The Complete Book of Men's Health - Part 3: Nutrition and Lifestyle

CARBOHYDRATE

The World Health Organization recommend that we obtain at least 50­60 per cent of our dietary energy from unrefined complex carbohydrates. If anything, the figure for athletes should be even higher. Carbohydrate is the most important fuel for athletes. It provides 4 kcal (energy) per gram and supplies muscles with energy twice as fast as dietary fat, which the body takes a long time to process and use. Carbohydrate is also the only fuel muscles can use during anaerobic exercise.

The basis of optimum sports nutrition is therefore carbo-hydrate. If the body is forced to burn fats instead, an athlete's performance will drop off.

What Are Carbohydrates?

Carbohydrates are molecules made up of carbon, hydrogen and oxygen atoms.

Monosaccharides are the simplest carbohydrates and consist of a single sugar molecule ­ a saccharide. The most important monosaccharide is glucose.

Disaccharides consist of two saccharide molecules linked together. The best known example is sucrose ­ ordinary table sugar ­ consisting of a glucose molecule linked to a fructose molecule.

Polysaccharides are long chains of saccharides linked together, of which the most important is starch. Poly- saccharides are also known as complex carbohydrates (while sugars are simple carbohydrates). Complex carbohydrates are found in cereals, root vegetables and fruits.

Some carbohydrates cannot be digested by humans ­ we lack the enzymes necessary to break down their chemical bonds. These 'unavailable' carbohydrates (e.g. cellulose) pass through our bowels virtually unchanged and make up the bulk of dietary fibre ­ also known as roughage.

How Our Body Uses Carbohydrate

The simple sugars (monosaccharides) are absorbed into the bloodstream unchanged. The body can use monosaccharides as instant energy after processing them by adding a high energy phosphate bond (phosphorylation).

The disaccharides and starches must be broken down into monosaccharides before the body can use them as an energy source. This process starts in the mouth, with salivary enzymes, and continues in the stomach and upper intestine.

Table sugar (sucrose) is broken down relatively easily and the monosaccharides that are released quickly increase blood sugar levels.

Starch digestion takes longer and causes a steady stream of sugars to enter the blood. Eating complex carbohydrates therefore provides a constant infusion of sugar energy into the bloodstream.

Once carbohydrates are broken down into their constituent monosaccharides (mostly glucose), they are absorbed through the digestinal tract wall into the blood for distribution to the tissues.

Some sugars, such as galactose and fructose, cannot be metabolized directly by the cells and are converted into glucose in the liver before they are used as a fuel.

Blood sugar levels are controlled by two pancreatic hormones, insulin and glucagon, and also by the liver's glycogen stores, which act as a sugar buffer. When blood glucose levels are high the liver synthesizes storage glycogen. When blood sugar levels are low, glycogen is broken down and glucose molecules released into the circulation.

The Carbohydrate High

Carbohydrate triggers the release of a chemical in a part of the brain known as the hypothalamus. This chemical, serotonin, is a neurotransmitter. It relays messages across important nerve connections (synapses) in the brain. One of the functions of serotonin is to influence appetite and food selection. It regulates satiety (that feeling of being replete) and therefore controls how much we eat. Carbohydrate, the only food source that stimulates serotonin release, therefore makes you feel fuller more quickly, and for longer, than any other dietary source of energy. Serotonin also triggers feelings of euphoria. In combination with other brain chemicals released during exercise (e.g. heroin-like endorphins) this alters an athlete's perception of fatigue and improves his or her motivation to keep going. This is especially important during long, endurance events.

Glycogen-Loading

Studies with endurance athletes show that the level of glycogen within the muscles before exercise starts is directly correlated with performance. The higher your muscle glycogen stores, the longer you can continue at your maximum pace. For long endurance events it is therefore important to ensure your muscles are loaded with glycogen before the event. This is only true of longer events ­ such as those that will last more than two hours. For shorter events glycogen-loading would only slow you down as you are not exercising enough to gain benefit from the additional weight (glycogen plus its associated water stores) you have to carry.

