Lenten abstainers who are counting the days to Easter and a return to various indulgences may be disappointed if they think their restraint will have any beneficial long-term effects on their health. For although there may be emotional and spiritual rewards to self-denial, any physical advantages disappear quickly. As nutritionist Dr Toni Speer puts it: "You can't bank healthy eating. You've got to do it all the time." And the same principle applies to those other two favourites: alcohol and tobacco.
Going without the foodstuffs that are most commonly foresworn, such as sugar and chocolate, for six weeks could, of course, lead to weight loss. "How much would depend on how much of those things you were consuming in the first place, plus other factors like your size and your levels of activity. If you are only cutting out the odd spoonful of sugar in tea it is not going to have a major effect," adds Speer, who works at the Medical Research Council's human nutrition research centre in Cambridge.
"You have to be careful not to replace, say, the missing chocolate with an extra portion of chips. If you swap it for more fruit and vegetables then that is obviously good but you can't somehow store those benefits to draw on after Lent is over.
"What you also want to avoid is the diet/binge cycle, which is where you deny yourself something, find it hard to stick with that in the long term and go back to your old eating habits, often over-consuming in compensation. The basic thing to remember is that there are no 'good' or 'bad' foods: just a balanced diet."
Complete fasting - going without food completely - for the 40 days of Lent would probably be dangerous, though 50 days tends to be the point at which hunger strikers die. But damage to the internal organs begins much earlier, after about a week, according to Professor Tom Sanders, professor of nutrition at King's College, London.
"Initially the body burns up glycogen, which is carbohydrate stored in watery form, so although you might lose a couple of kilos in the first week you are really only losing water. After that you start to burn up fat and the rate at which you do that will depend on your activity levels, but maybe you will lose another kilo in that next week. But the body makes adaptive responses: your metabolic rate drops and you start to feel the cold. You get a thermal surge after eating, which is why older people who don't eat enough are at risk of hypothermia.
"In the first couple of days of fasting people usually get irritable: you can see that even in children who miss breakfast and there have been tests where workers who have not eaten in the morning and therefore have low blood-sugar levels cope very badly and become stroppy when given an awkward task."
Women, it seems, are more prone to blood-sugar fluctuations, possibly due to hormonal interplay, and it is thought that mood swings during the menstrual cycle may be related to blood-sugar levels. They also have a lower metabolic rate. "A young woman might typically need 1,200 calories a day, whereas a young man might need 3,000," adds Sanders. "And that's just loafing around. Once you introduce activity it shoots up even more. Rowers, for example, can get through 6,000 to 7,000 a day.
"Once the body starts to break down fat it produces acetic acid which is then converted into substances called ketones, the body's way of detoxifying all that acetic acid. These can make the breath smell of pear drops or nail varnish. They can also have an effect on the brain to produce a mild high or feeling of elation. It is thought that this may have contributed to the mystical experiences of people who have fasted for religious reasons. If you continue without protein you then start to lose muscle, including that of the heart and lung: basically, the body begins to eat itself.
"If you are very obese you can last quite a long time without food. There was a case not long ago of a party trapped in the jungle for a couple of weeks and the fat westerner was OK but the poor Malays were at death's door," he says.
Although long-term fasting is highly dangerous, even fasting for a few days is not to be encouraged, he says, despite the current fad for detoxification. Both he and Speer doubt that it works. "I think it's a hype. In fact, fasting can actually reduce the body's ability to break down toxins and rid itself of non-nutritive stuff like spices, medication, chemicals and burned bits because that process requires proteins," says Sanders.
Speer adds: "We tend to underestimate our body's ability to rid itself of toxins but it is perfectly capable of doing so: the kidneys and the liver are sophisticated, complex organs. People talk about the toxins of modern life but there are some very strong toxins and carcinogens in nature and our bodies have evolved to deal with them."
A key carcinogen, of course, is the cigarette. Going without for six weeks would have considerable health benefits as well as saving a 20-a-day smoker around £180. By then an abstainer should have noticed easier breathing, a better sense of smell and taste, improved circulation and higher energy levels as smoking would no longer be sapping the body's oxygen supply. However, "If you start again at Easter, all the benefits rapidly fall away," says Amanda Sandford, research manager at Ash (Action on Smoking and Health). "But you should be over the worst side effects of quitting. If you can do without cigarettes for that length of time you may feel you can keep it up. If there's a relapse later, you shouldn't treat it as a disaster but try again."
The benefits of giving up booze for Lent are less clear cut. Since alcohol contains calories, going without can lead to weight loss. It may also, according to a spokesman for Alcohol Concern, promote better sleep. However, if you are drinking too heavily in the first place, six weeks of abstinence will not help; if you are drinking moderately you won't be at risk anyway. "If you have got to the point of damaging your liver then you have to stop completely in order to get any regeneration of the liver," says the spokesman.
"Otherwise the benefits are more in terms of overall well-being rather than directly physiological. So you might perform better at work if you are not going in with a hangover and you may just feel better generally. If these good feelings contrast to how you felt before, it may be a wake-up call to the fact that you had been drinking too much and that when you resume after Easter it should be at a lower level."
