The Big IF Debate: What Science Says About Time-Restricted Eating

Is restricting your food intake to certain hours or days the key to weight loss and longevity? Some swear by it, while others have doubts. Here, experts break down the myths and surprising benefits of time-restricted eating

1. Myth: Intermittent fasting is not safe long term.

Mythbuster: Many people can safely follow intermittent fasting (IF) for at least a year, says professor of nutrition Krista Varady. However, it’s not for everyone. Pregnant and breastfeeding women, along with those with certain health conditions like kidney stones or a history of disordered eating, should avoid IF, as it may trigger obsessive behaviours or eating disorders.

Popular types of IF include time-restricted eating—fasting for 16 hours and eating within an 8-hour window. There’s also alternate-day fasting, such as the 5:2 plan, where you eat regularly for five days and limit yourself to one 500- to 600-kilojoule meal on the other two days. Whatever method you choose, the key is ensuring you get enough food and variety to meet daily nutrient needs, says registered dietitian nutritionist Scott Keatley. He also advises speaking with a doctor before starting.

2. Myth: Fasting slows down your metabolism.

Mythbuster: Fasting may lower metabolism slightly, but no more than other diets. “Whenever someone loses weight, their metabolic rate—the number of kilojoules burned per day—drops slightly,” Varady says. That’s because metabolic rate is linked to body size.

In general, 20% to 30% of weight loss comes from muscle mass, which can slow metabolism. To maintain muscle and keep metabolism steady, Keatley recommends strength training two to three times a week and eating enough protein.

While intermittent fasting isn’t a magic fix, research suggests it can be an effective tool for weight management when done safely and consistently.

3. Myth: Intermittent fasting is the best way to lose weight.

Mythbuster: Simply changing when you eat doesn’t guarantee weight loss. Any diet that results in weight loss works because it helps you eat fewer kilojoules overall. Some people react to food restrictions by overcompensating on eating days, making weight loss harder.

IF isn’t necessarily more effective than other weight-loss methods either. A study led by Varady found that people who followed IF lost about the same amount of weight as those who restricted kilojoules—typically 3% to 10% of their body weight over three to 12 months.

The best diet is one you can maintain long-term. IF may work for some because it naturally limits kilojoule intake without feeling as restrictive as counting kilojoules. Research suggests that it can be easier to avoid food at certain times than to monitor intake throughout the day.

4. Myth: IF helps you live longer.

Mythbuster: The science doesn’t fully support this—at least not yet. Some animal studies suggest that kilojoule restriction or time-restricted eating can extend lifespan by about 3%. However, decades of research haven’t determined whether these benefits come from eating fewer kilojoules, the fasting window itself or a combination of both.

“Some research shows potential benefits for metabolic health, inflammation and cell repair processes in humans,” says Keatley. But more studies are needed to confirm whether IF actually leads to a longer life in people.

What is clear is that overall diet quality matters. A UK study found that switching from an unhealthy diet to one rich in whole grains, fruit, vegetables and nuts—while cutting back on red meat and sugary drinks—could add up to 10 years to life expectancy.

5. Myth: Fasting is bad for your heart.

Mythbuster: Research shows that intermittent fasting can improve factors that affect heart health such as cholesterol levels, blood pressure and insulin sensitivity. But what about the headlines claiming that IF raises heart disease risk by 91%? Experts say the study behind this claim had major limitations. Participants who fasted were more likely to be smokers and the study relied on self-reported data without analysing diet quality.

Whether IF benefits heart health depends on how it’s done. Keatley stresses the importance of managing fasting correctly—ensuring you get enough potassium and magnesium for heart function and avoiding excessive kilojoule deficits that could lead to muscle loss.