ENERGY BALANCE, FAT, OVERWEIGHT AND BODY CHEMISTRY

Energy expenditure

Energy expenditure ('Calories used') is the second aspect of energy balance, and altering this is also important for weight control. It is well established that increasing energy expenditure (through activity) at the same time as decreasing energy intake (through calorie restriction) is more likely to result in significant weight loss than doing either alone. Increasing activity is also a key factor in ensuring that weight lost is not regained over time.

Physical activity is the most important of the three forms of energy expenditure:

• resting metabolism
• thermogenesis
• physical activity

as it is the only one which is entirely under one's own control. However, resting metabolism and thermogenesis are not fixed, even though we cannot easily will them to change, and make up a large proportion of daily energy expenditure. So it is worth considering what factors may affect them.

Resting metabolic rate (basal metabolic rate) is the measure of energy expended in keeping our body processes working normally in a state of rest. It varies by up to about 10 per cent from one individual to another, mostly for genetic reasons, but does not differ much from day to day. It accounts for about 60-75 per cent of the body's daily energy expenditure.

Resting metabolic rate is mostly determined by body size - in particular, the fat-free mass of the body. Fat-free mass is made up of skeletal muscle, vital organs (heart, brain, lungs, kidneys, etc) and other tissues such as bones, skin and so on. Although skeletal muscle makes up about 40 per cent of body weight, it accounts for only about 20 per cent of energy expenditure in the resting state. By far the greatest amount of energy (about 60 per cent) is used by the vital organs, which make up only some six per cent of body weight. This is because these organs are very active in maintaining body processes, even at rest.

When weight is lost through calorie restriction alone, up to 30 per cent of the loss can be due to loss of lean muscle mass, which is part of fat-free mass. This tends to reduce resting metabolic rate, making continued weight loss more difficult. Weight training (or resistance/strength building activities like carrying shopping, digging in the garden, etc) during dieting helps to preserve or even increase muscle and may counteract this reduction in resting metabolic rate. Thus, even though it is not under direct voluntary control, this part of energy expenditure could, in principle, be influenced to help weight loss or prevent weight regain.

Thermogenesis is the name given to a variety of processes that expend energy through heat production. For example, heat production increases noticeably:

• following the consumption of a meal (diet-induced thermogenesis)
• after consuming caffeine, alcohol and nicotine (drug-induced thermogenesis)
• on exposure to cold, for example through shivering
• in response to psychological stress
• through changes in muscle tone
• during activities of daily living known as nonexercise activity thermogenesis

This last category has been found to be quite important, with people who show a substantial amount of non-exercise activity expending as much as 350 Calories/day more energy than people with a very sedentary lifestyle. While most of the factors influencing thermogenesis cannot be easily exploited to enhance weight loss, nonexercise activity (even something as simple as standing more, rather than sitting, using stairs rather than a lift, and so on) is something that can potentially be modified, and which could have a worthwhile effect on daily energy expenditure.

Physical activity is the third component of energy expenditure, and is the most easily modified. Physical exercise does not necessarily mean going to the gym or going jogging, which some people find an unattractive prospect, but covers a very wide range of activities that increase expenditure of energy, including:

• gardening
• dancing
• swimming
• playing sports
• brisk walking
• cycling
• roller/ice-skating
• doing housework

and many other activities that are sufficiently vigorous and protracted to raise the pulse rate for an extended period.

Most people can find one or more activity to carry out on a regular basis that will involve the expenditure of a significant amount of energy. For those with very inactive lifestyles, the first steps to take may involve simple substitution of one activity for another, such as:

• walking to the shops instead of taking the car
• using the stairs instead of the lift
• getting off the bus/train a couple of stops early and walking the rest of the journey.

While they are helpful initially, such changes are unlikely to bring major weight loss, and a more substantial programme of physical activity will be needed, combined with a reduced calorie diet.

Many studies have shown that aerobic exercise by itself does not produce significant weight loss - it must be combined with a lower calorie intake in order to have a worthwhile effect. Such regular exercise has, however, been found to be important in maintaining a stable weight after the initial weight loss period.

Repeated aerobic exercise over a period of weeks can affect the relative proportions of fat and carbohydrate burned during exercise. In non-overweight people, training increases the amount of fat burned and reduces the amount of carbohydrate burned at a given exercise intensity. Studies in overweight people have given less certain answers, but some show that endurance training increases the amount of fat burned, especially in men and women with an apple-shaped fat distribution.