Sports Nutrition

Sports-Nutrition-1

Carbohydrate metabolism
Metabolic pathways
Carbohydrates as a critical fuel for athletes
Central Fatigue Theory
Carbohydrate needs
Protein needs
Hydration
Carbohydrate Loading
Training and events
Sourced from
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Carbohydrate metabolism

  • Carbohydrates include complex and simple sugars. Carbohydrates are a superior short-term fuel because they are simpler to metabolize than fats or those amino acid portions of proteins that are used for fuel. The most important carbohydrate is glucose
  • Glucose is the preferred fuel for creation of muscular energy (as ATP). The body has limited storage for carbohydrate and depletion of stores leads to mental and physical fatigue and loss of athletic performance (“blowing out” and “hitting the wall”)
  • Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy. Energy obtained from metabolism (e.g. oxidation of glucose) is usually stored temporarily within cells in the form of ATP
  • Simple sugars can be broken down directly in cells. More complex carbohydrates such as sucrose require specific enzymes that break the chain and release simple sugars. Starch is a polymer of glucose units and is broken down to glucose. The simpler the sugar, the easier it can be absorbed into the blood stream and the faster its effect in raising blood sugar
  • Excess blood sugar is regulated by insulin which transfers glucose into cells for storage or use. Exercise and the use of cellular energy sensitizes and allows cells to uptake glucose directly from the blood
  • Low blood sugar stimulates the pancreas to release glucagon and the breakdown of glycogen in the liver
  • Breakdown of liver and muscle glycogen is also stimulated by adrenalin, whilst cortisol breaks down muscle protein. Athletes should thus manage stress in order to preserve pre-race energy stores (as glycogen) and also muscle mass
  • Glycogen and blood glucose are easily depleted and athletes feel the effect of this as first as mental fatigue followed by muscular fatigue and athletic performance impairment
  • The strong affinity of most carbohydrates for water (hydrophilic) makes storage of large quantities of carbohydrates inefficient. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates

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Metabolic pathways

  • Glycogenesis – the conversion of excess glucose into glycogen as a cellular storage mechanism
  • Glycogen – storage form of glucose primarily found in either the muscles or the liver
  • Glycolysis – the aerobic or anaerobic metabolism of glycogen or glucose molecules to obtain ATP and pyruvic acid or lactic acid
  • Gluconeogenesis – generation of glucose molecules from non-glucose molecules such as amino acids and fat

The role of glycolysis for the different type of athletic training

  • Aerobic (with oxygen) glycolysis is the most efficient form of metabolism of glucose or glycogen into ATP. This is used for low intensity workouts
  • Anaerobic (without oxygen) glycolysis generates lactic acid that the body can tolerate only marginally before muscular ability and athletic performance is impaired (approximately 2 minutes). This is used for high intensity workouts
  • Lactic acid is converted back to pyruvic acid and acid once the oxygen debt has passed. This conversion is made more difficult with dehydration due to the volume of fluid needed for it to occur. “The solution to pollution is dilution”: hydration is essential during intense exercise

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Carbohydrates as a critical fuel for athletes and their role in fat burning at different intensities of workouts

  • All modes of exercise (high or low intensity) require carbohydrate in the metabolism of energy
  • Carbohydrate is the most efficient fuel source, which explains why muscular (and mental) fatigue occurs quickly during high-intensity workouts when blood glucose and glycogen stores are rapidly depleted
  • Increased reliance on carbohydrates is needed for
    • High-intensity activity, long-duration activity, exercise in hot and cold extremes, exercise at high altitude, activities performed by younger people compared to older
  • Decreased reliance on carbohydrates is needed for
    • Endurance training, good aerobic conditioning, temperature adaptation, different genders
  • Both carbohydrate and fat are burned during all types of exercise
  • Changes in the intensity of exercise affects the relative proportions of energy derived either from fat or carbohydrate
    • During low-intensity work-outs, fat is the preferred fuel and the volume of fat burned is low
    • During high-intensity workouts, carbohydrate is the preferred fuel but fat is still burned and the overall volume of fat burned is high

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 “Central Fatigue Theory”

