MRC Elsie Widdowson Laboratory

EWL Impacts: Energy requirements for extreme exercise

The energy expenditure of a round-the-world solo sailor and British Army Infantry soldiers has been measured using techniques pioneered by scientists at MRC Human Nutrition Research. Their energy expenditures over weeks of high exertion were far greater than the energy in their food – explaining the weight loss they experienced and providing guidance for improvements in rations.

SONY DSCArduous solo sailing around the world

The scientists from EWL measured the total energy expenditure of Denise ‘Dee’ Caffari MBE as she completed the Vendèe Globe round-the-world single-handed sailing race. Solo yacht racing is physically demanding and arduous, and Dee is the only woman to have sailed solo, non-stop, around the world in both directions. In the 99 days of the Vendèe Globe race during the winter of 2008-09, Dee travelled 34,600 km in rough weather conditions and was one of only 11 out of the 30 competitors who finished the race.

Doubly labelled water technique

The doubly labelled water (DLW) technique aims to determine how much of our body’s fuel is converted into energy, called the metabolic rate, using stable isotopes. The technique replaces the oxygen and hydrogen in water (H2O) with non-radioactive stable isotopes – deuterium (2H) and oxygen-18 (18O).

During the test, the labelled water is drunk and enters the body and the ratio of labelled isotopes to ‘normal’ unlabelled isotopes in the body’s water can be measured in samples of urine (or saliva).

The oxygen-18 is lost from the body in two ways:

  • The water is used when the body burns fuel (food) to create energy – called ‘metabolism’. During metabolism, the oxygen in the water is converted into carbon dioxide (CO2), which is breathed out from the lungs.
  • Through water loss, such as in urine and sweat.

So that the rate of water loss through urine and sweat can be taken into account, the labelled hydrogen is measured in a urine sample.

Once water loss is excluded, the rate of loss of the labelled oxygen from the body’s water will tell us what the person’s metabolic rate was – and therefore what their energy expenditure has been.

This stable isotope technique was developed by scientists at the Dunn Nutrition Unit (MRC HNR’s predecessor) in the 1980s.

Total energy expenditure from such racing was not well-known, and was needed to inform dietary planning to sustain the sailors’ bodies at peak performance.

Dee’s energy expenditure was measured with doubly labelled water testing kits (see box), which she took on the voyage and self-administered twice, during the first and sixth weeks of the race. At the end of the race, the urine samples were analysed at HNR in Cambridge.

Her energy expenditure was 3,800 calories per day, which significantly exceeded the 3,000 calories per day energy content of her food rations. This explains why she lost 8 kg during the race – more than 9% of her body weight.

However, her lean body mass was measured before and after the race and it was maintained, therefore the weight loss was mostly attributable to loss of body fat. It was concluded that the diet provided for her by the race support team, whilst not being adequate to maintain her body weight, was adequate for the level and duration of physical activity.

Battle training with the British Army

Forty soldiers completing the British Army Section Commander’s Battle Course were tested to find out if their rations were adequate for their energy expenditure. The UK Ministry of Defence funded the research and their energy expenditure was measured using the DLW technique by EWL in Cambridge.

The testing was carried out during eight weeks of high physical intensity tactics training, involving infantry soldiers learning and practicing the field skills necessary to be a Section Commander.

During the training, their energy expenditure ranged from 4,700 calories per day in weeks two to three of the course, up to 5,100 calories per day in weeks six to seven. This was in contrast to the army rations, which contained only around 3,400 calories per day at the earlier time-point and 3,700 calories per day at the later time-point – which would mean in theory that the soldiers had a calorie deficit of more than 1,300 calories per day.

OGL Ministry of Defence 2013The researchers found that the soldiers lost on average 5 kg during the 8 week course, and by measuring their weight loss and body composition, the researchers found that their actual energy deficit was only 640 calories per day. This revealed that the soldiers were choosing to supplement their rations with approximately 700 calories per day.

It was concluded that the rations provided during the course should be modified to have a higher energy content, such as by introducing a fourth meal, increasing portion size, carbohydrate and protein content, and decreasing fat content.