Sportspeople and athletes need to take special care with their diet. To perform at optimal levels, you must obtain adequate amounts of vitamins, minerals, certain amino acids and essential fatty acids from your diet.
The harder you train, the more nutrients you need to:
• maintain red blood cell status so enough oxygen is supplied during exercise
• maintain muscle bulk and optimize contractile power
• keep metabolic reactions ticking over at a high rate.
Shortage of even a single nutrient can cause a metabolic imbalance that might impair your performance.
While you need these substances in adequate amounts, it is just as important not to overdose. Some vitamins and minerals are toxic at only 510 times their recommended intake (e.g. vitamin A, selenium, chromium).
It is also important to obtain the nutrients in the right balance. Too much of one vitamin can interfere with the metabolism of another. Excess zinc, for example, disrupts iron metabolism; excess iron upsets the way the body handles copper.
Too little of one nutrient can also render adequate supplies of another useless; for example, iron is no use if folic acid intakes are low both are needed to make the blood-oxygen-carrying pigment, haemoglobin.
Everyone differs in exact nutrient needs, depending on his or her inherited metabolic traits, lifestyle and levels of exercise.
The following information is a general overview of metabolism and sports nutrition. A serious athlete will need his or her nutrient intake individually assessed and any supplements prescribed by a sports nutritionist.
Body Composition of an Average Young Adult Male
• Water: 60 per cent body weight
• Protein: 18 per cent body weight (50 per cent dry body weight)
• Fat: 15 per cent body weight
• Minerals: 6 per cent body weight
• Glycogen stores: 1 per cent body weight
• 2,500 calories (kcal) energy stored as carbohydrate
• 112,000 kcal (80 per cent of energy reserves) stored as fat
• Remaining fuel is stored as protein (e.g. muscle).
THE METABOLISM
The word metabolism literally means change. It describes all the chemical and energy transformations that occur in the body, including those that convert food into energy stores and their utilization during exercise.
Ultimately, all building blocks essential for our metabolism are derived from dietary sources. We are literally what we eat.
Our body is a giant chemical reactor that oxidizes food in a complex, slow, multi-step process that liberates energy in small, usable parcels.
Enzymes (complex molecules consisting of proteins, minerals and vitamins) are essential for controlling these metabolic reactions. They act as catalysts to trigger chemical interactions which would otherwise not occur, or would only happen slowly in their absence.
During digestion, dietary proteins are broken down into smaller units called amino acids, fats to fatty acids and carbohydrates to glucose.
These smaller units are then oxidized (combined with oxygen) to release carbon dioxide, water and energy. Some are recombined to make new complex proteins (e.g. enzymes), carbohydrates (e.g. glycogen) and fats (e.g. cholesterol) that are needed for the smooth functioning of the body. Others are stored in the form of energy-rich molecules containing phosphates such as adenosine triphosphate (ATP).
Under normal circumstances, when food is plentiful, 40 per cent of dietary energy is stored as chemical energy and used to fuel exercise and metabolism. The remaining 60 per cent is dissipated as heat.
The Metabolic Rate
The speed at which the metabolism ticks over (amount of energy liberated per unit of time) is known as the metabolic rate. This varies from person to person, from day to day and even from hour to hour. It is regulated by the nervous system and the chemical messengers known as hormones.
The metabolic rate can be estimated by measuring the amount of oxygen consumed. This is a relatively simple scientific task as oxygen is not stored in the body and consumption usually keeps pace with immediate metabolic needs.
Approximately 4.82 kcal of energy are liberated for every litre of oxygen consumed. More accurate measurements require information about the food source (protein, carbohydrate or fat) being oxidized. This is obtained by analysing the amounts of carbon dioxide and nitrogenous waste substances excreted.
