How do I lose weight?

Weight loss occurs when there is a chronic decrease on caloric intake, compared to our energy expenditure. It takes a loss of approximately 7700 calories to lose 1 kilo of body fat. We should all know the energy balance equation

Change in energy stores = energy intake – energy expenditure

What this equation does not take into account is the effect that weight loss will have on energy expenditure.

The energy balance equation can be expressed in a way that will account for the dynamic nature in humans.

Rate of change = rate of change of – rate of change of

energy stores energy intake energy expenditure

When body weight decreases due to chronically depleted energy intake, there is a compensatory decrease in the amount of energy used at rest, as well as during activity when body weight is being carried around.

What does this mean for weight loss?

It means that just decreasing your calorie intake by 250 calories a day below your energy intake will amount to a 1.6kg weight loss at some time. Once this occurs you will need to decrease your calorie intake again to lose weight.

The other side of the energy equation involves the expenditure of energy and includes basal metabolic rate, thermogenesis (heat production) and exercise.

Basal metabolic rate (BMR) is the rate of energy expenditure under standardised conditions. It is important because it represents 60% - 75% of total energy expenditure in the average sedentary person. It is proportional to fat-free mass and after age 20 it decreases approximately 2% for women and 3% for men per decade. Prolonged dieting also influences BMR by decreasing it.

Estimating Basal Metabolic Rate

Males Females

15 x Body weight [kg] + 716 = Calories 12 x Body weight [kg] + 716 = Calories

To determine your total daily calorie needs, multiply your BMR by the appropriate activity factor:

1. If you are sedentary (little or no exercise) : BMR x 1.2
2. If you are lightly active (light exercise/sports 1-3 days/week) : BMR x 1.375
3. If you are moderately active (moderate exercise/sports 3-5 days/week) : BMR x 1.55
4. If you are very active (hard exercise/sports 6-7 days a week) : BMR x 1.725
5. If you are extra active (very hard exercise/sports & physical job or training twice a day) : BMR x 1.9

Adjustments for age

30-35yr reduce BMR by 5%

36-50yr reduce BMR by 10%

51-69yr reduce BMR by 15%

70+ reduce BMR by 20%

The advantage of using exercise compared to caloric restriction alone in weight loss programs is that there is a higher proportion of body fat lost than lean tissue.

Execise and Energy expenditure

Physical activity constitutes the most variable part of the energy balance equation, being 5% to 40% of daily energy expenditure. A strenuous exercise session can expend 700-800 calories for a 70kg person. Do the maths. 800 caloris times five days equals 4000 caloris. Half a kilo! The important thing to remember is to not increase you food intake otherwise you won’t lose it