It is Ramadan, a month when Muslims around the world fast during daylight hours. They are not the only group who practise fasting, as practitioners of other religions also fast from time to time.
Intermittent fasting - taking breaks from eating - is also increasingly practised for health reasons.
Fasting patterns vary. For instance, it can mean eating less on two days a week or every other day.
LOSE WEIGHT FAST?
Some people opt for intermittent fasting to lose weight. When we eat, the body stores sugar as glycogen.
A body that is deprived of food will start to use its glycogen stores. Once these stores are used up, the body burns fat for energy, resulting in weight loss, said Ms Bibi Chia, the principal dietitian at Raffles Diabetes & Endocrine Centre.
Indeed, intermittent fasting that involves eating less on alternate days or two days a week has been shown to lead to weight loss in overweight people, said Dr Abdul Shakoor S.K., a senior consultant at Tan Tock Seng Hospital's department of endocrinology.
But while fasting has definite benefits for people who are obese, there is no definitive proof of its benefits for healthy people, he said.
Intermittent fasting may also lower the risk of metabolic and cardiovacular diseases in obese and non-obese people, he said.
However, there is insufficient evidence to show that it is a way for diabetic patients to control blood glucose levels, he added.
Besides, most people find it hard to restrict their calorie intake over the long term. It is also natural to overeat during non-fasting hours, which negates any weight loss.
Dr Quah Boon Leong, a staff physician in general surgery at Khoo Teck Puat Hospital, said: "According to one study, weight changes during Ramadan were relatively small and mostly reversed after that, gradually returning to pre- Ramadan status. Consistent lifestyle modifications are necessary to achieve lasting weight loss."
BEYOND WEIGHT LOSS
Intermittent fasting is not just about weight loss. "Some studies indicate a decreased risk of coronary heart disease and diabetes among those who fast routinely," said Dr Sueziani Zainudin, a consultant endocrinologist at Seng- kang Health's general medicine department. "Studies also report behavioural changes, including improvement in mood, increased alertness and mental acuity, and tranquillity."
Dr Abdul Shakoor said alternate- day fasts in animals have been shown to lead to longer lifespans. It has also been shown to prevent or delay heart disease, diabetes, cancer, neurological disorders and stroke.
"However, there is no convincing evidence from human studies to show that fasting or caloric restriction help to ward off or delay the onset of chronic diseases," he said.
Therefore, while healthy adults may fast for religious reasons, they do not need to fast specifically for health purposes as there is insufficient evidence to show that intermittent fasting will prevent certain diseases or prolong life in humans, he said.
NOT EVERYONE CAN FAST
Doctors said fasting is not suitable for people who are ill, elderly, pre-pubescent or have eating disorders, as well as women who are pregnant or breastfeeding.
It may affect diabetic patients on medication or those with heart, kidney and liver conditions who take medications that induce water loss, said Dr Sueziani.
Dr Quah said the health risks of fasting during Ramadan are hypoglycaemia (low blood glucose levels) and dehydration. "Severe hypoglycaemia could lead to convulsions and falls, and may be fatal."
While intermittent fasting shows promise, a lot of questions remain unanswered. Ms Chia said these include which is the most effective fasting pattern, the optimal calorie consumption during the fasting period and how sustainable it is in the long term.
Dr Abdul Shakoor said if intermittent fasting with low-calorie diets is practised frequently, it may affect one's ability to get adequate essential nutrients.
Most international dietetic associations do not recommend following a very low-calorie diet for more than 12 weeks, he said.
When fasting, look out for signs like dehydration, heartburn, bad breath, headaches, increased irritability and dizziness, said Ms Chia. "Stop when there are signs of shivering, extreme thirst, heart palpitation or extreme weakness."