Studies show that the metabolic rate is influenced by a number of factors including:
• muscular exertion (exercise)
• metabolic effects (Specific Dynamic Action) of recently
ingested food
• high or low environmental temperature
• body temperature
• age
• height
• weight and surface area
• whether male or female
• emotional state
• circulating thyroid and adrenaline hormone levels
• the time of day or night
• drugs.
The most important factor is exercise. This raises the metabolic rate both during exercise and for a significant amount of time afterwards.
Muscles
Muscles are essentially processors that convert chemical energy into mechanical energy. They are the biggest contributor to the basal metabolic rate.
During exercise, blood vessels within the muscles dilate and blood flow is significantly increased to bring in the additional oxygen needed to oxidize fuel stores and release energy. This accounts for the rapid 'pumping up' effect observed during a good workout.
Mitochondria
The energy-producing reactions that occur within muscle cells take place in tiny units called mitochondria.
These are the cellular equivalent of rechargeable batteries. They are thought to have evolved many millennia ago in the primordial soup from a symbiotic species of bacteria that entered primitive one-celled organisms and set up home.
Mitochondria possess their own DNA, separate from that present in the cell nucleus, which codes for proteins and enzymes needed during energy processing.
The shape, size and DNA-makeup of the mitochondria present in your cells are identical to those of your mother. They are passed down from generation to generation within the cytoplasm of the egg from which you developed.
Interestingly, a sustained exercise programme can increase both the number and size of mitochondria within each muscle cell. This increases potential energy production and raises the basal metabolic rate.
Energy Storage Compounds
The body can easily convert protein to carbohydrate (glucose) for instant energy but lacks the enzymes and metabolic pathways needed to convert fat into glucose.
Fat stores have to be mobilized and broken down into fatty acids before they can be oxidized and used as a muscle energy source. This takes time; in an emergency situation the body resorts to burning extra protein as this is easier.
In addition, the body finds it difficult to mobilize the energy from fat molecules without a plentiful supply of dietary carbohydrate.
To use a common analogy, if excess fat stores are the logs to be burned in the fire of our metabolism, we need carbohydrate kindling to get the flames going.
Small amounts of dietary glucose significantly counteract the breakdown of muscle protein as an emergency energy source. This is called the protein-sparing effect of glucose. It is therefore essential to obtain carbohydrates before and during strenuous periods of exercise (see sports nutrition).
Muscle Fuel Sources
Resting muscles prefer to use fatty acids as fuel. During exercise, muscle energy needs increase and these are initially met by taking up additional glucose from the bloodstream and through breaking down carbohydrate stores (glycogen) within the muscle itself.
Glycogen is a storage form of glucose. It is present in most body tissues but is mainly concentrated in the muscles and liver.
Muscle glycogen stores are expandable. A male with a sedentary lifestyle will have around 1 g of glycogen per 100 g of muscle. A trained athlete may have as much as 4 g of glycogen per 100 g of muscle weight.
Muscles with a plentiful supply of glycogen are able to exercise longer without tiring. In turn, this increases muscle bulk and increases the amount of glycogen you can store.
After exercise, the muscle glycogen stores are replenished either from carbohydrate in the diet or by breaking down protein from lean tissues if food supplies are scarce.
Fatty acids, glycogen and glucose do not fuel muscle contraction directly. The mitochondria get the energy for this from the organic molecule adenosine triphosphate (ATP).
When the molecular bonds in ATP are broken down by enzyme reactions, energy is released to fuel muscle contraction and a molecule called ADP (adenosine diphosphate) is formed.
Usually, ADP is immediately converted back into the energy store ATP to provide muscle cells with fresh energy stores for the next contraction. It is for this chemical conversion that oxygen plus fatty acids or glucose are needed. Exercise is then said to be aerobic.
If exercise is so strenuous that oxygen is used up more rapidly than it can be brought in by the blood, or so that glucose stores run out, ATP the muscles' energy source rapidly depletes. Muscles then tire and lose the ability to contract any further.
