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Some intermittent fasting diets advocates eating without restrictions for five days and then consuming just 500 calories on each of the other two days of the week.Getty Images/iStockphoto

The question

What is intermittent fasting?

What does intermittent fasting mean? I have heard that it can help you lose weight and live longer. Is that true? Is it safe?

The answer

Intermittent fasting is an alternative to calorie counting

Whether long-term calorie restriction increases life expectancy in humans isn't known. Data do show, however, that people who eat a nutrient-dense, 25-per-cent calorie-restricted diet are leaner and have marked reductions in cardiovascular risk factors such as blood pressure, fasting blood glucose, fasting insulin and C-reactive protein, a marker of inflammation.

These findings sound encouraging, but eating less than you want every day can be difficult if you feel hungry, deprived and/or tempted to cheat.

What are the effects of intermittent fasting?

Intermittent fasting is an alternative to constant calorie restriction. It allows you to eat normally on certain days and then fast (water and low-calorie drinks are permitted) or drastically cut your calorie intake on other days.

What are the common eating window for intermittent fasting?

Many variations of intermittent fasting exist. The 5:2 diet lets you eat normally five days of the week and then fast on the remaining two days (no more than 500 calories to 600 calories on fasting days).

Other protocols have you eat 500 calories to 600 calories every other day (alternate-day fasting), every third day or fast once a week.

Another approach, called time-restricted eating, requires you to fast for 16 hours of the day and eat only during an eight-hour window. For instance, you might skip breakfast, eat lunch at noon and then finish eating dinner by 8 pm. During the fasting period, water is allowed.

What are the health benefits of fasting?

A number of studies published have associated Ramadan fasting with lower blood pressure, blood glucose, cholesterol and triglycerides levels and less inflammation. (Ramadan is the ninth month of the Islamic calendar when Muslims abstain from food and drink during daylight hours.)

A small study from Louisiana State University Medical Center, published in 2007, found alternate-day fasting resulted in a rapid improvement of asthma symptoms, inflammation and oxidative stress in overweight asthma patients.

In experimental animals, intermittent fasting has been shown to reduce the risk of diabetes and lower blood pressure, cholesterol and triglyceride levels. Studies from the U.S. National Institute on Aging found alternate-day fasting defended rats' brains from toxins, protected against stroke damage and slowed cognitive decline in mice bred to develop Alzheimer's disease.

Two larger studies involving women concluded that a 5:2 intermittent fasting plan was effective for weight loss and improving how the body uses insulin.

In one investigation, published in 2013, researchers from the University Hospital in South Manchester, England, instructed 116 overweight women to cut calories by 25 per cent seven days a week (1,200 to 1,800 calories/day) or restrict calorie intake to 600 calories to 650 calories two days per week and eat normally the remaining five days.

After three months, the intermittent fasting group experienced significantly greater improvements in insulin sensitivity and lost more body fat than the daily calorie restriction group.

These short-term effects seem promising, but the long-term effects are unclear.

It's thought that intermittent fasting acts like a low-grade stress that triggers cellular defences against molecular damage and disease. In animals, fasting has also been shown to rev up autophagy, a process that cleans up and recycles damaged proteins in brain cells.

Who should avoid fasting?

Intermittent fasting isn’t for everyone. For many people, it can be a struggle to go for extended periods without food or eating very little. Headache, fatigue and irritability are reported side effects of the change to your eating pattern, though they usually diminish over time.

Growing children and teenagers, pregnant and nursing women, people with diabetes who take insulin or oral medications and individuals with hypoglycemia (low blood sugar) should not fast.

If you have a health condition of any kind, consult your doctor before dramatically altering your diet.

If you decide to try intermittent fasting, do it correctly. Ensure your calorie intake is high enough to maintain a healthy weight and that the calories you are eating – on fasting and normal eating days – come from nutrient-packed lean proteins, legumes, vegetables, fruit and whole grains. Consider consulting a nutritionist.

Resist the urge to binge when you finish your fast. Fasting for several hours doesn’t give you permission to eat whatever you want. And be consistent; intermittent fasting is not intended to be a quick fix to weight loss.

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD

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