Muscle glycogen stores are optimized by an intake of around 650 g carbohydrate per day. This varies from athlete to athlete depending on body weight, your chosen sport, how hard and long you train, plus your inherited level of enzyme activity. Some athletes need double this amount of carbohydrate, some need only half. In order to utilize this dietary carbohydrate, muscles must be worked to glycogen depletion so that glycogen synthesis is triggered. Otherwise, excess carbohydrates are converted into fatty acids rather than glycogen. You will need to experiment with how much carbohydrate you can eat without tiring and without losing or gaining weight in the form of body fat. By depleting muscle glycogen during training, muscles subsequently become loaded with more glycogen than they started with. During training this depletion/loading cycle is repeated to maximize muscle glycogen stores prior to an event. It is only the muscles which are exercised that will deplete and load with glycogen, however. The process does not automatically occur in all muscles of the body.

Widespread glycogen-loading is best achieved by doing high repetitions with light weights in the widest selection of exercises possible. Many athletes glycogen-load their leg muscles successfully (e.g. by using a cycle, treadmill, etc.) but are let down by neglecting to exercise their arms, neck, shoulder and even back muscles ­ these muscles then become depleted of glycogen during the event itself.

It is best to obtain the assistance of a personal trainer to help you glycogen-load successfully.

Carbohydrates Before Exercise

To fuel the event itself you will need to start loading on carbohydrates 3­4 hours before strenuous exercise. This is best achieved in the form of an energy drink containing 100 g of carbohydrate plus obtaining adequate fluid.

Carbohydrates During Exercise

Carbohydrates taken during exercise can be a useful source of fuel, but only if your muscle glycogen stores are deficient ­ because whereas glycogen is in a chemical form that mitochondria can use straight away, glucose taken up from the bloodstream must first be processed by adding a phosphate chemical bond. This is what allows it to drive the regeneration of ATP from ADP.

It is much better to load with carbohydrate several hours before the event so the carbohydrate is converted into glycogen which is then already sitting in the muscle cells waiting to be used. Also, if muscle glycogen stores are low before exercise, glucose during exercise is important to prevent muscles turning to protein as an emergency energy source. Even small amounts of glucose have a protein-sparing effect (see energy storing compounds).

During a long endurance event you will need 40­90 g carbohydrate (e.g. glucose, maltodextrin) per hour as well as an adequate fluid intake for optimum performance. Research suggests absorption of both is best using 5­10 per cent carbohydrate solutions (sip around a litre per hour depending on level of exercise).

Carbohydrates After Exercise

As soon as you finish exercise, muscle cells must repay any oxygen debt they have incurred. They also start replenishing their diminished stocks of glycogen.

Glycogen synthesis is rapid for the first two hours, slightly less rapid for the next four hours, and occurs at a slow rate for the following 24 hours.

During the first two hours an enzyme (glycogen synthetase) is activated which triggers rapid synthesis of glycogen. This is when you need to hit the carbs in a big way.

It has been found experimentally that an athlete obtaining 225 g glucose polymer in liquid form during the first four hours synthesizes the maximum amount of glycogen. This saturates the enzyme systems, so any more than this does not increase the amount of glycogen synthesized.

So after you have replenished your water intake, start drinking a carbohydrate energy drink which contains high quantities of sugars (10 to 20 g per 100 ml).

You need to maintain a constant, steady intake of carbo-hydrate (up to 1,000 g in total depending on duration and level of exercise) during the 24 hours after a strenuous event. This is best achieved by eating little and often, for example six small meals spread throughout the day rather than three larger ones, which is the Western norm. This will minimize sugar swings that can trigger insulin fluctuations. If these occur, glycogen synthesis is inhibited and some newly generated glycogen may get broken down.

