Within any group of animals or people there are known to be wide variations in the degree of metabolic efficiency, with some maintaining their normal body weight whilst eating twice the quantity of another. This individuality factor makes it difficult to give specific recommendations of quantities to be eaten, and causes guidelines to be at best vague. We have what scientists term a 'set point, which is the weight our body tries to maintain, whatever the dietary pattern, whether this involves increased or decreased calorie intake.

Weindruch and Walford suggest that the ideal weight loss to aim for when following a calorie restriction diet is one which causes not less than a 10 per cent, and not more than 25 per cent, reduction in the subject's own set point.

How can we know what our 'set point' is?
According to Weindruch and Walford:

The set point for individuals whose body weight has been stable since ages 20 to 30 is precisely that body weight. For those who have gradually gained body weight with age, the appropriate set point in terms of dietary restriction regimen is uncertain.

Clear guidelines

1. If your weight has remained stable since your 20s you need to ensure that any dietary intervention you decide to adopt, using calorie restriction with full nutrition, achieves not less than a 10 per cent and not more than a 25 per cent reduction from your normal weight.

2. If you have gained weight steadily since your 20s the reduction you need to aim for, should you decide to apply this dietary approach, is one which takes your body weight to a level which is not less than 10 per cent and not more than 25 per cent below the average weight you were when in your 20s.

What are your energy needs?
A further consideration is also necessary: how efficient is your use of energy? Evidence of wide variations in calorie intake exist between people of the same size and weight, efficiently performing much the same rate of work. Even an individual's own energy requirement may vary from time to time by fairly wide degrees (up to 30 per cent higher or lower requirement of energy-based food) for no apparent reason, and with no apparent ill-effects in terms of weight change or functional ability.

A number of human studies have shown that energy requirement adapts to the diet being followed. Research reported on by Weindruch and Walford confirms that a steady reduction in intake of calories produces a lowering of metabolic activity (and therefore of required energy from food) which tends to stabilize when the right level of food intake is achieved to allow for the sort of weight loss described above. What the calorie intake should be really needs individual assessment, they maintain, but nevertheless they do give broad guidelines: 'It seems probable that for a US population [it would be no different in the UK], a daily intake of around 1,800 to 2,000 calories would induce a very gradual body weight loss which we surmise to be the best procedure.'

They suggest that an average individual would adapt in this way, and that once body weight loss of 10 to 25 per cent (from 'set point' weight) had been achieved, there would be no further loss of weight on a calorie-restricted pattern involving 1,800 calories per day. On such a diet there should be no ill-effects such as tiredness, apathy, weakness or swellings, all of which would indicate the likelihood of nutritional deficiencies in the diet, something which clearly should be avoided if and when calorie restriction is introduced.