Energy is life. Energy is power. And power is what we need to run, mountain bike, paddle, climb…
As the basal metabolic rate is the smallest amount of energy required by the body purely to keep you alive at rest, an additional quota will enable your body to react to environmental stressors like heat and cold by sweating and shivering to regulate body temperature and to perform work. In order to be active, we have to provide the body with fuel. Food is the body’s fuel. Carbohydrates, fats and, to a lesser extent, proteins1 provide the body’s metabolic systems with the means to generate energy, measured in kilocalories (kCal) and kilojoules (kJ)2, via numerous metabolic pathways. High intensity exercise, like marathon running, lasting longer than two minutes and up to three hours is predominantly powered by the aerobic glycolytic system. Here glucose is broken down in the presence of oxygen to generate power for muscle contraction.
For longer, less-intense ultra-endurance events, fats (lipids) – in the presence of oxygen – provide most of the energy for muscle metabolism. An efficient system, this aerobic lipolytic system delivers 9 kCal per gram of lipid3. During prolonged activity the use of fats relative to carbohydrates varies according to the fitness of the individual, the intensity and duration of activity and their diet.
Using and losing energy
As distance covered increases, an athlete’s running speed will decrease due to the mismatch between the demand for energy by the working muscles and the supply of energy from the metabolic breakdown of carbohydrates and fats.
In events over five hours fatigue results when muscle glycogen stores are depleted the body switches to using fats. But, as the supply of fats from the adipose tissue is slower than that of glycogen the athlete is forced to run slower, reducing his intensity and thus matching his energy expenditure with the rate at which fats can be metabolised.
| In 20mins a 70kg person will use: Running @ 5.5min/km – 272kCal Walking @ 8min/km – 167kCal Cycling @ 15km/hr – 134kCal |
Now consider a 130km adventure race where activity intensity is generally low to medium but the racers are active for the entire time. Using a sport wristwatch, which is able to accurately calculate the energy expended during activity from the heart rate, we can assess energy expenditure. At this particular event, the racer used 13 000kCal of energy to complete the 130km race in 20hr15min.
13 000kCal equates to 38 peanut butter sandwiches (or 112 bananas) – an impossible amount to eat in a 20hr period.
This would also mean that where a team is unable to replenish food stores from their race crates (unsupported event), or from their support crews, each individual would need to carry 3kg of food (at a 65:15:20 carbohydrate, protein and fat ratio) to fulfill their energy requirements in a 20hr period.
In reality, even with hi-tech food supplements, the racers don’t consume the necessary volume of food to obtain sufficient calories and yet they still manage to maintain performance for days on end. Thus, by using stored lipids, particularly where glycogen stores have been depleted and carbohydrate intake is insufficient, the body is able to fulfill its energy requirements for a reasonable period of time. Post-race munchies are common, serving to correct the accumulated nutritional deficit.
| ENERGY ON THE ICE: self-sufficient trans-Antarctic crossing In 1985 Robert Swan, Roger Mear and Gareth Wood undertook a one-way self-sufficient expedition to the South Pole, hauling sledges carrying all their food and gear. They covered the 900miles to the Pole in 70 days (approx. 13 miles/day) expending 5500-6000kCal/day. They each lost around 6.5kg in body weight.Ranulph Fiennes and Mike Stroud used these results to plan for their 1991 self-sufficient trans-Antarctic crossing. They had calculated 6500kCal per day to be their ideal intake as they would travel a greater distance to traverse the Antarctic continent from the Weddell Sea to the Ross Sea in 100days. Limited by weight they would be able to haul on their sledges and the volume of gear they would be able to fit on it, they decided to compromise on 5500kCal/day, accepting the resulting body-weight losses.But, they had not factored in that they would be hauling for more hours each day (10-12) than the members of the 1985 expedition. They had also assumed that they would be working at 40% of their maximum aerobic capacity and that the first 250-miles across the Weddell Sea ice-shelf would be easy. But, the ice was not smooth creating friction on their super-slick sledge skis and the ice shelf was split with great crevasses. Add elevation gain and driving winds in which they were struggling to do 1.5miles/hr and it is little wonder that their energy expenditure often exceeded 8000kCal/day. From Day 20-30, climbing up the Ice Shelf onto the plateau, Fiennes and Stroud expended 10670kCal/day and 11650kCal/day respectively, while ingesting only 5500kCal/day. By the time they reached the South Pole on day 68, they had each lost 20kg. Still, they decided to continue, tucking into an extra 1500kCal from their ration packs each day. They had planned to eat the additional quota until they reached the edge of the plateau, dropping to 3000kCal rations as they descended off the continent, hoping for winds, which they didn’t get, to aid their journey. Onto the Beardmore Glacier and half-rations, they grew steadily weaker. Along with body fat, they had also lost much of their muscle mass. On day 95, on the Ross Sea ice-shelf – having successfully crossed the Antarctic continent and over 1000-miles – they called for assistance and were airlifted to safety. They had each lost almost 25kg – a third of their body weight and the point at which starvation victims and hunger strikers die.« |
1 Protein building blocks, amino acids, are primarily used to build, repair and maintain the body’s protein-based substances (muscles, cellular structures, hormones, enzymes etc). But, if a sufficient amount of carbohydrates and lipids are unavailable, then this amino acid pool is used for energy – compromising the primary function of repair and maintenance. Endurance athletes do use a small amount of protein for fuel in their normal endurance activities. 2 Most commonly mentioned in books and on food labels in kCal. If you read something as Cal, it is likely to mean kCal. There are approximately 4.2kJ in 1kCal. 3 The aerobic glycolytic and proteolytic (protein) systems produce 4 kCal per gram of carbohydrate and protein respectively.