DPT-605- Nutrition: Exercise Nutrition

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The best nutritional recommendations to accompany an exercise regime will vary according to the physiological demands placed upon the body, and therefore must be relevant to specific training programmes.

Aerobic training requires fuel, which can be provided by fat, carbohydrate and even protein. Whilst working at lower intensities, a greater percentage of fat will be utilised for energy, but with rising intensity of exercise a progressively higher amount of carbohydrate will be drawn into the fuel equation. 

The serious exerciser or the dedicated athlete is faced with an array of information and possible options to best suit their needs. These choices can be narrowed down to the following:

  • the use of food alone
  • the sole reliance on sports drinks and/or protein shakes
  • a combination of food plus sports drinks 
  • a combination of food plus protein shakes
  • a combination of food plus water

Pre-exercise Meal – High or Low Glycaemic Index? 

It has often been quoted that a pre exercise meal is best chosen from low GI foods. The rationale for this belief is that, if a high GI carbohydrate meal is taken before training or competing, the relatively rapid rise in blood glucose will cause corresponding release of insulin. 

 The feared effects of high GI CHO were:  

  • an increased rate of early glucose oxidation
  • a ‘rebound hypoglycaemia’ effect where the increased insulin causes a rapid fall in glucose levels possibly even before the session has begun

The original research reported that cyclists given a low GI meal consisting of lentils eaten one hour before intensive exercise, performed for longer before fatiguing, when compared to those fed on a high GI meal (Thomas et al, 1991). The researchers suggested that glycogen sparing may have occurred with the low glycaemic trial, thus promoting better performance. However, post training glycogen levels were never measured, and subsequent studies have failed to prove any clear benefit from pre-feeding on a low glycaemic meal, including repeated work from Dr Thomas’ team. 

The majority of studies show that there may be slightly more favourable metabolic conditions with regards to insulin levels during exercise associated with low GI foods than with high GI alternatives. But these differences are small and short lived. The conclusion is that athletes probably perform the same on both pre-race meals.

As will be discussed later, the real difference in performance appears to be related to carbohydrate feeding during exercise, which seems to over ride any metabolic or performance effects arising from the type of pre-event meal. Athletes should consume adequate amounts of carbohydrate drinks during endurance exercise, and may feel free to choose their pre-exercise meal according to their personal preferences (Burke et al, 1998).

Carbohydrate taken during exercise or competition

Different studies have indicated that ingesting carbohydrates is acceptable:

  • if the session is longer than an hour
  • if the match or race is longer than 90 minutes 
  • if a pre-exercise meal is not possible (such as early morning intensive training)

The consumption of isotonic drinks during exercise has been shown to delay the onset of fatigue and to improve performance in endurance athletes (Tsintzas et al, 1995). Many athletes find it difficult to consume even a light meal before exercise without causing gastrointestinal discomfort, or they simply may not have time to eat before their planned training session. Often people have to train early in the morning, making a pre-exercise meal impossible. One study found that ingesting an isotonic drink during endurance training is as effective as a pre training carbohydrate meal (Chryssanthopoulos et al, 1994). The replacement of fluid provided by the isotonic drink is also a direct advantage. 

Post-Exercise Meal Guidelines

After intensive exercise, the muscles are more sensitive to the effects of insulin thus enabling a more efficient replacement of lost glycogen. This process is particularly evident during the first two hours following the training session. The rapid synthesis of muscle glycogen stores is aided by the immediate intake of high GI carbohydrate. 

The frequency of carbohydrate meals post-training does not appear to exert an effect on glycogen replenishment. Small regular intakes or three larger meals appear to gain the same results. 

General Exercise

This category refers to people under taking lower intensities of training, where a greater amount of fat will be utilised with some possible loss of glycogen. These guidelines are somewhat similar to normal guidelines for eating, except that it should be timed appropriately around the exercise session.

Guidelines for general exercise
aim to stay within energy balance
create energy deficit of 250 kcal if trying to lose body fat
fulfil CHO needs, chose from moderate to low GI foods
try to provide energy that can be metabolised, don’t mix high CHO with high fat
smaller portions and regular meals favour the oxidation of nutrients
micronutrients should be high
fibre should be adequateinclude adequate protein
EFA’s should be eaten in balance 

What is an Isotonic Drink?

 

Sports drinks are now widely used in order to improve performance and recovery. They serve two main roles, notably the replacement of fluid and the provision of fuel in the form of carbohydrate. Isotonic drinks have a similar balance of dissolved solids to the blood. This helps provide a faster rate of absorption of fuel whilst maintaining reasonable hydration. They also contain the necessary electrolytes or salts lost through increased sweating during intensive exertion. 

