(1), 17. http://dx.doi.org/10.1186/1550-2783-5-17
Kerksick, C., Harvey, T., Stout, J., Campbell, B., Wilborn, C., & Kreider, R. et al. (2008). International Society of Sports Nutrition position stand: Nutrient timing. Journal Of The International Society Of Sports Nutrition, 5(1), 17. http://dx.doi.org/10.1186/1550-2783-5-17