Complex carbohydrates are best obtained in the form of whole grains (e.g. brown rice, wholewheat pasta, wholemeal bread), legumes (e.g. soybeans, baked beans, lentils, kidney beans), and vegetables (e.g. potatoes, yams).

Your fluid intake must remain high during this glycogen replenishing phase, as each gram of glycogen is associated inside your cells with 2.7 g of water.

After the high intake during the first 24 hours, revert to your normal calorie intake (otherwise you will start putting on fat), but ensure that at least 60 per cent is in the form of carbo-hydrates.

Dedicated athletes eat as much as 80 per cent of their cal-ories in this form during training periods, which would certainly please the World Health Organization.

FOCUS ON: BODY BUILDERS

Body builders use carbohydrates and glycogen-loading to increase muscle bulk and definition before competition rather than as an energy source.

Starting a week before the event, body builders initially deplete their diet of carbohydrates and increase protein intake to provide around 60­70 per cent of their calories. As soon as glycogen stores are depleted no more can be synthesized due to the lack of dietary carbohydrate. The body preferentially switches to burning fat stores as fuel and the body builder deliberately enters an unpleasant state known as ketosis.

Ketones are a by-product of fat metabolism. They are metabolized with difficulty in the liver and are normally rapidly metabolized in other body tissues. This process requires products of glucose metabolism however, so when carbo-hydrate intake is low and excess ketones are formed, they spill over into the circulation.

Ketones are an important source of energy in some emergency situations such as during temporary periods of starvation or after prolonged vomiting.

If too many build up in the body, however, they affect the body's acid balance and can cause cell damage. Ketones also affect the brain by acting as false neurotransmitters. This leads to irritability, lethargy and confusion.

Body builders force themselves to work through these feelings by burning up ketones as fuel until they reach the point of exhaustion and maximum glycogen depletion. This is a potentially dangerous situation and should always be done under the strict supervision of a personal trainer and preferably a physician. If ketosis is handled incorrectly, it can be lethal.

Three days before the event, body builders change tactics and increase their carbohydrate intake to 70­80 per cent of energy intake and drop protein right down to only 10 per cent of calories.

Sodium intake is also heavily restricted, with potassium intake raised to compensate. Together with a lowered fluid intake, this encourages mild dehydration to obtain a leaner, more sculpted look.

Ketosis disappears now that carbohydrates are being used as a fuel source, and exercise is kept very light.

Over these three days, muscle glycogen stores rapidly increase from the depleted state so that 2­3 kg in weight are gained.

The combination of glycogen-loading and slight dehydration has to be handled carefully so that water is diverted into the muscles (bound to glycogen) from other body cavities, without causing symptoms of dehydration (dry mouth, dizziness, rapid pulse, increased breathing rate, confusion ­ even coma).

Some body builders risk their health by taking diuretics or having a sauna to lose even more fluid before the competition. Don't do it!

Body builders are in a highly abnormal physiological state while preparing for competition. Never do this alone without advice from a properly qualified trainer and the input of a sports physician.

DIETARY FATS

The average fat intake in the Western world makes up at least 40 per cent of daily calories. This is far too high. The World Health Organization recommends that fats be reduced to no more than 30 per cent of energy intake ­ preferably nearer 20 per cent.

For athletes, fat intake should be less than for even the healthiest eating sedentary adult. A certain amount of the right sorts of fats is important, however. The fat (lipid) fraction of our diet supplies us with:

• building blocks for cell membranes

• fatty acids necessary for the central nervous system

• precursors for important hormone-like chemicals called prostaglandins

• substrates for hormone production

• molecules from which to make bile salts

• fat-soluble vitamins A, D, E

• essential fatty acids (linoleic and linolenic acids)

• energy ­ 9 kcal per g

We lack the enzymes and metabolic pathways needed to convert fat into glucose, therefore dietary fat is shunted into pathways that convert it to body fat ­ that is, flab.

