Some dietitians argue that the diet outlined was deficient, unlike the isonutrient diets of Drs Weindruch and Walford. Dr Bircher would disagree, saying that the high enzyme content of raw food compensates for an apparent lack of protein or other nutrients. The fact is that many people have survived in excellent health for many years on just such a diet.

Where does fasting fit into all this?
Later in this book, after evaluating the life extension effects of animal studies, I suggest strategies which mimic these experiments and which you can put into daily practice. For now, the purpose of this chapter is to highlight a different aspect of the potential which this knowledge offers us, the use of fasting and dietary modification as a means of health promotion, rather than with the aim of life extension.

Fasting is not starvation
During starvation (once fatty tissue has been used up) the body draws on its own essential protein reserves for fuel, whereas in fasting it is the non-essential fat and protein stores which are used for this purpose. Clearly, if fasting continues for too long a period, starvation will take over, but no such risk exists when fasting is used according to certain strict guidelines which I will explain.

One definition of fasting is of a period during which no solid food is taken and when (ideally) water only is consumed. Fasting in the treatment of chronic disease has been used for centuries, and research into its effectiveness has been carried out for at least 100 years. ¹ A number of university studies have been conducted which show quite clearly just what happens to the various body systems when humans and animals fast.^2,3 In some of these strictly controlled studies prolonged fasting (months in some cases) was shown to produce no harmful effects, only benefits. Some of the diseases which have been found to improve with fasting are listed at the end of this chapter.

What happens to the body on a fast?
The body's basic metabolic rate (BMR), which is an index of the rate at which the body burns fuel to create energy, is seen to slowly reduce, by around one per cent daily until it stabilizes at 75 per cent of its normal rate.⁴ In animal studies a number of ways have been found to slow BMR, induding dietary (calorie) restriction and the cooling of core temperature (such as occurs during hibernation)⁵ and indeed one of the major markers of animals and humans whose potential life spans are extended by use of reduced calorie intake is a slowing down of the rate at which they 'burn' oxygen; in other words their BMR slows down. The effect of fasting, in slowing BMR, is therefore one way in which it promotes longevity. Just how this is achieved is of some importance for it brings into play a degree of adaptation in which energy is conserved, making the process more 'thrifty'. Weindrudh and Walford have shown that longevity is directly linked to efficient energy consumption ('thrifty' as opposed to 'burner' animals and people).

When fasting begins, the first source of energy which is tapped is the stored glucose in the liver (glucose is vital for brain function and red blood cells). When its own stores are used up, and whatever remaining food in the digestive tract has been used as an energy source, the body begins to synthesize more glucose, taken as stored glycogen from muscle tissues. After about 24 hours these sources will be depleted, and free amino acids and protein, and later fat stores (triglycerides), from various nonessential sites will be turned into energy by the liver and the kidneys.