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How Does Fasting Affect the Human Body?

Millions of Muslims mark the month of Ramadan by fasting from sunrise to sunset each day. But what effects do those fasts have on the human body?
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The month of Ramadan is a time of fasting and prayer for an estimated 8 million American Muslims and 1.6 billion Muslims worldwide. For thirty days, observers forgo both food and water from dawn until dusk, only breaking their fast once the sun has set.

The practice of fasting isn't unique to Islam. For Hindus and Jains, single-day fasts mark auspicious occasions. Over the forty days of Lent, Christians undertake a partial fast. On the night before Yom Kippur each year, Jews begin a 25-hour period of fasting and prayer. Mormons are encouraged to fast for a day each month.

For most faiths, the sacrifice of food and water -- for hours, days, or weeks at a time -- is understood to be an intensely spiritual practice that allows for reflection and asceticism.

But while the spiritual importance of fasting is widely known, its physical effects on the body are less clear. How does the human body begin to change when it is systematically deprived of food and water, particularly over the long, hot, summer days of this year's Ramadan? Are there any biological benefits that accompany spiritual ones? Here are some answers.

Muslims throughout the world are marking Ramadan, a month of fasting, during which observant Muslims abstain from food, drink and other pleasures from sunrise to sunset.Shakil Adil / AP
Heart Health and Diabetes Prevention

A 2008 study conducted in Utah found that people who fast on a regular basis lower their risk of contracting coronary disease. In 2014, a follow up study found that fasting instigates metabolic changes and lowers “bad” cholesterol levels, which in turn can reduce the chance of heart disease by as much as 58%. That study also showed a decrease in blood sugar levels among people who fast.

Weight Loss

According to nutritionist Adam Brown, “Fasting should never be undertaken to lose weight. At the same time, some weight loss is reported by most people who fast.” Once the body has used up its reserves of glucose, it burns fat for energy, which can result in some weight loss. Nutritionists warn, however, that excess fasting can lead to starvation and should be avoided at all costs.

Detoxification and Cleansing

Adherents of alternative food movements and medicine often undertake partial fasts during which they only consume vegetable juices. Proponents of juice fasts believe that they allow the body to cleanse itself of toxins absorbed from processed and fast foods.

Mental Health

Fasting may be religiously-mandated, but the social and communal traditions that accompany the practice carry just as much benefit. During Ramadan, families sit down to break their fast each night together. They visit relatives' and friends' homes for the nightly ritual, known as iftar. The communal aspect of such fasting has been shown to impact the mental health of observers in a positive way. “Engaging in fasting can bring families and social groups closer together," said psychologist Susan Jones. "This often helps people suffering from depression and loneliness by reassuring them that they are not alone.”

Breaking Addiction

Fasting, some say, gives the body time to pause and reset. That pause -- breaking from dietary routines -- can help to break food habits like sugar or caffeine addictions. Razia Ahmed, a student at New York University with a self-diagnosed "sweet tooth," says Ramadan helps her to break her chocolate-habit. “After Ramadan ends each year, it becomes much easier for me to control myself and stay away from the M&Ms," said Ahmed. "At least for a few weeks.”