  • Inadequate carbohydrate, glycogen and blood glucose lowers the rate of ADP to ATP and energy available to cells leading to fatigue
  • Buildup of ADP also leads to muscle fatigue
  • Increased tryptophan uptake by the brain causes relaxation, mental fatigue and feelings of muscle fatigue
    • When tryptophan crosses the Blood Brain Barrier (BBB) it stimulates the formation of 5-HT (serotonin). Increase in serotonin (5-HT) causes relaxation
    • BCAAs and tryptophan compete to use the same carrier to cross the Blood Brain Barrier (BBB). BCAAs are preferred. The following factors will allow tryptophan a better chance to cross the BBB.
      • When blood sugar is low, BCAAs (Branch Chain Amino Acids) are converted for use as energy, and as gluconeogenesis occurs the level of BCAAs in the blood lowers
      • Other factors that affect the level of BCAAs in the blood are the level of BCAAs or tryptophan in food eaten
        • Eating a meal high in carbohydrates (and thus low in BCAAs)
        • Eating foods high in tryptophan (eg turkey or milk)

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Carbohydrate needs with regard to timing and utilization to enhance training outcomes

  • Carbohydrates are needed in order to:
    • Provide cellular energy, optimize glycogen stores, allow for muscle recovery
    • Optimize source of energy during training and events, optimize blood sugar at all times and provide an ease of energy release when required
  • In order to meet these needs, athletes should
    • Eat a blend of carbohydrates including low-glycemic foods high in dietary fiber
    • Hydrate effectively as glycogen replenishment requires effective hydration
    • Choose foods that contain variety of nutrients beside carbohydrates as well as protein which aids in both glycogen recovery and muscle repair
  • Carbohydrate intake based upon intensity and duration of exercise with higher requirements for greater duration and intensity
  • Athletes should consume carbohydrates at frequent intervals within 4 hrs of exercise to ensure recovery
  • Athletes who have limited time between training or events (eg 8hrs) should consume high carbohydrate and nutrient dense foods
  • Athletes may consume simple carbohydrates both during and after exercise to sustain blood glucose and ensure optimal glycogen stores

Why fat is not a primary source of energy for the athlete

  • Fatty acids require both oxygen and carbohydrate for complete oxidation and conversion to energy thus they can only be metabolized aerobically
  • Omega 3 fatty acids act to enhance aerobic metabolic processes through the delivery of oxygen to cells as well as acting to reduce inflammation caused by muscular fatigue and over-exertion, thus speeding up recovery and reducing excess inflammation
  • Fat is a required energy source with an advantage of being compact and a disadvantage of being slowly absorbed
  • For fat to be most useful to an athlete, endurance training must occur, whereby the size and number of mitochondria and oxidative enzymes increase, this increasing athletic ability to use fat as an energy source. This is advantageous as it leads to carbohydrate stores being spared whilst fat is burned, leading to greater endurance
  • If fat replaces carbohydrate as an energy source, athletic performance is impaired
  • Some types of fat (eg Medium Chain Triglycerides) and fat structures (Glycerol) can be converted directly into energy in the liver. MCTs may enhance athletic performance as well as weight loss
  • For body fat to be metabolized most effectively and lead to weight loss, VO2 max should be 65% or more of maximum during the workout. A lower intensity workout burns a lower volume of fat

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Protein needs of an athlete

  • Protein is required for tissue repair as well as for energy and endurance athletes require as much protein as power athletes
  • Non-athletes require 0.8g of protein per kg body weight (or 60g for a 75kg / 165lb adult) per day. Athletes require more protein due to greater lean mass, greater need for tissue repair and as a form of fuel during activity (especially during endurance activities)
  • Athletes require 1.2 to 1.7g per kg = 120g (or 120g for a 75kg / 165lb adult) per day
  • Sufficient protein is required to avoid catabolic breakdown of muscles as well as to ensure anabolic requirements for muscle synthesis
  • When protein is used as fuel, more water is needed to excrete metabolic waste (nitrogen) leading to enhanced potential for dehydration. In addition, calcium is lost in the urine during high protein intake, leading to bone health issues. Thus carbohydrate intake should be optimized
  • Carbohydrate intake should accompany protein intake to ensure enhanced recovery via enhanced muscular protein uptake
  • Young athletes have an enhanced need for protein due to enhanced growth
  • Vegetarians should monitor their diet to ensure all essential aminos are present in the same meals so they can be presented to cells at the same time for effective protein synthesis (eg via combining grains with legumes)

