Countless websites extol the benefits of fasting. The theory is that it allows the body to stop its usual metabolic treadmill and concentrate on rebuilding parts that are begin-ning to wear.
But how true is this? There is little good scientific evidence. What's available involves animals and so may not apply to humans. The limited evidence for pure fasting (with only water) shows immediate and pro-found effects on the body. But the older you get, the less good these effects may be for you.
A water-only fast can bring on rapid weight loss, loss of total body protein and cell mass, and a severe negative nitrogen balance. In animal studies, certain bodily processes don't revert to normal on reintro-ducing food. In rats fasted for two days and then re-fed for 24 hours, there were significant decreases in leptin, insulin, glucose and other factors, and an increase of free fatty acids. On reintroducing food, blood glucose and insulin reverted to starting levels in all age groups, but the free fatty acid and leptin levels failed to return to normal in older rats (Gerontology, 2005; 51: 357-62).
Leptin appears to signal the brain that there is fat in the body, and fasting can decrease blood leptin by 58 per cent (J Clin Endocrinol Metab, 2000; 85: 2499-503). Without leptin, the brain believes there is no fat in the body and this, in turn, can stimulate the appetite to eat more, once the fast is broken.
Other studies show that fasting and reintroducing food show up age-related changes in carbohydrate and fat metabolism. Fasting also causes changes in blood corticosterone-the key hormone of a prolonged stress response-and alters kidney activity (J Physiol Pharmacol, 2006; 57 [suppl 6]: 77-84; Mech Ageing Dev, 1997; 97: 237-48). What's more, it can tempor-arily lower thyroid hormone product-ion (Can Med Assoc J, 1968; 98: 1031-3).
Fasting is particularly problem-atical for diabetics, who may develop high blood glucose if they don't take their medication or are less physic-ally active than usual.
However, when 16 healthy subjects (compared with 16 matched con-trols) underwent a 44-day pure fast, they lost 25 per cent of their body weight. On reintroducing food, red cell counts dropped as blood plasma increased, a condition often seen with an enlarged spleen. They also had too-low blood phosphates-seen in hyperparathyroidism, rickets or osteomalacia (weak bones due to a lack of vitamin D), and kidney dys-function-and slight fluid retention, with depletion of vitamins B1, B12 and B6. Blood levels of fatty acids and zinc were abnormally high, with abnormal liver function after a week. Finally, levels of the hormone orexin, which controls the sleep-wake cycle and food cravings, were altered (Eur J Endocrinol, 2007; 157: 157-66).
Unlike fasting, there's no doubt of the health benefits of restricting calories while continuing to consume nutrient-dense foods, but avoiding refined carbohydrates and processed foods. Studies in many animal species show that lifespan is extend-ed by restricting normal caloric intake by up to 40 per cent (Weindruch R, Walford RL. The Retardation of Aging and Disease by Dietary Restriction. Springfield, IL: C.C. Thomas, 1988), and rodents lived 50 per cent longer (Ann NY Acad Sci, 2000; 908: 180-98).
Other health conditions, and risk factors for cardiovascular and degenerative diseases, may also be reversed. In a small two-year study, blood cholesterol was reduced by 38 per cent and body mass by 15 per cent, while blood pressure was significantly lowered (J Gerontol A Biol Sci Med Sci, 2002; 57: B211-24). Indeed, similar results were seen after only 10 weeks of caloric restriction (Eur J Clin Nutr, 1994; 48: 138-48).
Restricting calories appears to protect cells against the effects of ageing, preventing damage due to higher energy expenditure and allow-ing cellular repair (Biomed Pharmaco-ther, 2003; 57: 203-8), while avoiding oxidative stress. And, at least in animals, calorie restriction also appears to prevent cancer (Med Sci [Paris], 2005; 21: 175-80; Cancer Res, 1945; 5: 616-25).
So far, we cannot show, except in isolated reports, that restricting food altogether leads to greater longevity. What the evidence does suggest is that, for people over 40, cutting out food may actually be detrimental (J Gerontol A Biol Sci Med Sci, 2001; 56: 45-54).
- Start slow and end slow. Cut down on food gradually by eating light protein, cooked vegetables and rice. When you break the fast, do the same in reverse.
- Consider a juice fast (but not orange juice). Juice provides calories and important nutrients. Choose either raw fresh fruit or vegetables.
- Ensure that you drink enough water-in fact, drink more than usual so that your urine isn't dark.
- Try one meal a day in the mid-afternoon in place of a fast (say, fish or chicken with steamed vegetables or rice). The rest of the day, consume water, juice, teas, and fresh fruit and vegetables.
- Move on to a fast from one meal a day, then on to a fast for five days and, finally, for a week or two.- A light dinner followed by a 12- to 14-hour break from food can constitute calorie restriction and a daily mini-fast.