From The University of New Mexico
What Is Nutrient Timing?
Nutrient timing is the application of knowing when to eat and what to eat before, during and after exercise.It is designed to help athletes, recreational competitors, and exercise enthusiasts achieve their most advantageous exercise performance and recovery. There are three distinct phases in the nutrient timing system that are based on muscle, its nutritional requirements, and its recovery demands for best strength and endurance results. But first, a basic review of the hormones of exercise is warranted.
The Hormonal Responses To Exercise
Within the body are numerous catabolic (breaking down) and anabolic (building up) hormones that are stimulated by exercise.Catabolic hormones aid in the disassembly of nutrients for energy production or cells needs. The main catabolic hormones of exercise are epinephrine, norepinephrine, cortisol and glucagon. Anabolic hormones support muscle hypertrophy (growth), tissue repair, inflammation control, and facilitate the regulation of carbohydrate, fat and protein metabolism. The anabolic hormones of interest during exercise are insulin, testosterone, IGF-I and growth hormone.
The Catabolic Hormones
During aerobic exercise, levels of epinephrine and norepinephrine are elevated to prepare (or mobilize) the cells for the breakdown of glycogen (converting it to glucose for fuel) in the liver and muscle. These hormones also increase the heart rate, blood pressure, heart contractility, blood redistribution to muscle, and respiration rate to meet the physiological needs of the continuous dynamic exercise.
Cortisol is largely responsible for the breakdown of carbohydrate and fat for energy during exercise. It is a very important catabolic hormone that is activated when low blood glucose levels are present, such as during exhaustive exercise. If the body is low in glucose and glycogen, cortisol will send amino acids to the liver to make new glucose, referred to as gluconeogeneses. Thus, in exercise, when carbohydrate sources are dwindling, cortisol takes the building blocks of proteins (amino acids) and uses them for new glucose synthesis. Glucagon stimulates fat breakdown and also helps to raise blood glucose levels by increasing the release of glucose and the rate of gluconeogeneses (Ivy & Portman, 2004).
The Anabolic Hormones
One widely known anabolic hormone is insulin. Insulin sensitivity is increased during aerobic and resistance exercise, which literally means there is an enhanced glucose uptake for muscle contraction. It also accelerates the transport of amino acids into muscle and stimulates protein synthesis in muscles (Levenhagen et al., 2001). However, during sustained aerobic exercise insulin levels in the blood decrease slightly because epinephrine and norepinephrine inhibit the release of insulin from the pancreas.
Another important anabolic hormone is testosterone. Testosterone is a powerful hormone for protein synthesis and muscle hypertrophy. The amount of testosterone within the body increases with exercise (Ivy & Portman, 2004). Growth hormone is an anabolic hormone that promotes bone and cartilage growth. It is also responsible for stimulating IGF-I, a hormone responsible for the development of muscle cells from myoblasts (immature muscle cells) into myotubes (growing muscles cells) and then into mature muscle fibers. High levels of IGF-I are needed in order to promote muscle hypertrophy. Growth hormone also increases protein synthesis (Volek, 2004).
The Three Nutrient Timing Phases
The nutrient timing system is split into three distinct phases:
1) Energy Phase (just before and during workout)
2) Anabolic Phase (post 45 minutes of workout)
3) Growth Phase (remainder of the day)
The Energy Phase
Muscle glycogen is the primary fuel (followed by fat) used by the body during exercise. Low muscle glycogen stores result in muscle fatigue and the body's inability to complete high intensity exercise (Levenhagen et al., 2001). The depletion of muscle glycogen is also a major contributing factor in acute muscle weakness and reduced force production (Haff et al., 2000). Both aerobic and anaerobic exercise decrease glycogen stores, so the need for carbohydrates is high for all types of exercise during this energy phase.
Several hormonal and physiological responses occur during the energy phase. Cortisol levels are increasing, insulin levels are slightly decreasing, and muscle glycogen levels are becoming depleted, making the energy phase catabolic (Ivy & Portman, 2004). Therefore, the goals with the energy phase are to increase nutrient (primarily carbohydrate and some protein) delivery to muscles, spare glycogen and protein loss, limit immune system suppression, minimize muscle damage and prepare nutritionally for a faster recovery (Ivy & Portman, 2004).
Prior to aerobic exercise, protein intake with carbohydrate supplementation has been shown to stimulate protein synthesis post-exercise (Volek et al., 2006). The combined intake of carbohydrate and protein (pre-exercise) also aids in the rate of muscle recovery after exercise (Ivy & Portman, 2004).
Carbohydrate supplementation prior to resistance training can increase the body's capacity to perform more sets, repetitions and prolong a resistance training workout (Haff et al., 2000). It will help to maintain blood glucose levels, sustain immune function, and aid in the suppressing of cortisol (Ivy & Portman, 2004).
The Anabolic Phase: The 45-Minute Optimal Window
The anabolic phase is a critical phase occurring within 45 minutes post-exercise. It is during this time that muscle cells are particularly sensitive to insulin, making it necessary to ingest the proper nutrients in order to make gains in muscle endurance and strength. If the proper nutrients are ingested 2 - 4 hours post-exercise they will not have the same effect. It is also during this time in which the anabolic hormones begin working to repair the muscle and decrease its inflammation.
Immediate ingestion of carbohydrate is important because insulin sensitivity causes the muscle cell membranes to be more permeable to glucose within 45 minutes post-exercise. This results in faster rates of glycogen storage and provides the body with enough glucose to initiate the recovery process (Burke et al., 2003). Muscle glycogen stores are replenished the fastest within the first hour after exercise. Consuming carbohydrate within an hour after exercise also helps to increase protein synthesis (Gibala, 2000).
The Growth Phase
The growth phase consists of the 18 - 20 hours post-exercise when muscle repair, growth and strength occur. According to authors Ivy and Portman, the goals of this phase are to maintain insulin sensitivity in order to continue to replenish glycogen stores and to maintain the anabolic state. Consuming a protein and carbohydrate meal within 1 - 3 hours after resistance training has a positive stimulating effect on protein synthesis (Volek, 2004).
Carbohydrate meals with moderate to high glycemic indexes are more favorable to enhance post-exercise fueling. Higher levels of glycogen storage (post-exercise) are found in individuals who have eaten high glycemic foods when compared to those that have eaten low glycemic foods (Burke et al., 2003).
