For years, intermittent fasting has ranked among the most talked-about diet trends, attracting millions of followers with claims of weight loss, better metabolic health and even the possibility of a longer lifespan.
However, while much of the public conversation highlights its upsides, recent guidance suggests the potential downsides deserve just as much attention before anyone decides to try it.
Harvard Health Publishing, which serves as the consumer health publishing division of Harvard Medical School, has flagged a number of notable side effects linked to intermittent fasting and says people should speak with a healthcare professional before beginning any version of it.
It serves as a useful reminder that one of the most heavily promoted eating strategies of recent years is not without concerns.
The label ‘intermittent fasting’ applies to several different eating patterns rather than one single method.
Some people use alternate-day fasting, which involves not eating every other day. Others take a less strict approach, consuming roughly 25 percent of their normal calories on fasting days instead.
Another option is periodic fasting, commonly known as the 5:2 plan, where calorie intake is cut to about 500 to 600 calories on two days each week. Time-restricted eating is also popular, focusing on limiting the hours during the day when food is consumed.
Harvard says each of these approaches comes with its own possible risks, and long-term research into their safety remains limited overall.
Among the most frequently mentioned side effects is simply feeling unwell. Depending on the length of the fasting window, people often experience headaches, tiredness, irritability and constipation, especially when they are just starting out.
In some cases those problems settle as the body adjusts, but for others they continue long enough to make the approach difficult to maintain.
There is also a physiological issue that can undermine the plan. When food intake is restricted, hunger hormones increase and the brain’s appetite centres become more active, which can make overeating once the fast ends much more likely. Combined with the common urge to treat yourself after a period of restriction, this can result in poor eating habits on non-fasting days and may wipe out any calorie deficit created during the fast.
Harvard also notes that two classic effects seen with calorie-restricted diets, a slower metabolism and increased appetite, can show up with intermittent fasting too, which helps explain why some people struggle to stick with it or fail to see the results they expected.
Researchers are also paying closer attention to time-restricted eating, with some evidence indicating that eating at times that clash with the body’s natural circadian rhythm, the internal clock that regulates daily cycles, may contribute to metabolic issues instead of improving them.
That point matters because time-restricted eating is often promoted as the simplest version of fasting. But newer research has produced a more mixed picture: some trials have found modest improvements in weight and blood sugar, while larger reviews have concluded that intermittent fasting tends to perform similarly to standard calorie-cutting diets rather than dramatically outperforming them.
One major 2024 review co-authored by Harvard T.H. Chan School of Public Health researchers analysed 99 clinical trials involving more than 6,500 participants and found that intermittent fasting and traditional calorie restriction were broadly comparable for weight loss and cardiometabolic health, although alternate-day fasting appeared to produce slightly greater weight loss than standard calorie restriction in some comparisons.
The outlook becomes even less clear for older adults. A large share of intermittent fasting studies have involved younger or middle-aged people and have only followed them for short periods, which means there are still major unanswered questions about its effects later in life. Experts do know that too much weight loss can harm bone density, weaken immune function and reduce energy, which may make fasting especially risky for people who are already quite lean.
That concern is not just theoretical. In older adults, losing weight too quickly can sometimes mean losing muscle as well as fat, and preserving muscle mass becomes increasingly important with age for mobility, balance and general health.
Anyone living with certain medical conditions should be especially cautious.
For people with diabetes, missing meals or sharply reducing calories can create particular dangers. Those taking medication for blood pressure or heart conditions may also face a greater risk of unhealthy sodium and potassium imbalances during longer fasting periods.
Fasting may also be unsuitable for anyone who has to take medication with meals in order to prevent nausea or stomach discomfort.
Recent studies have added more nuance rather than eliminating those concerns. For example, an NIH-funded trial published in 2024 found that an 8- to 10-hour eating window produced modest improvements in hemoglobin A1C, body weight and trunk fat in adults with metabolic syndrome over three months, but the researchers also stressed that larger and longer trials are still needed to clarify the long-term effects.
Separate NIH-backed research in adults with obesity and type 2 diabetes found that time-restricted eating led to average weight loss of 3.6 percent over six months, with blood sugar improvements similar to those seen with a conventional calorie-restricted diet. Even so, federal health experts still advise people with diabetes to speak with their doctor before trying intermittent fasting because of the risk of blood sugar problems.
Doctors are broadly in agreement on the main point: anyone considering intermittent fasting should get medical advice before making the change.
That is particularly important for older adults, people who are underweight, anyone with diabetes, and those taking medicines that depend on regular meals or can affect fluid and electrolyte balance.
Starting slowly, drinking plenty of calorie-free fluids and avoiding any medication changes unless advised by a doctor are all seen as essential precautions rather than optional extras.