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Hydration

  • Sodium concentration causes enlargement of blood volume due to body’s uptake of water into the blood to normalize sodium concentration (nb water follows sodium)
  • Exercise leads to increased Antidiuretic Hormone (ADH) and Aldosterone which act to conserve water and sodium
  • The better the state of hydration of athlete, the greater the sweat potential. As athlete becomes dehydrated, sweat rate is reduced, thermic conduction from body declines, and body temperature rises
  • Thirst sensation occurs after body is dehydrated (and after body has lost 1.5 to 2 litres of water) and is therefore a poor indicator of fluid needs
  • Factors that regulate gastric emptying time and thus affect presentation of fluids for uptake in the small intestine
    • Concentration of carbohydrate: above 7%, gastric emptying time decreases. Levels below 7% have gastric emptying time similar to water and are recommended
    • 6 to 7% concentration appears to optimize absorption rate: above 7%, water is encouraged to leave muscles and intestines to dilute solution before absorption
    • Small carbohydrate chain length: table sugar takes longer than a simple starch
    • Large volume of fluid: gastric emptying is made faster until volume reduces, thus athletes should initially intake a large volume then sip constantly to maintain faster rate of gastric emptying
    • Cool fluids leave stomach faster
    • Progressive dehydration and rising body temperatures cause slower gastric emptying
    • High mental stress, anxiety and high intensity activity associated with slower gastric emptying
  • Fat deters heat loss thus causing lower heat tolerance
  • Superhydration prior to event optimizes chance for enhanced performance and endurance due to higher blood volume levels resulting in lower core temperatures and heart rates during activity
  • Carbohydrate in fluid consumed during exercise maintains mental function. Lack of blood glucose affect brain first and mental fatigue leads to muscle fatigue, even if muscles have sufficient glucose
  • Rehydration takes time and carbohydrate and sodium in beverages restore fluid balance faster than water. Faster recovery of both fluid balance and glucagon to muscles enhances recovery and prepares for next event
  • Hydration strategy
    • Do not rely on thirst sensation: consume fluids in advance of sensation
    • Ensure clear urine through sufficient fluid intake
    • 2 to 3 hours before exercise, foods that have long gastric emptying time should be avoided: fat, protein and fiber
    • 1 to 1.5 hours before event, consume 0.5l of water to improve gastric emptying followed by constant sipping (half cup every 10 minutes)
    • Avoid diuretics such as tea, coffee, chocolate and sodas
    • Consume carbohydrate in fluid during exercise with concentration of 6 to 7%
    • Consume carbohydrate and sodium in rehydration beverage at end of exercise and consume 0.5l immediately after event to increase rate of gastric emptying

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Carbohydrate Loading

  • Muscles filled with glycogen and water lead to stiffness: for every gram of stored glycogen, body stores 3 grams of water. Sports requiring flexibility should adjust carbohydrate strategy
  • Women have higher lipid and lower protein and carbohydrate oxidation than men, thus carbohydrate loading may not be as beneficial toward athletic performance
  • Carbohydrate loading is advised as carbohydrate seen as the limiting substrate in metabolic activity. Muscle glycogen depletion results in dramatically reduced performance
    • Carbohydrate required regardless of modality for power and endurance. Carbohydrate required for
      • High intensity exercise as primary fuel
      • Low intensity exercise for complete oxidation of fat (the primary fuel)
    • Low glycemic foods provide slower but more sustained release of glucose to the blood, thus improving endurance during prolonged exercise. Low glycemic foods / high carbohydrate polymer foods eaten close to and during events do not improve performance and may cause gastric disruption and slow gastric emptying
    • Carbohydrate foods advised after exercise for use in replenishing glycogen stores and to reduce muscle protein breakdown and increase availability of protein for muscle recovery

Frequent eating strategy

  • Exercise and fasting compromises immune function and anti-oxidant systems
    • Glucose and glutamine are only fuel used by immune system: hypoglycemia acts to lower macrophage and lymphocyte function
    • Athletes should avoid low blood sugar and nutrient deficiencies both during and immediately after prolonged exercise (within 1 hour) to prevent immune degradation
  • Body reduces metabolic mass (muscle mass) to decrease metabolic rate and the need for calories in response to inadequate caloric intake
    • Lowering of lean mass triggers fat gaining when normal eating resumes
    • Timing of meals that delays eating and places athlete in energy deficit triggers fat gaining (eg main meal in evening)
  • Delayed eating causes blood sugar drops. Low blood sugar leads to:
    • Catabolism of amino acid alanine in muscle tissue for gluconeogenesis in liver
    • Hyperinsulinemia when combined with large meals which leads to fat gaining
    • Athletes seeking weight loss should aim for subtle overall daily energy deficit and ensure frequent meals to ensure large energy deficits
    • Increased risk of amenorrhea in females
    • Release of stress hormones adrenalin (which releases muscle and liver glycogen) and cortisol (which triggers muscle catabolism) leading to increased risk of both loss of muscle and bone mass

Frequent eating is thus advised as the best strategy with 6 meals per day at 2.5 to 3.5 intervals due to