If carbohydrate is in short supply and fat must be used as an energy source, it must first be mobilized and broken down into fatty acids. This takes time and, in an emergency situation, the body resorts to converting protein stores into glucose to plug the gap.

In addition, the body finds it difficult to mobilize the energy from fat molecules without a plentiful supply of dietary carbohydrate.

Fat in the diet ­ although ostensibly providing twice as much energy as carbohydrate (9 kcal/g versus 4 kcal/g) is therefore not the best source of energy for anyone ­ especially athletes. Carbohydrate is the prime energy source.

But athletes do need some fats, and it is important that those they eat should provide the most nutritional benefits.

Fatty acids are made up of chains of carbon and hydrogen atoms. Different fatty acids have chains of different lengths. Fats can be classified into saturated, monounsaturated and polyunsaturated according to their chemical structure. In saturated fats, all the carbon bonds are attached to hydrogen atoms. In mono- and polyunsaturated fats, there are some hydrogen atoms missing. This allows them to interact with the body's metabolism more easily. Monounsaturated fats are only missing one hydrogen atom. Polyunsaturated fats are missing two or more hydrogen atoms.

Most natural fats and oils contain a blend of saturates, monounsaturates and polyunsaturates.

Cholesterol

Cholesterol is an important fat-based structural molecule that is a building block for cell membranes, hormones and bile salts. Two forms of cholesterol exist in the bloodstream:

1. low-density LDL-cholesterol molecules which are small enough to seep into artery walls and trigger hardening and furring up of the arteries (atherosclerosis).

2. high-density HDL-cholesterol molecules that are too large to seep into artery walls and therefore stay in the bloodstream to act as important carrier molecules. They help to mobilize and mop up the risky LDL-cholesterol.

High blood levels of LDL-cholesterol are linked with an increased risk of coronary heart disease, while high levels of HDL-cholesterol are protective (see Chapter 11).

It is important to realize that most of the cholesterol in our blood is made in our liver from fats in our diet.

Eating preformed dietary cholesterol (e.g. in egg yolks) has relatively little impact on the amount of cholesterol in our blood.

Saturated Fats

Saturated (hydrogenated) fats tend to be solid at room tempera-ture. They are mainly derived from animal fat and are found in high quantities in cream, butter, egg yolk and red meat.

Saturated fats are converted in the body to harmful LDL-cholesterol. We would be better off without eating any saturated fats, yet they currently make up around 15 per cent of energy intake. Everyone should reduced the intake of saturated fats to less than 10 per cent of calories ­ preferably zero. This is especially important for optimal sports nutrition.

Unsaturated Fats

Monounsaturated and polyunsaturated fats tend to be liquid at room temperature ­ i.e. oils. Olive oil is the richest source of monounsaturates, while other vegetable oils such as wheatgerm, sunflowerseed and safflower tend to be rich in poly- unsaturates.

Monounsaturated fats help to lower harmful blood LDL-cholesterol levels and raise levels of beneficial high density HDL-cholesterol. Athletes should preferentially use Extra Virgin olive oil or rapeseed oil for cooking and salad dressings.

Polyunsaturated fats are now viewed with some suspicion. They are highly reactive and rapidly oxidize (go rancid) to form carcinogenic (cancer-forming) chemicals. They also affect cell membranes, interfere with cellular transport and possibly promote early ageing.

When polyunsaturates are partially hydrogenated, trans-fatty acids are produced. These are associated with an increased risk of coronary heart disease and may be an even greater health hazard than natural saturated fats.

Trans-fatty acids are also thought to interfere with the way the body handles other essential fatty acids, so that their beneficial effects are not fully realized.

The amount of trans-fatty acids in our diet varies but average consumption is around 5­7 g per day. Some people eat as much as 25­30 g of trans-fatty acids per day, particularly if they use cheap margarine and eat lots of processed foods.