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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