A back-up molecule (phosphorylcreatine) is present in small amounts and can be used as a rich source of energy when oxygen availability is compromised. This molecule can be broken down to provide energy for the reformation of ATP from ADP without the need of oxygen. Exercise is then said to be anaerobic.
We cannot exercise anaerobically for long, as a waste product of metabolism, lactic acid, builds up within the muscle. As conditions become more and more acidic, muscles rapidly tire and painful cramping results.
Anaerobic metabolism in muscles is useful for providing an additional 'spurt' of strength in times of danger, stress or when quick responses are required. For example:
• in a 100-m sprint taking 10 seconds, 85 per cent of energy consumed is formed anaerobically because of the sudden increase in oxygen required.
• In a 3-km race taking 10 minutes, 20 per cent of energy is formed anaerobically. A more sustained effort is required, which the bloodstream can just keep primed with a constant supply of oxygen.
• In a slower race occurring at a more constant speed (e.g. a long-distance race lasting 60 minutes) only 5 per cent of energy would be derived anaerobically. A raised but constant oxygen demand can be met by the blood supply to the tissues.
The Oxygen Debt
Usually, the increased amount of oxygen consumed by muscles is proportionate to the amount of energy expended (i.e. 4.82 kcal energy per litre of oxygen) and all energy needs are met by aerobic processes.
When muscle exertion is great (e.g. the 100-metre sprint) and anaerobic reactions kick in to provide the emergency, self-limiting supply of energy, additional oxygen is still needed afterwards to:
• remove the build-up lactic acid
• replenish the stores of energy-rich molecules used in aerobic metabolism (ATP)
• replenish the emergency stores of phosphorylcreatine molecules.
The amount of extra oxygen needed is proportionate to the oxygen deficiency encountered during the period of intense exercise. An oxygen debt has been incurred.
Experimental measurements show that by forming an oxygen debt which can be repaid later, the human body is capable of six times the exertion that would have been possible without this mechanism.
There is a limit to the oxygen debt we can build up, however. Violent exertion is possible only for short periods of time, though with less strenuous forms of exercise, the debt can be incurred over longer periods.
Athletes in the peak of condition can increase the oxygen supply to their muscles much more than those who are less fit. They have:
• a larger muscle bulk
• more glycogen stores with which they can replenish ATP fromADP
• a larger number of mitochondria that are bigger and contain many more metabolic enzymes and ATP molecules
• a better network of blood vessels within the muscles bringing in blood and oxygen
• a more efficient respiratory and cardiovascular system.
Fit males can therefore exert themselves more strenuously without a build-up of lactic acid in their tissues. They will incur a much smaller oxygen debt for a given amount of exertion than would someone who was unfit.
In order for the metabolism to work efficiently, an optimum amount of nutrients are required. These include water, carbohydrates, vitamins (see Chapter 19), minerals (see Chapter 20) and co-enzymes.
Water
We don't usually think of water as a dietary nutrient, yet it is one of the most important substances an athlete needs.
The body is made up of 60 per cent water. A third of this is outside the cells (extracellular) and two thirds inside body cells (intracellular).
During an average inactive day in temperate climates we lose around 2.4 litres through our lungs, skin and kidneys. In a hot climate, or if partaking in strenuous exercise, it is easy to lose twice this amount.
Athletes undergoing a rigorous training programme will lose over 9.6 litres per day, which needs constant replenishing. If a muscle is dehydrated by just 3 per cent it loses as much as 10 per cent contractile strength, which will reduce your speed. Your performance literally dries up.
Intensive exercise speeds up the metabolic activity within mitochondria and increases the heat output of muscles to over 20 times their resting rate. As you start to overheat, several mechanisms are activated to help cool you down.
Water is lost through the skin (sweat) to cool you by dissipating heat energy through evaporation. Peripheral vessels dilate and blood is shunted away from the intestines and muscles towards the skin. This is so that heat can pass from the blood to assist the process of sweat evaporation.
