Article by Ruth McKean, Sports Nutritionist

3. Optimal Fuel - fat, carbohydrates or protein?


Initial exposure to altitude appears to cause an increase use in blood glucose (carbohydrate) as fuel at rest and during exercise. Muscle glycogen use (stored form of carbohydrate in muscle) is not decreased but the reliance on fat as a substrate declines. It has also been shown that even after longer-term exposure to altitude, glucose clearance during exercise is increased by approximately 25%. In other words, fatigue and low blood sugar levels will occur more quickly at the same intensity of activity at altitude compared to sea level if increased carbohydrate needs are not met.

In activities such as skiing, carbohydrate is the main fuel and that glycogen depletion will occur after a few hours activity. Although the level and intensity of the skiing will have great effect on the rates of carbohydrate use.

In addition to reducing fatigue and preventing low blood sugar levels it has been shown that a diet high in carbohydrate resulted in fewer symptoms of acute mountain sickness. Studies have shown that a high carbohydrate intake can reduce the effects of altitude by 300m - 600m at a height of 4000m and 5200m respectively. This is most likely due to the fact that carbohydrate requires 8-10% less oxygen for use, compared to fat and protein.

Therefore, the ideal fuel at high altitude is carbohydrate. This is often not the most climber-friendly answer to those who have to carry all their food as carbohydrate, which is not as energy dense as high fat foods. Carbohydrates and protein provide 16kJ (4 kcal) of energy per gram compared with 37kJ (8 kcal) of energy per gram for fat. As a result, you have to eat more carbohydrate and therefore carry more to obtain the same amount of energy as fat. Since appetite is often reduced and many carbohydrate foods are bulky it would be sensible to ingest carbohydrate dense food when exercising or competing at altitude. However, it is perhaps more sensible whilst on long treks to carry foods highest in energy (such as carbohydrate electrolyte drinks) as well as some carbohydrate dense foods. Extreme situations such as polar explorations (those seen by Captain Scott and Mike Strouds & Ranulph Fiennes) have demonstrated the importance of using energy dense foods in an attempt to maintain body fat and minimise weight loss. However, such feats are extreme and neither Scott, who died, nor Strouds & Fiennes finished malnourished and near to death managed to realise their huge energy expenditure.

Although carbohydrate would appear to be an important fuel at attitude it should be emphasised that the most important of all nutritional considerations at high altitude is that of maintaining an energy balance. Not only may an energy deficit enhance the risk of acute mountain sickness (see section 6), risk of dehydration (see next section 4) it also results in an augmented use of protein as a metabolic fuel (unfortunately not fat). A negative protein balance shows this when energy intake is inadequate at high altitude.

Hence, it is recommended to increase carbohydrate and protein intake if the intensity of exercise and better performance at high altitude are desired. Again because appetite is often suppressed it is advisable to eat foods which contain both protein and carbohydrates such as, flavoured yoghurts, milkshakes, smoothies and liquid meal supplements/meal replacements, some cereal and sports bars.


Carbohydrate Dense Foods

Many of these foods may be useful for those who need to carry their own food supplies since they provide a compact source of fuel. It may also help to present food in smaller portions such as sandwiches cut into small pieces to encourage nibbling.

  • Honey, jam, syrup
  • Sports bars and sports gels
  • Energy tablets (dextrose tablets)
  • Boiled sweets, sugary lollies
  • Powdered versions of sports drinks (sprinkled over breakfast cereals and pasta etc…)
  • Energy drinks
  • Liquid meal supplements
  • Dried fruits
  • Rice & pasta
  • Malt loaf
  • Mint cake
  • Bagels

CHO dense/rich foods with a high fluid content may be more appealing to those who have reduced appetites. Although, some of these foods are likely to only be appealing and/or available if food is cold or readily accessible. Perhaps more useful to athletes training at altitude rather than those who have to carry their own food.

Carbohydrate dense foods also high in fluid

  • Fruit smoothies
  • Liquid meal supplements
  • Commercial high CHO drinks
  • Sports gels
  • Flavoured yoghurt
  • Ice cream
  • Milk shakes
  • Frozen yoghurt
  • Tinned fruit in syrup

 

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