As previously stated there are several studies showing that the use of sports drinks has been beneficial in prolonging activity and has particularly helped as fuel replacement during exercise. However, it should be noted that many of the commercial sports drinks have other additives which are less desirable. It is not uncommon to find isotonic drinks that have sweeteners and colourings in them. Aspartame and acesulfame K are common sweeteners (see food additives chapter) and have undesirable side effects.

Another option is to make your own equivalent of a sports drink. Below are 2 options:

  • dissolve 60g glucose into a litre of water and add a fifth teaspoon of natural unprocessed salt
  • mix 500ml of unsweetened fruit juice with 500ml of water and add a fifth teaspoon of natural unprocessed salt (paralympics.org.uk)

Protein Shakes

Protein shakes have become a popular training supplement in the last 20 years, partially due to the increased profile of bodybuilding, but also because of the significant investment in advertising in men’s magazines and the internet. The appeal is a quick, easy to use supplement that enables individuals seeking hypertrophy to achieve their increased protein needs. Much of the advertising implies that their supplement is the ‘answer’ and is a ‘highly advanced’ or ‘precision engineered’ muscle building formula. So how much is marketing and how much is truth?

Whey protein is found in milk, which averages about 6.5% protein, of which about 20% is whey protein. In its natural state it has the highest biological value to the body of any protein, due to its high concentration of essential and branched chain amino acids. This makes it useful to the body in many ways, one of which is in the repair and growth of muscle tissues (Kadey, 2005).  However, by the time a tub of whey powder is purchased it is often vastly different from the original product. Consider the following points:

  • whey is a waste liquid by-product from cheese manufacture.  Traditionally it was disposed of by farmers into animal feeds (Fallon, 2003)
  • often dried at high temperatures for speed of manufacture – above 60ºC these fragile proteins become denatured, which destroys their ability to function (Fallon, 2003)
  • manufacturers use sugars, sweeteners, colours and flavours to improve palatability
  • often very low in fat – proteins need fat for proper metabolism and use. Some studies indicate that skimmed milk can lead to increased body fat storage compared to whole milk (westonaprice.org)
  • often backed up by self-funded research, if any – this does not provide an independent, objective view
  • prices now are highly inflated due to market demand generated by clever advertising 

It is important to recognise that protein shakes were only intended to supplement, not replace good food. The body is designed to absorb and metabolise real, untainted food and protein sources. If a supplement is required then consider the following points before purchasing:

  • seek cold processed protein powders, manufactured below 50 ºC
  • no added sugars, sweeteners, colours or flavours
  • mix with whole organic milk, as fats are necessary for protein metabolism 

Key learning points

1. Pre-exercise feeding:

  • traditional thought was to use low glycaemic CHO two hours before session or event
  • now appears that the glycaemic index of CHO before training or an event makes little difference
  • CHO of personal preference can be eaten some time prior to training
  • isotonic sports drink can be taken within 15 min before session
  • consider home made isotonic drinks
  • fuelling during session or event is important for sessions or events longer than an hour
  • Post exercise feeding should take advantage of active glycogen storing enzymes:
  • increased glucose availability
  • increased insulin 
  • increased glucose uptake by muscles
  • increased glycogen synthesis
  • Isotonic drinks
    • same concentration as blood
    • offer fast absorption of CHO
    • contains important electrolytes/salts
  • Protein shakes are driven by advertising
    • often processed at high temperatures, and have many additives
    • should only be used as a supplement to natural, quality food

References

Burke LM, Claassen A, Hawley JA, Noakes TD. (1998). No effect of glycemic index of pre-exercise meals with carbohydrate intake during exercise. Med Sci Sports Eexerc 30, S82: 471. 

Chryssanthopoulos C, Williams C, Wilson W, Asher I, Hearne I. (1994). Comparison between carbohydrate feeding before and during exercise on running performance during a 30 km treadmill time trial. International Journal of Sports Nutrition. 4: 374-386. 

Murray R, Paul GL, Seifert JG, Eddy DE, Halaby GA. (1989). The effects of glucose, fructose, and sucrose ingestion during exercise. Med Sci Sports Eexerc. 21:275-282.

Thomas D.E, Brotherhood J.R, Brand J.C. (1991). Carbohydrate feeding before exercise: effect of glycemic index. Int J Sports Med. 12: 180-186. 

Tsintzas O.K, Williams C, Boobis L, Greenhaff  P. (1995). Influence of carbohydrate supplementation early in exercise on endurance running capacity. Carbohydrate ingestion and glycogen utilisation in different muscle types in man. Journal of Physiology. 489: 243-250. 

Tsintzas O.K, Williams C, Singh R, Wilson W, Burrin J. (1995). Influence of carbohydrate-electrolyte drinks on marathon running performance. 70: 154-160. 

Kadey M. (2005) Whey Protein: Not just for Bodybuilders. Ptonthenet.com

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