By Dr. Colin Michie of AUC Medical School
Dr. Colin Michie has worked as a paediatrician in the United Kingdom, Africa, the Caribbean and the Middle East. He is specialised in nutrition, haematology and infectious diseases. Now the Associate Academic Dean for the American University of the Caribbean Medical School in St. Maarten, his enthusiasm is training medical students and healthcare teams to ensure they deliver better value health care.
Words, like images, have power over all of us. They produce sensations and emotions way beyond their few assorted letters. “Calorie” is one of these words. Whether after a big festive meal or a weekend party-fest, we may worry that we have had too many calories.
“Calories” are mentioned daily in our newspapers, most often exhorting us to look out for them.
A French engineer is responsible for this use of the word. Nicholas Clement-Desormes, alias Nicholas “Calorie” to his colleagues, used the term to define how much heat energy (or “calor”) could be derived from coal to power a steam engine in 1824. The French led early steam engine design: Their work was employed in the ‘Rocket’ engine that won a race in England in 1930, and in Tom Thumb which lost a race against a racehorse in Baltimore in the same year.
Clement used “calorie” as it is Latin for heat. A single calorie would heat a kilogram of water by a single degree Celsius. “Calories” could then be used to measure the energy in other fuels, including that in foods. Using the term “calorie” for food energy was developed by Wilbur Olin Attwater in Connecticut. He developed a system for estimating how many calories were available in carbohydrates, proteins and fats.
His system has been refined to include the fibre content and the digestibility of foods. The term calorie was also redefined in the 19th century as the energy required to heat a gram of water. The term kilocalorie is now used by food nutritionists with reference to the energy to heat a kilogram of water.
In the nineteenth century, the system of joules to measure all types of energy was developed, the Joule being named after an English physicist. A kilojoule or kJ is the same as 4.18 kCal. Many food labels will therefore have energy measurements in both kCal and kJ.
Calories or Joules allow estimates of how we use them. A mobile ’phone app or a wearable device can do the math too. Asleep, we consume some 70 kCal an hour. This is sufficient for keeping us warm and breathing with a heartbeat and some dreaming, as well as radiance of heat from our face where warmed air leaves our airway.
Our brains use about 20% of this – more energy than our hearts, even if when we are asleep! So you are always paying your brain’s energy bill. This basal metabolic rate is a little higher in children, growing teenagers, pregnant women and those living in very cold environments. The base rate of your metabolism can be estimated from your weight and height.
The business of burning calories to stay warm is evident if one watches animals in nature. A crocodile is a cold-blooded reptile or ectotherm: it collects heat from its environment. A large croc only had to eat once a year. A lion or tiger has to eat well every two weeks, however, as they need to stay warm, consuming about 20 times more calories.
As relatively expensive, high octane warm-blooded primates, our physiology needs more food than an iguana. Food molecules are burned in the fuel compartments of our cells, the mitochondria. Mitochondria “waste” a proportion of all glucose supplied to them in order to generate heat. Heat is generated mostly by our internal organs; fat in our skin acts as insulation to keep it in (just as the furred animals’ pelt keeps heat in their body).
Walking consumes about 100 kCal an hour, swimming five times this, sprinting a 14-15 times greater consumption. On exercising, our organs generate heat – hence our sophisticated system of skin circulation and sweating. This means that as machines, we are not particularly efficient. An internal combustion energy can develop greater mechanical power or speed without producing quite so much wasteful heat.
During physical activity, muscles begin to consume energy, using oxygen.
Some energy is made without oxygen – anaerobic energy – as when you run up the stairs. Exercise will burn up food stored in the muscles first, then mobilise stores from the liver and then from body fat. Fat is a particularly effective and form of food storage – you will have to work for 3,500 calories extra, above your basal metabolic rate, in order to lose a pound of fat. This would equate to sprinting for two-three hours. Gym equipment should let you know the metabolic equivalent in kCal or kJ how much fuel your workout has used.