  • Thermic effect of food and metabolic rate is higher with frequent eating: thermic effects of food is the calorie burn rate as opposed to conversion to fat
  • Frequent eating leads to higher muscle mass and decreased catabolism due to energy deficits and decreased insulin and leptin response to frequent food intake (both of which associated with fat production)
  • Blood sugar rises and falls in 3 hour patterns: meals should be timed to coincide
  • Muscles requiring blood flow and fluid volume for nutrient delivery and removal of metabolic waste. Compromise in either system results in decreased performance. Therefore dietary intake of foods and fluids should be spread throughout the day at frequent intervals to minimize possibility blood and water diversion. Blood flow and water in blood may be diverted to:
    • Skin to increase sweat rate
    • GI tract for dilution of carbohydrate in gut or GI activity for use in digestion

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Consideration for energy intake prior to training and events

  • Athlete should gradually taper off training to increase rest during week before event
  • Rest is to build up muscle glycogen stores to capacity
  • Athlete should become hydrated in advance. Note that hydration increases glycogen storage
  • Athletes should strictly avoid increasing training as an event draws near
  • Athletes should complete eating of high-carbohydrate meal at least 90 minutes before event or training
  • Athletes should avoid hypoglycemia triggered by high-glycemic foods or from consuming no carbohydrates or delayed eating
  • Athletes should similarly avoid hyperinsulemic response triggered by an excess energy intake that leads to fat storage
  • Hypoglycemia and low blood sugar leads to the release of cortisol and the catabolism of muscle tissue
  • Upon waking, blood sugar is marginal and liver glycogen stores are depleted: eating is therefore necessary before training
  • Waiting too long after training to eat can diminish the efficiency of muscle glycogen replacement

During event, carbohydrate containing beverages and food should be taken to delay fatigue, enhance performance and shorten post event recovery time in order to

  • Maintain blood glucose and preserving liver glucose
  • Maintain BCAA levels, which prevent central fatigue via excess transit of tryptophan across BBB
  • Inhibit production of cortisol which is catabolic to muscle tissue
  • Reduce usage of muscle glycogen by providing constant source of glucose to the blood

In addition, glucose polymers may improve gastric emptying and have enhanced absorption and prove useful for extremely high-intensity activity over a long duration

After exercise

  • Glycogen and fluid as well as protein are depleted to some degree
  • As glycogen levels deplete, enzyme glycogen synthethase (GS) becomes elevated in blood and reaches its peak at point of greatest glycogen depletion, which is right after exercise
    • Providing sucrose or glucose (and NOT fructose which causes GI distress) whilst GS is elevated efficiently replaces glycogen
    • Athletes should therefore consume high-glycemic foods during 2 hours after exercise

The Paleo Diet for Athletes

The importance of rapid recovery for an athlete

  • Required to ensure preparation for next workout or event and to optimize health for life
  • Athlete is defined as someone who trains 10 to 30 or more hours per week of rigorous exercise
  • Keys to optimal recovery are sleep and inclusion of low to moderate glycemic carbohydrate to Paleo diet

The dietary recommendations for the week prior to a race

  • Eat slightly more carbohydrate than usual to ensure glycogen stores are full
  • Ensure low glycemic meals by including protein and fat in all meals
  • Reduce dietary fiber to allow for easier digestion in the day prior to a race
  • Eat at least 2 hours in advance of exercise
  • Ensure energy stores depleted at night are re-stocked
  • Ensure fluid levels are re-established at all times, especially sleep
  • Prepare the body to recover from a workout by pre-loading nutrients

 What an athlete should eat the day of the race

  • Consume 200 to 300 calories per hour up to 2 hours before the event
  • Take in most carbohydrate to ensure easy digestibility
  • Reduce the glycemic index to ensure blood sugar levels are optimized and insulin and adrenalin surging is avoided
  • Reduce fiber intake as this swells, takes up gastric space and delays gastric emptying
  • Include proteins, especially BCAAs, to improve time to exhaustion and also enhance recovery as well as to lower the glycemic index of carbohydrates ingested with them
  • Ensure early hydration as this also reduces protein breakdown during performance
  • Food ideas include Fruit with egg, applesauce with protein powder, baby food and chopped animal products, liquid meals, sports bar with protein
  • In the last hour, avoid carbohydrate in order to avoid hypoglycemic reactions including dizziness and adrenic surging causing nervousness
  • In the last 10 minutes before an event, eat a high glycemic food with water to ensure glycogen stores are replenished: the energy will be directly taken up by cells, thus avoiding insulin surging

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Sourced from:

  • D. (2012). Advanced Sports Nutrition (2nd Ed).  Champaign, Il: Human Kinetics
  • Cordain, L. (2005). Chapter: “Stage I: Eating Before Exercise” (p.15-26). The Paleo Diet for Athletes. Rodale

 

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