Faith and fasting have a curious connection. Be it Karwa Chauth, Mahashivratri or Paryushan, Ramzan or Lent, fasting is a constant. Of course, there exist variants. There is intermittent fasting when one does not eat for 12 to 14 hours, partial fasting in which one meal a day is allowed and complete fasting, wherein one gives up food and water completely for days. Then there are fasts where you eat only a certain food, such as fruit fasts and juice fasts. But fasting is tricky business. If not observed properly — that is, without the requisite medical advice — it will damage the body.

While there are some pros of fasting — weight loss, better insulin resistance and longevity among them — the cons outweigh them by far.

Says Delhi-based naturopath and nutritionist Swati Srivastava, “Fasting once in 7-10 days usually doesn’t lead to major health issues. But prolonged fasting can cause problems.” Then there are nirjala (without water) fasts, which Jains observe during Paryushan and Hindus during auspicious days such as Chhath. “At the end of these fasts, and in some cases even in-between, people complain of giddiness, headache, low blood pressure, slow or fast pulse and hypoglycemia (low blood sugar),” she adds.

Dr Pankaj Puri, director of gastroenterology and hepatology at the Fortis Escorts Heart Institute in New Delhi, adds muscle aches, dizziness, weakness and fatigue to that list. “Long-term fasting can cause deficiencies in vitamins and minerals, especially Vitamin D and B 12. It also triggers migraine, leads to anaemia, osteoporosis, a weakened immune system, muscle breakdown, liver diseases, electrolytes imbalance and ketosis,” he continues.

There can also be issues with digestion. Says Dr Shamik Banerjee, a consultant gastroenterologist at ILS Hospital, Howrah, “Bloating, indigestion and heartburn are common, especially in those who practise unhealthy eating habits such as excessive eating during iftar or suhoor meals during Ramzan. We often come across patients with peptic ulcer who are usually asymptomatic but whose symptoms get aggravated during Ramzan. Patients with chronic liver disease may have a fall in blood sugar during fasting and need to be hospitalised.”

In certain kinds of fasting, people don’t eat or drink for 10-12 hours and then indulge in rich food, mostly during Ramzan and Navratra. This leads to digestion problems as well as weight gain. Dr Banerjee says eating late at night is also associated with reduced sleep and poor sleep quality.

Why then do people fast? What is the science behind fasting? Dr Puri puts it in medical terms, “Reduced oxidative stress, reduced inflammation, increased thermogenesis and browning of adipose tissue, circadian rhythm restoration and beneficial hormonal changes.” Dr Banerjee has a simplified explanation, “It entails the reduction of calorie intake and blood sugar levels, and subsequent effects such as weight loss with beneficial metabolic parameters and improved inflammatory markers.”

According to these experts, fasting is a strict no-no for diabetics, patients with low blood pressure or a chronic disease, pregnant women, the elderly and underweight. In case diabetics must fast, Dr Banerjee suggests they seek medical opinion and check their blood glucose levels multiple times a day. Srivastava says all chronic patients should gradually change their diet before starting prolonged fasting.

With intermittent fasting becoming a trend to lose weight and improve metabolic health, the key to fasting lies in the way one does it.

As a rule of thumb, fasting is only healthy if you do not feast after it. As Srivastava puts it, “The most important thing is the eating pattern post the fast. Avoid overeating and indulging in rich food. Break your fast with liquids such as coconut water or vegetable soup. When you start eating, chew each bite of food properly before you swallow.”

Dr Banerjee advises against spicy food and citrus fruits as they may increase reflux and heartburn. Avoid excessive tea, coffee or cola. “During fasting, withdraw medication only under a physician’s supervision,” he warns. “And prepare yourself for temporary mental discomforts such as impatience, crankiness and fleeting hunger pangs,” he adds.

Dr Puri advises meditation to handle the mental issues and staying hydrated. “Intermittent fasting is good but a fast should not last more than 24-48 hours. Keep a check on the signs your body gives you,” he says. Srivastava suggests quality sleep and lemon juice with water to cleanse the body.

“Limit your activity but exercise moderately. Walk 1.5-5km a day if comfortable. Rest as much as your schedule will permit,” says Dr Banerjee. Srivastava suggests you keep as active as your body allows. She adds, “I strongly recommend fasting once in 7-10 days as it helps flush out toxins and impurities. It also helps the digestive system get some rest.”

So the next time you go on a fast, you know what to look out for.