Running from Shadows

My Marathon des Sables . . . and beyond

The Calorie Conundrum

The perennial question: how many calories are needed to run, walk or crawl through 153 miles in 7 days?

If I were to wake up and then simply lay in bed for 24 hours staring at the ceiling the following formula (there are other variations) gives me a good idea of how many calories I would need just to maintain my body weight (i.e., my basal metabolic rate or BMR):

BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) – (6.8 x age in years)

though the formula for women is slightly different:

BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years).

For me, the formula gives 1896 calories per 24 hour period.

The Harris Benedict Formula is then applied to the sum above to give a total calorie requirement dependent on the level of physical activity:

multiply by

  • 1.2 for little or zero exercise;
  • 1.375 for light exercise;
  • 1.55 for moderate exercise (regarded as 3 to 5 days a week);
  • 1.725 if very active (6 to 7 days per week); and
  • 1.9 if being very active and working in a physically demanding job.

I reckon I fall into being “very active” when starting the Sahara Race (Jordan) on Sunday, so I get a daily calorific need of approximately 3,271 calories per day (BMR 1896 calories x 1.725).

And here’s the conundrum: if I were to take that literally I would start the race with around 23,000 calories. If I were able to average out at 4.5 calories per gramme of weight that would mean I’d start carrying 5.1 kilos in weight just for food! By the time I add all the other kit in my rucksack, plus at least 1.5 litres of water at the start, I’d be starting with a rucksack weight in excess of 10.5 kilos.

Too much for my liking!

The race-legal minimum for the entire event is 14,000 calories, as it is with most desert multi-stage races. For the Marathon des Sables 2012 I took something like 16,800 calories.

I’ve spent the weekend biting the bullet to trim my calories for the event down to 14,248, which weighs a tad over 3.3 kilos. Theoretically, with a pound of body fat containing around 3,000 calories, I should only lose around 3lbs in weight but, with inevitable added dehydration, it’s likely to be more. On the Marathon des Sables I lost around 7lbs (or 3 kilos) in body weight.

We shall see!

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