Mountaineering: hill walking training

Diet risks for hill walkers who cut back on rations

Recreational hill walkers who hold back on their rations may be at increased risk of injury as well as cold and fatigue, according to a combined British and Canadian study.

Hill walking is one of several kinds of prolonged exercise that tends to induce a negative energy balance. Studies on military personnel have suggested that only severe energy deficits are likely to impact on performance. But hill walking is a recreational activity that attracts a wide range of participants, of varying age and fitness levels, for whom energy deficits might have more drastic effects.

The purpose of the current study was to compare the effects of high and low energy intakes on some relevant responses that are important to the safety of hill walkers, including thermal stress, performance, mood and blood glucose levels.

Sixteen male subjects – all active and experienced hill walkers – completed a strenuous 21k hill walk under two different dietary conditions:

  • A low-energy intake, consisting of snack foods to the value of 616 calories;
  • A high-energy intake, consisting of similar foods to the value of 3,019 calories.

Subjects consumed a standardised 595-calorie breakfast before setting off on both walks. Both the in-walk diets included macronutrients in similar proportions and were divided into three equal amounts, with subjects encouraged to consume one portion by 7k, the second by 14k and the final portion by the end of the walk. In both conditions, subjects were instructed to consume about 400ml per hour of water.

All subjects completed both walks in a mean duration of 7hrs 28mins, with no differences between the different energy conditions. The differences in time to complete the walk were due mainly to variations in weather conditions and terrain, with 10 subjects experiencing very sustained wet and windy weather during both of their walks.

The key differences between the two dietary conditions were as follows:

  • There was a clear trend towards lower core temperature during the low-energy intake condition, but this reached significance only in the wet and windy conditions;
  • In a range of performance measurements carried out before and after the walks, the only significant difference between the energy intakes were improved reaction time in the high-energy group and deterioration in balance in the low-energy group;
  • During the low-energy condition, nine of the 16 subjects showed marked changes in behaviour – including withdrawal, slowing down and, in a few cases, aggression during the walk, especially after the first 10k. These symptoms are generally considered to be among the early signs and symptoms of exposure and hypoglycaemia (low blood sugar) and were present predominantly in the adverse weather conditions;
  • Four of the subjects on the low-energy intakes sustained minor injuries during the walk. This didn’t happen to anyone during the high-energy condition;
  • Blood tests post-walk showed increased mobilisation of fat, a greater hormonal ‘stress’ response and significantly lowered blood glucose concentrations in the low-energy condition.

The researchers suggest a link between glucose depletion and the injuries that occurred in the low-energy condition. ‘There is some evidence,’ they point out, ‘that low muscle glycogen levels, as may be anticipated during the low-energy intake, are associated with increased injury risk in alpine skiing, especially in recreational skiers. The explanation is that glycogen depletion of the fast-twitch [muscle] fibres will limit the ability to develop a high muscle tension in a short period of time (needed to correct false turns or inadequate timing). Physical inability to correct movements will in time lead to increased injury risks. This relationship between low muscle glycogen levels and a physical inability to correct moves could well be mediated through balance impairment.’

They conclude that ‘subjects consuming a low-energy intake may become compromised in their ability to operate safely in the mountainous environment. Although the impairment in the low-energy intake…was somewhat moderate, this impairment may well be an influencing factor in susceptibility to both fatigue and injury while outdoor recreational activity is pursued.’

J Appl Physiol, vol 94, pp11075-1083, 2003