Even with constant fluid replenishment and a cool environment, heavy exercise can increase core body temperature to as much as 39.4°C (103°F) within 15 minutes (around 37°C/ 98.6°F is normal).
If air humidity is high, so sweat cannot easily evaporate away, overheating occurs even faster.
Adequate supplies of water are essential. If you become dehydrated, your body temperature will rise even further.
If body temperature goes over 40°C (104°F) your metabolism becomes inefficient and your athletic performance will rapidly fall off. More blood is pumped to your skin and away from your muscles and heart, which therefore receive less oxygen. Without oxygen, your muscles shift into burning fuel anaerobically, and more heat is generated.
Signs that you are starting to overheat include feelings of radiant heat in the face, a throbbing in the temples and feelings of coldness over the chest.
Always aim to keep your core body temperature below 40°C (104°F) during exercise. This is important when environmental temperature and humidity is high. To help maintain a safe body temperature:
• Drink plenty of water.
• Drink water that is as cold as you can stand it.
• Expose as much skin as possible to maximize sweat evaporation.
• Wear lightweight clothing.
• Wear light-coloured clothing.
• Avoid exercising in the sun. Stick to the shade as much as possible.
Preloading with Water Before Exercise
If you are training seriously for a particular endurance event such as a marathon, you can improve your body hydration beforehand by:
• eating lots of carbohydrates during the week prior to the event (see carbohydrates). Carbohydrates are stored in the body as glycogen, which mops up water like a sponge. Each gram of glycogen is associated with 2.7 g of water. An athlete can store at least an extra litre of water this way.
• Drink extra water during the 48 hours before the event. Starting four hours beforehand, drink 200 ml of water every 15 minutes until half an hour before. Then drink nothing in the final half hour to ensure all the fluid is absorbed and your stomach is empty.
You will need to empty your bladder just before the event, but once you start exercising heavily your urine output will fall drastically. Anti-diuretic hormone is secreted by the brain to switch off fluid loss through the kidneys so that water is conserved. You will not have to worry about being caught short during the event.
Everyone has a different capacity to store water so it is worth trying this waterload regime during training to ensure it suits you.
Drinking During the Event
During the event itself it is also important to drink as much as you can. Sweat loss can be as high as 180240 ml per mile in a marathon, which can lead to severe dehydration. This will cut your performance drastically as well as harming your health.
Even if you are water-loaded before the event, you need to drink as much cold fluid as possible during longer events. When water intake matches sweat loss, temperature rises the least and athletic performance is maximized. You will put in better times, recover faster and feel better during and after the event.
It is important to sip the water, however, not gulp it down. If you gulp you will swallow air which will bloat your stomach, reduce absorption and perhaps even trigger cramps.
Water taken before and during the event should be either plain water or contain only low quantities of sugar (hypotonic see below). Carbonation or water containing more than 7 per cent sugar solutions will slow absorption down.
Fluid Intake After the Event
After a tough endurance event you will be dehydrated, even if you were preloaded with water and maintained a good intake throughout the event.
Although salts are dissolved in sweat and urine, you will have lost much more water than salts. You are therefore overloaded with electrolytes (sodium, potassium, etc.) and ideally need to take in plain water to correct the imbalance. Hypotonic solutions containing low amounts of glucose should be reserved until you have drunk at least a pint of plain, bottled (pure) and preferably distilled water.
Sports drinks are specially designed to replace fluid loss (after the plain water intake) as well as providing an instant energy boost to replenish those ATP molecules.
There are many brands available, which come in three main types:
1. Hypotonic Drinks which are less concentrated than body fluids. These contain 2 to 3 g of carbohydrate per 100 ml.
2. Isotonic Drinks containing the same concentration of salts as body fluids plus 6 to 7 g of carbohydrate per 100 ml.
3. Carbohydrate or Energy Drinks which contain high quantities of sugars (10 to 20 g per 100 ml).
