Tuesday, 12 December 2017
Review of Sport Nutrition, 2nd Edition

Sport Nutrition 2nd Edition

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Review of Sport Nutrition – 2nd Edition – written by Asker Jeukendrup and Michael Gleeson.

The first chapters of this impressive book explain some of the important principles of nutrition, including definitions and terms used to describe such appropriate intakes as the RDA – the recommended daily allowance. The RDA is for 97-98% of a certain age and gender group and is above the EAR, the Estimated Average Requirement, for that age and gender group, but well below the UL or Tolerable Upper Level. At the lower intake end of the bell curve, there is a risk of inadequacy, compared with a risk of adverse effects at the upper end of the intake bell curve.

Nutrition intake of Dutch athletes

While explaining some complex science in terms that can be understood by the non-scientist, the authors describe the biochemistry of exercise, emphasising the metabolic pathways that provide muscles with energy to perform physical work. This is followed by a consideration of how the energy content of foods and the energy needs of different sports can be estimated where we have selected the cycling related ones in this chart. 

Of particular interest to cyclists who have found it difficult to digest certain foods while on long rides, Chapter 5 is devoted to how food is digested and absorbed in the gastrointestinal tract and how sport can affect these processes. This chapter is preceded by an examination of the energy content of foods and their coefficient of digestibility, based on early pioneering work by Wilbur Olin Atwater (1844-1907), and known as Atwater factors. There is a useful reference to the US Department of Agriculture’s US Nutrient Data Bank (USNDB) which contains over 6000 items, higher than the 3500 items in the equivalent UK database.

Nutrition energy cost by body weight

This section is followed by the role of each of the major macronutrients – fat, carbohydrate and protein – as well as the importance of adequate fluid intake in relation to exercise performance.

Moving further into the book you realise that this 2nd edition focuses more on body composition and weight management than the earlier edition.

Often a problem for cyclists and runners alike is the problem of acid leaking from the stomach if the oesophageal sphincter is not working. This leads to frequent reports of heartburn and vomiting, which are upper gastrointestinal problems, and the urge to defecate, possible with stools or diarrhoea and bleeding which are lower gastrointestinal problems. Related symptoms are nausea, dizziness, stitch and the urge to urinate. The book gives some tips on prevention of gastrointestinal problems:

• Avoid milk products that contain lactose as even mild lactose intolerance while exercising can cause problems
• Avoid high fibre products on the day of the event and for a day or more beforehand
• Avoid aspiring and Non-Steroidal Anti-Inflammatory drugs (NSAIDs) such as ibuprofen prior and during events
• Avoid high-fructose foods and drinks as fructose is less tolerated than glucose and may lead to cramping and loose stools
• Avoid dehydration
• Practice new nutrition strategies (you need to buy the book!)

The authors point out that chewing food more while exercising reduces the size of food particles which also increases the rate of gastric emptying. The increased surface area of the well-chewed food increases the contact area of digestive enzymes, increasing the rate of digestion, and also mixes the food with saliva, which again helps the digestive enzymes.

Many long distance and event cyclists have experienced “the bonk”. Hypoglycaemia is caused if blood glucose levels drop below a critical level and the rate of glucose uptake by the brain is insufficient to meet its metabolic requirements. Hypoglycaemia is characterised by a variety of symptoms which include dizziness, nausea, cold sweat, reduced mental alertness and ability to concentrate, loss of motor skill, increased heart rate, excess hunger and disorientation. Fortunately, hypoglycaemia can be treated simply by carbohydrate consumption, although one section of Chapter 6 discusses “Supercompensation”.

Cyclists used to referring to the Cyclists Touring Club as the CTC who explain that the initials really stand for Coffee, Tea and Cakes, will be pleased to know that there is a chapter on Carbohydrate intake with a recommended intake of 70 g / hour which surely justifies a cake at elevensies and another mid afternoon, preferable surely to the authors' suggestion of 600ml of Cola or 1 litre of sports drink or 3 medium bananas; though their alternative of 120-150g of wine gums sounds quite good!

Another chapter which I felt to be of particular interest to the long-distance cyclist is chapter 9 on water and hydration.

The sensation of thirst is not perceived until a person has lost at least 2% of body weight through sweating.

With a water loss of just 3% of total body weight, blood volume decreases and exercise performance deteriorates. A 5% loss can result in confusion and disorientation and a loss of 10% or more can be life threatening. Water intake is typically 2.0 – 2.8 litres per day, where 1.0 – 1.5 is taken as water and the remainder in food. The authors show an example of cycling in 30°C temperatures causing a 2,000 ml per hour sweat loss and in an example of running in 20°C temperatures an endurance time of 77 minutes with no water intake being pushed out to 103 minutes when taking in 2 ml per kilogram of body weight every 15 minutes.

nutrition time trial hydrationThe image shows the time to complete a cycling time trial performed at the end of a prolonged exercise at 31°C in which either a small (200 ml) or large (1330 ml) fluid volume with either zero or 79g of carbohydrate was given. Little wonder therefore that the US and Canadian Dietetic Associations recommend drinking 500 ml 2 hours before exertion and 500 ml 15 minutes before prolonged exercise where the book is more specific in saying that it should be 6-8 ml per kilogram of body weight 2 hours before exercise to allow sufficient time for fluid absorption. During cycling, you should start drinking early and at regular intervals to prevent dehydration.

“The ideal drink for fluid replacement during exercise is one that tastes good to the athlete, does not cause gastrointestinal discomfort when consumed in large volumes (this rules out all fizzy drinks), promotes gastric emptying and fluid absorption to help maintain extracellular volume and provides some energy in the form of carbohydrate for the working muscles. Exercising subjects prefer cool, pleasantly flavoured, sweetened beverages, and the presence of sodium in the drinks seems to promote their consumption, probably by maintaining thirst. “ (P. 214)

Nutrition supplements are supplements and not replacements

A key statement is made that nutrition supplements are supplements and not replacements, which is obvious when written so simply, but which is a topic surrounded by confusion and fallacies.

“An abundance of information can be found in advertising and on the Internet about nutritional supplements. But most claims are not backed up by scientific studies and many of them are unrealistic of even impossible.” (P. 258)

Some nutrients are described as “conditionally essential” or “conditionally indispensable”, terms which were introduced in 1984 when it was recognised that some nutrients that were not normally essential became so under certain criteria. There are now more than 40 different nutrients which can meet those conditional criteria.

As a coffee addict, I was delighted to read that caffeine ingested 1 hour before the start of an exercise “bout” increased plasma concentration and improved performance and then let down by the statement that “caffeine in coffee was less potent than caffeine given in a capsule”. Still, the authors more than made up for it by saying that more research is needed before they can say anything conclusive about fasting training – going out for a bike ride prior to breakfast, which I have always thought should be a criminal offence! 


"Sport Nutrition" is published by Human Kinetics.

 

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