According to Google Trends data, “intermittent fasting” was the most popular diet search in Google 2019.
That doesn’t surprise me. People regularly ask me about intermittent fasting – how to do it, its benefits – and I’ve incorporated the protocol into a number of my clients’ meal plans. Its potential health benefits are a draw. Intermittent fasting is thought to promote weight loss, improve blood sugar, reduce cholesterol levels and dampen inflammation. While research has been limited very small groups of people, the findings, so far, are intriguing.
Here’s what you need to know about this popular diet trend.
Intermittent fasting is a pattern of eating that involves completely or partially abstaining from eating for a certain amount of time before eating again. Several variations exist. The 5:2 approach advocates eating normally five days of the week and then fasting on the remaining two days (eating no more than 500 to 600 calories per day). Other protocols have you eat 500 to 600 calories every other day (alternate day fasting), every third day or once a week.
The most popular form of intermittent fasting, and perhaps the easiest, is called time-restricted eating. It requires you to eat meals within an 8 to 10 hour window and then fast for 14 to 16 hours.
You might eat breakfast at 9 am and dinner at 5 pm or you might delay your first meal until 11 am and then eat your last meal by 7 pm. During the fasting period you can drink water, coffee, tea or other non-caloric beverages.
Time-restricted eating is not about skipping meals, restricting food choices or cutting calories (although you may inadvertently eat fewer calories). It’s about meeting daily calorie and nutrient requirements within an 8 to 10 hour window.
Research suggests that intermittent fasting is an effective weight loss strategy, but not more so than restricting daily calories. A review of 12 clinical trials, published in 2015, found no significant difference in weight loss between participants who fasted and those who ate a calorie-reduced diet.
A recent randomized controlled trial, published in JAMA Internal Medicine (2017), tracked 100 obese participants for one year and also concluded that intermittent fasting was no more effective for weight loss than daily calorie restriction.
In experimental animals, intermittent fasting has been shown to reduce the risk of diabetes and lower blood pressure, cholesterol and triglyceride (blood fats) levels. Data for the health benefits of fasting in people is limited but growing.
A small study published last year found that early time-restricted eating – restricting food intake to the daytime – improved how the body used insulin and lowered blood pressure in men with prediabetes. Another study conducted in overweight men revealed that early time-restricted eating lowered triglycerides and reduced fasting blood sugar.
Research published earlier this month looked at time-restricted eating in 19 men and women with metabolic syndrome, most who were already taking a medication to lower cholesterol or blood pressure. (Metabolic syndrome is a cluster of risk factors that occur together, increasing your risk of heart disease, stroke and type 2 diabetes)
The researchers found that combining time-restricted eating with medication helped participants better manage their condition. Eating within a 10 hour window resulted in weight loss, improved blood sugar control and reductions in blood pressure and cholesterol. These short-term effects seem promising, however the long-term effects of intermittent fasting in humans are unclear.
Even so, restricting our food intake to the daytime hours makes sense. Early time-restricted eating matches our body’s natural circadian rhythm, its 24-hour internal clock that governs things like blood sugar, cholesterol, hormones and digestion.
Our metabolism has adapted to eating during the day and sleeping at night. Eating late dinners and snacking into the night can disrupt circadian rhythm, affecting how the body processes carbohydrates and fats.
It’s thought that over time, misalignment of the body’s sleep-wake cycle can lead to weight gain and contribute to type 2 diabetes and cardiovascular disease.
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, however 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.
Intermittent fasting might seem like an appealing way to lose weight and improve certain health measures. If you decide to try it, consider consulting with a dietitian to ensure that you meet your daily nutrient needs.
There are simple dietary tweaks you can make to reap some of the potential benefits of intermittent fasting. Avoid, as much as possible, late dinners and do not snack after dinner. If you must eat dinner late, stick with protein and vegetables.
Eat carbohydrates earlier in the day when the body metabolizes them more efficiently. Avoid refined (white) grains and added sugars.
Leslie Beck is Medcan’s Director of Food and Nutrition.
She writes a weekly column in The Globe and Mail and is a contributor to CTV News and CBC Radio. Follow her on Twitter @LeslieBeckRD