The practice of fasting dates back millennia, to the time of Hippocrates and Plato. Even long before that, food scarcity was a common occurrence, and it's unlikely our predecessors ate three meals per day plus snacks, a situation that created involuntary periods of fasting. Today, fasting is generally defined as the abstinence from some or all food or drink, or both, for a specified period of time. However, fasting can take many forms, from the currently popular 5:2 diet (eat normally, but healthfully, for five days of the week and consume only 500 to 600 kcal on two non-consecutive days) to the fasting pattern of Muslims during Ramadan, in which fasting (no food or drink) occurs from sunrise to sunset for a month. Some type of fasting is a common practice among many faiths, including Judaism, evangelical Christianity, Hinduism, and Catholicism. But today, fasting has become a popular dietary practice, regardless of religious affiliation.

A large body of research exists on laboratory rats and mice to show that intermittent fasting can have profound beneficial effects on everything from diabetes and cardiovascular diseases to Parkinson's disease and stroke. However, many findings in mice won't translate directly to humans. While animal studies far outnumber those in humans, there have been some clinical trials designed to test the efficacy of fasting. Two studies in obese women at risk of breast cancer compared a calorie-restricted diet (25% reduction in daily calories) with a 5:2 fasting regimen. While both groups lost similar amounts of weight, those on the fasting regimen lost more abdominal fat and experienced a greater improvement in insulin sensitivity. Another study of obese women and men found that following an alternate-day fast (consuming 25% of energy needs on the fast day and regular intake on the following day) resulted in significant weight loss, significant decreases in body fat, and a reduction in triglycerides, LDL cholesterol, and systolic blood pressure.

The American Heart Association (AHA) published a scientific statement on meal timing and frequency and concluded that while there's evidence that both alternate-day fasting and periodic fasting may be effective for short-term weight loss, there isn't enough evidence to determine whether it's effective long term. It also was concluded that a weight loss of 6% or more may be required for fasting to be effective for lowering blood pressure.

On the other hand, there are potential harms to fasting, too. If one already has a diet poor in vitamins and protein, eating less food could lead to vitamin deficiency and muscle loss. Of course, this can be offset by taking multivitamins and doing strength training. Fasting can also lead to malnourishment if taken to an extreme. In addition, intermittent fasting is not recommended for people who are taking beta blockers or diabetes medication (insulin-dependent) or for those with hypoglycemia or a history of eating disorders.

To Fast or Not to Fast

Before trying to adopt fasting for weight-loss, you should get the green light from your primary health care provider, especially if you have any existing health conditions. If you have been given the OK and want to fast, the following approaches are recommended:  • Two non-consecutive days each week, eat only one meal (500-600 calories), and the other five days eat normally; or  • Five days each week, don't eat between the hours of 6 PM and 10 AM.  Fasting, as with any other lifestyle change, requires discipline, but reducing intake a few days per month may be more acceptable than chronic calorie reduction. And if more clinical studies bear out the findings in animals, fasting could be another tool in the weight management toolbox.

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