There is still much to be understood about maintaining weight - a complex issue determined by physiological, psychological and social factors.
We know we can maintain our weight as long as energy inntake equals energy output. But more often than not, psychological factors are responsible for changed patterns of eating resulting in large losses or gains in weight.
Energy balance
The chief factor in determining body weight is the amount of energy stored as fat. This depends on the energy balance, shown by a simple equation:
| Energy stored = energy intake - energy output |
Stable weight occurs when the energy balance is such that energy intake from foods equals energy output.
This balance can be tightly controlled, without conscious regulation, as the energy consumed through food is converted in the body to heat and movement energy.
We can compensate for overeating by periods of restriction. Indeed the animal kingdom uses this balancing act to perfection; migratory birds store up just enough food reserve as fat for their migratory flight.
Yet the situation is not so predictable and simple as the equation. We have habits, desires and emotions that alter the balance, swinging it into either a positive or negative state.
Energy intake
Our recommended energy intake depends on age, sex, and lifestyle. Once the body size, tissue composition, and level of physical activity have been taken into account, the energy requirements of the human body are remarkably constant.
In general, women require 2000 2500 kcal per day, and men require 2500 3000 kcal, the variance largely due to the different levels of energy expenditure.
| Food components | Calorific content |
| Fat | 9 kcal/g |
| Carbohydrate | 4 kcal/g |
| Protein | 4 kcal/g |
| Alcohol | 7 kcal/g |
Heat released when 1 g of the substance is completely oxidised. |
For details of how food provides energy, see the Essential nutrition topic.
Energy expenditure
We expend energy in three different ways, through our basal metabolism, thermogenesis, and physical activity.
Basal metabolic rate (BMR)
| BMR is the energy required for all resting activities, including breathing, maintenance of body temperature, nerve cells firing, heart pumping etc. |
Our BMR accounts for 60-70% of our total energy expenditure, a large proportion being used to maintain our electrolyte concentrations and for the synthesis of protein and other molecules.
BMR varies with age, sex and body weight. Men have a higher BMR due to their higher level of lean muscle tissue, which is metabolically active. BMR decreases with age in both sexes, as the amount of lean tissue decreases.
Finally, the more we weigh, the higher our BMR due to the metabolic cost of supporting excess body weight (heart pumping harder etc).
How to calculate your BMR Daily BMR = weight (kg) x 24 (hr) For example: A person who weighs 60kg will have a daily BMR of 1440 kcal at complete rest. This is worked out by the following calculation: 60 x 24 = 1440 kcal |
Thermogenesis
Thermogenesis is the heating of our body by the released energy from food digestion, resulting in an increase in metabolic rate after a meal. The degree of thermogenesis is approximately related to the size of the meal consumed.
Thermogenesis accounts for 10% of our total energy expenditure, the energy being spent on secretion of digestive enzymes and the transport and storage of nutrients.
BAT Theory - there is no evidence to support the theory that some people have higher levels of thermogenesis, so greater energy expenditure, due to a larger proportion of brown adipose tissue (BAT).
Physical activity
Finally, physical activity accounts for 15-30% of energy expenditure and is the factor under our control. From walking to gardening, dancing to swimming, different activities burn off a different number of calories, depending on intensity and duration.
The following chart shows the amount of calories that can be burnt in one hour whilst doing different activities.
| Activity | Female
Calories / hour | Male
Calories / hour |
| Bowling | 207 | 270 |
| Cycling: moderate | 192 | 256 |
| Cycling: hard | 507 | 660 |
| Domestic work | 153 | 200 |
| Driving | 108 | 144 |
| Eating | 84 | 112 |
| Gardening: active | 276 | 368 |
| Golf | 144 | 192 |
| Ironing | 120 | 160 |
| Office work: active | 120 | 160 |
| Rowing | 600 | 800 |
| Running: moderate | 444 | 592 |
| Running: hard | 692 | 900 |
| Sewing and knitting | 84 | 112 |
| Sitting at rest | 84 | 112 |
| Skiing | 461 | 600 |
| Squash | 461 | 600 |
| Swimming: moderate | 230 | 300 |
| Swimming: hard | 480 | 640 |
| Table tennis | 300 | 400 |
| Tennis | 336 | 448 |
| Typing | 108 | 144 |
| Walking: moderate | 168 | 224 |
20/05/2009