Harvard details four concerning side effects of intermittent fasting that everyone should be aware of

Intermittent fasting has spent the last few years at the top of every diet trend list going, with millions of people swearing by its promises of weight loss, improved metabolism and even a longer life.

But while the headlines tend to focus on the benefits, new research is urging people to pay closer attention to the risks before jumping in.

Harvard Health Publishing, the consumer health publishing arm of Harvard Medical School, has outlined several significant side effects associated with intermittent fasting that it says people should discuss with a medical professional before starting any plan.

The findings are a timely reality check for one of the most hyped dietary approaches of the last decade.

The term 'intermittent fasting' covers several different approaches.

Some people practise alternate day fasting, where they fast every other day entirely. Others follow a modified version that allows them to eat around 25% of their usual intake on fasting days.

There's also periodic fasting, often called the 5:2, which restricts calories to around 500 to 600 a day on just two days per week, and time-restricted eating, which simply limits the daily window during which you eat.

Harvard notes that each method carries its own risk profile, and that the long-term evidence on their safety is still limited across the board.

Intermittent fasting has spent the last few years at the top of every diet trend list going, with millions of people swearing by its promises of weight loss, improved metabolism, and even longer life. (Getty stock image)

What are the dangers of Intermittent fasting?

One of the most commonly reported issues is simply feeling rough. Depending on how long fasting periods last, people regularly report headaches, low energy, irritability, and constipation, particularly in the early stages of a new fasting plan.

For many, these symptoms ease over time, but for others they can be persistent enough to make the diet unsustainable.

There's also a biological catch that often gets overlooked. When the body is deprived of food, appetite hormones surge and the hunger centres in the brain go into overdrive, creating a powerful urge to overeat once the fasting window ends. Add to that the very human tendency to reward yourself after a hard stretch of restriction, and it becomes easy to see how fasting days can lead to unhealthy eating on non-fasting days, potentially cancelling out any calorie deficit achieved.

Research is also building around time-restricted eating specifically, with evidence suggesting that eating patterns which fall out of sync with your body's natural circadian rhythm, its internal daily clock, can actually cause metabolic problems rather than solve them.

For older adults, the effects of intermittent fasting has not been fully researched (Getty stock image)

Is intermittent fasting safe for older adults?

For older adults, the picture is murkier still. Most studies on intermittent fasting have focused on younger or middle-aged participants over relatively short periods, leaving significant gaps in what we know about how it affects people later in life. What experts do know is that losing too much weight can have serious consequences for bone density, immune function, and energy levels, making intermittent fasting potentially risky for anyone who is already on the lighter side.

People managing certain health conditions need to be particularly careful.

Those with diabetes face specific risks from skipping meals or severely limiting calories, while people on blood pressure or heart medication may be more vulnerable to dangerous imbalances in sodium and potassium during extended fasting periods.

Anyone who needs to take medication with food to avoid nausea or stomach irritation may find fasting genuinely incompatible with their health needs.

The advice from medical professionals is consistent: if you want to try intermittent fasting, speak to your doctor before you start.

Easing in gradually, staying well hydrated with calorie-free drinks, and never adjusting your medication routine without medical guidance are all considered essential steps, not optional ones.