The engines driving your body are called muscles, and they create every movement within the body. The obvious movements are those that are caused by the skeletal muscles that combine with the skeleton to move your body around when you walk, run, or push other things around. There are lots of less obvious movements going on in your body.
Examples are the cardiac muscles creating the beating of your heart, or the smooth or “visceral” muscles causing the constriction and relaxation of the blood vessels such as arteries and veins, or the raising of your ribs to draw air into your lungs, or the pushing of food down your gullet and through your digestive system, or the squeezing of the glands of your body to release hormones, or the focusing of your eyes, or even the movements of your tongue when you’re eating or talking.
These movements are controlled by the autonomous, or automatic, nervous system. They are preprogrammed into your body so that at birth you automatically breathe, and swallow when food is put into your mouth.
The control of these processes is located well down the brain, even in the spinal cord itself.
Others have to be learned, such as the ability to control your bowels. As a kid you were probably fascinated by the process of learning to control your body, but as you get older you become unaware of these processes as they become more and more automatic.
The control of these functions is programed into the middle part of the brain, where the nerves of the brain biological neural networks or patterns that can be learned and reused.
As well as all this muscular activity, each cell of your body is like a little chemical factory constantly remixing the chemical soup inside its cell walls in response to the commands sent from the brain, or in response to its external environment.
Think of the cells in the hair follicles under your skin reacting to the hormones secreted by the gonads to mix up chemicals (amino acids in this case) that will combine to form the coarse secondary sexual hairs that poke through your skin. This chemical processing goes on every moment of every day of your life.
All this chemical activity requires energy, and the fuel for energy production is provided by carbohydrates, in the form of glucose, and fats, in the form of triglycerides.
The amount of fuel needed to supply all the energy needs for all this muscle and chemical activity in the resting body is called the “basal metabolic rate”, or BMR. BMR accounts for most of the energy used by the body every day, except for extreme elite endurance athletes, such as those in the Tour de France.
Every body function requires energy. The amount of energy required for basal metabolism peaks during adolescence and then decreases with age.
This makes sense if you think about the energy required by a growing adolescent to create bigger and stronger bones and muscles, and fuel the chemical processes triggered by the hormones flushing through the body of the adolescent who is growing a new adult body.
It takes five to 10 years to learn to efficiently use this new adult body, and efficiency increases with age. So a 35-year-old will require less energy to perform a particular task than a 25-year-old. Practice has made them more efficient.
The decrease in metabolic rate is mainly associated with a decrease in lean body mass, decrease in body weight, and increase in body fat. BMR can be measured directly by measuring the amount of heat produced by the body. Every time the metabolic fuels of glucose and triglycerides are broken down to release energy, heat is also released.
This heat production is measured in units called “calories”, or more recently “joules”. An instrument the size of a room is required to measure the changes in heat of the body, and is called a “calorimeter” or “a measurer of calories”.
The more energy being created in the body, the more heat is released. So your body heats up after eating a meal, as energy is required to break down, digest, and absorb the food.
Locking someone in a sealed room for days on end is not easy, so usually the metabolic rate of the body is calculated indirectly from the amount of oxygen being absorbed and utilised by the body. This is called measuring the “oxygen uptake”, or “volume of oxygen (VO2)” of the body.
This is a little easier, as all that’s required is to collect and store samples of inspired and expired air from the lungs, and to measure the total amount of air breathed in and out per minute.
The collected air is compared with room air to see the difference in oxygen content. The reduction in oxygen in the expired air obviously represent the amount of oxygen absorbed by the body to create aerobic energy.
These surveys clearly show that VO2 decreases linearly with age, both at rest and during work. If VO2 decreases, potential metabolic rate decreases.
The implication is that as you age you require less and less energy intake in the form of food. If you don’t reduce your food intake as you age, you’ll get fatter, no matter if you remain active. As Shakespeare says — “Leave gormandising; know the grave does gape for thee thrice wider than for other men”.
It seems to be at about age 35 that most people notice that even though their exercise and nutrition habits haven’t changed, they’re getting fatter and fatter.
This can be accounted for by a small reduction in the energy requirement of BMR, meaning that you now have a small energy surplus, which, as your body is so efficient, it will store as body fat for later use.
A little stored every day builds up over the months and years to become noticeable one day when those pants get a bit too tight.
The theories that try to explain this decrease in metabolic activity with ageing include a decrease in physical growth and repair, decrease in physical activity, and decrease in total muscle mass of the body. There is no real “right” theory.
All these factors contribute to a decrease in metabolic rate with age, and a need for increased activity. As you age it becomes more and more important to keep your body efficient with 20 minutes a day of physical activity that gets you breathing faster but still able to talk easily, and to have a nutritious but less energy-rich diet.