Fad Diets: Intermittent fasting
These days new diet fads come along roughly as often as MTR trains during rush hour, so it's no surprise that the latest to grip the popular imagination is actually a reboot of a couple of existing ideas, a kind of a diet mash-up. Intermittent fasting, as the name suggests, combines elements of fasting - not eating for prolonged periods - with elements of the Paleolithic diet, which emphasises eating like our ancestors before the development of settled agriculture about 10,000 years ago, consuming foods such as meat, fish, fruits and vegetables.
The Paleolithic diet is an attempt to eat the foods our digestive systems are best adapted to consume. Intermittent fasting has a similar aim: to mimic pre-agricultural eating patterns, when food was not so regularly available, by not eating on particular days or for a certain period each day. That runs contrary to a lot of recent nutritional wisdom, which tends to favour eating little and often.
"We encourage small, frequent meals as some studies have showed that they might increase metabolic rate," says Sylvia Lam See-way, chairwoman of the Hong Kong Dietitians Association. "[Frequent meals] can keep you satiated so that you can reduce the chance of overeating. And they can stabilise blood sugar, preventing big fluctuations in insulin secretion, which [could] reduce the risk of diabetes and obesity."
Intermittent fasting proponent Robb Wolf, in contrast, says that our eating patterns are those of herbivores, rather than the omnivores we're designed to be. Wolf is a nutritional expert and author of
The Paleo Solution: The Original Human Diet. He says that your diet is as much a matter of when you eat as what you eat.
Wolf claims that "for almost every disease under the sun, systematic inflammation is a factor. And every time you eat, there is an inflammatory response. We're constantly bombarding our digestive tract with food, and we need to give it a regular rest."
Fans of the diet claim it can do everything from regulating blood glucose and controlling blood lipids to improving cardiovascular function and reducing blood pressure and the risk of cancer.
But sceptics say there are risks. "Potential dangers from fasting include an increased chance of eating disorders, inadequate intake of micro- and macronutrients, and the possibility of overeating after a fast," says Lam, who doesn't recommend any form of fasting as a diet. "Some acute symptoms [also include] weakness, dizziness, low blood sugar and agitation."
Of course, the effects depend on what intermittent fasting regime you follow because this is an ill-defined diet with several manifestations. Sometimes it means not eating for 24 hours once or twice a week; sometimes it's so-called alternate day fasting - eating for 24 hours and then not eating for 24 hours; sometimes it involves randomly skipping meals in an attempt to more accurately mimic the unpredictability of pre-agrarian revolution eating patterns.
But probably most common is not to eat for a set number of hours per day - anything from 16 to 20 is common. Of course, all of us have a period of fasting each day: when we're asleep, and usually for a while before and after, too, which can easily make up 12 hours out of 24.
Wolf prefers this, among the milder of the various fasting regimes. "Going a full day without eating seems austere. Don't eat your last meal of the day after 5pm or 6pm, and don't eat the first before 9am or 10am, and that's 16 hours. Doing that two or three days a week is fine at first. Eat proteins, fruits and vegetables. It's the old wisdom: eat good food that's not processed, and don't eat too close to bedtime."
Karen Chong, dietitian at Matilda International Hospital, says that fasting for 16, 18 or even 20 hours a day "should be fine". But for longer fasts, of say 30 hours, people might not get an adequate amount of nutrients or calories."
Weight restriction is the most common reason for taking up the diet, but weight loss is probably the result of consuming fewer calories.
The element of calorie restriction makes it difficult to test the efficacy of intermittent fasting. "There are not many human studies to support fasting as a good diet to reduce the risk of diseases," says Lam. "Fasting has been shown to improve health and longevity only in animals such as dogs, fruits flies, rodents and non-human primates."
Other studies have focused on Ramadan, when more than a billion people undertake a form of an intermittent fasting regime. Results have been mixed, with some finding improvements in metabolic risk factors including cholesterol levels, triglycerides and blood sugar, but others finding no effect.
Moderate intermittent fasting is unlikely to do you too much harm, but the more extreme versions might, plus at the moment there's no proof that it does you any good.
Among the people who definitely shouldn't take up the diet, says Lam, are pregnant and lactating women, elderly people, children and people with chronic conditions such as diabetes and heart disease. For everyone else, the jury is still out.