Pre and post workout nutrition

Our bodies require fuel to maintain our bodies and day to day actions but also for any exercise that you might do. Pre workout nutrition is mainly based around providing enough energy for our muscles to work efficiently. Post workout nutrition is based around repairing our muscles and restocking our muscle fuel supply.

Your metabolism is the energy required for your body to maintain all its basic processes. There are two types of metabolism: anabolism – the formation of larger molecules, and catabolism – the breakdown of large molecules into smaller molecules.  Metabolic rate is the rate at which your body expends energy; basal metabolic rate is the number of calories requires to maintain essential bodily processes such as breathing and organ function whilst sleeping.  The amount of muscle you have along with the exercise you perform contributes to your metabolic rates and body’s requirement for energy.

Our energy comes from the macronutrients; carbohydrates, fat and protein and alcohol. Carbohydrates are broken down to glucose, galactose and fructose which are simple sugars. Fat is broken down into fatty acids and proteins into amino acids.  Carbohydrates and alcohol are short term energy sources whilst fats are long term energy sources. We can use protein as an energy source, but mainly when we do not have enough carbohydrates – this is the basis of low carbohydrate diets. The main problem with this is that you may sacrifice muscle mass.

The body is not very efficient at converting food into power and most of the energy produced is released as heat. Energy is measured in calories or joules – one calorie is defined as the amount of heat required to increase the temperature of 1 gram or millilitre of water by 1 degree centigrade.

Different macronutrients and foods contain different amounts of energy. 1g of carbohydrate and protein supply 4 kcal whilst 1g of fat provides 9kcal. Alcohol provides 7kcal of energy per gram.  Fat is the most concentrated form of energy but not necessarily the best form of energy for exercise.

Your body stores fuel in two ways; the primary store is in the muscles and liver in the form of glycogen and the secondary store is in the fat cells in the form of fat. The body utilises glycogen from the muscles first when exercising. When this supply has been exhausted (we might quite literally feel exhausted!), the body utilises the fat stores.

The muscles of the body store most of the glycogen (approx. 400g) compared the liver (100g). Glycogen is a long molecule made up of many units of glucose joined together. Glycogen storage requires about three times the volume of water. The total glycogen store of 500g is equivalent to 1600 – 2000 kcal which would last you one day if you did not eat anything.  Low carbohydrate diets seem to work quickly as you lose the glycogen and associated water.

If you are an endurance athlete such as a marathon runner or Iron Man triathlete, you will usually have a higher level of muscle glycogen compared to sedentary people. People with higher levels of muscle mass such as those who lift weights will also have increased storage capacity for glycogen.  Liver glycogen maintains blood glucose levels whilst you are resting or in prolonged exercise.

The body stores fat in adipose tissue both under the skin and around the organs.  Protein is the basis of muscles and organs – it is mainly a building material rather than a store.  If you do not have any food, the body can use the muscles and organise to release energy from protein – this is common in starvation. In very prolonged exercise, the proteins in muscles might make up about 10% of the body’s fuel.

Our muscles can use different combinations of carbohydrate and fat to provide energy for physical exercise. The most important factors are:

  1. Intensity of exercise
  2. Duration of exercise
  3. Fitness level
  4. Pre-exercise nutrition

 
The proportion of muscle glycogen used increases with exercise intensity and decreases with exercise duration. For short sharp bursts of energy, the aerobic systems using glycogen dominate whereas slower, lower intensity exercise uses fat. Performance is usually limited by the amount of glycogen in the muscles.  This demonstrates that glycogen is the most valuable fuel for all types of exercise. If you don’t have enough carbohydrates or glycogen, you will fatigue more quickly.

Ideally, you should eat 2 – 4 hours before exercising so that blood glucose levels are optimum, glycogen levels are restored and your digestive system has settled.  If you leave it too long after eating, your blood glucose levels may be too low and you might feel faint. However, other studies[i] have shown that fasted exercise may help you to burn more fat as insulin levels are at their lowest. This may be useful for fat loss but not if your main goal is performance as you may end up using protein from the muscles as fuel! Wu et al found that a low GI meal helped athletes to burn more fat than if you eat a high GI meal. (GI – glycaemic index – for glossary).

During most exercise, unless it exceeds one hour, you do not need to drink more than water as long as your glycogen levels are high[ii]. If exercising for more than one hour, you may want to consume additional carbohydrates to maintain blood glucose levels and allow you to exercise longer.

After exercise, you will need to refuel to refill glycogen stores and repair muscle. The more depleted your glycogen stores, the longer they will take to refuel. Muscle damage occurs after heavy weight training, plyometric exercise or hard running may also delay glycogen stores refilling.  Ideally, you will refuel as soon as possible i.e. within the first two hour after training[iii]. Glyocgen recovery usually takes about 20 hours but it can be kick started by eating 1g of carbohydrate per kg of body weight in the first post training meal.  Again, I think that this is quite high unless you are an athlete. Combining protein with carbohydrates will help the muscles to recover more quickly in terms of glycogen storage and muscle repair.

Protein is required for exercise, particularly if prolonged as exercise triggers the activation of an enzyme which promotes protein as a fuel.  The greater the intensity and length of the exercise, the more protein is used as a fuel source. Protein is also required for maintenance of the body including metabolism, fluid balance, replacement of body tissue and nutrient transport. This means that the body’s requirement for protein increases as you exercise.  Protein breakdown is increased when glycogen storage levels are low i.e. when exercise lasts more than one hour.

The recommended protein intake by the government is 0.75g per kg body weight. However, this is quite low. If you are strength training and want to lose fat or gain muscle, 1g – 2g of protein per kg of body weight is recommended. Each person is individual and your energy requirements are different depending on your body and the type and intensity of exercise and the time of day that you exercise.

Whilst we have not discussed this in much detail in this article, we have discussed that the body requires fat and the American Dietetic Association suggests that 20 – 25% of our energy comes from fat. This means that the remaining 75% or 80% of the diet comprises of carbohydrates and protein.  As each person may have varying goals, we cannot come up with a one size fits all formula but can suggest amounts of food. The number of meals a day consumed will also make a difference to the amounts of each macronutrient consumed in each meal.

References
[i] J Physiol. 2010 Nov 1;588(Pt 21):4289-302. doi: 10.1113/jphysiol.2010.196493.

Training in the fasted state improves glucose tolerance during fat-rich diet.

Van Proeyen K1, Szlufcik K, Nielens H, Pelgrim K, Deldicque L, Hesselink M, Van Veldhoven PP, Hespel P.

[ii] Sports Med. 2001;31(4):267-99.

Guidelines for daily carbohydrate intake: do athletes achieve them?

Burke LM1, Cox GR, Culmmings NK, Desbrow B.

[iii] J Appl Physiol (1985). 1988 Nov;65(5):2018-23.

Muscle glycogen storage after different amounts of carbohydrate ingestion.

Ivy JL1, Lee MC, Brozinick JT Jr, Reed MJ.