Intermittent fasting: benefits, risks and how to start safely

Intermittent fasting is less about what you eat and more about when. According to the Harvard T.H. Chan School of Public Health, the three most studied types are:

• Time-restricted eating (16:8 intermittent fasting): you eat within a set window each day and fast the rest. In 16:8 that is 16 hours without food and an 8-hour eating window, for example 10 a.m. to 6 p.m.
• The 5:2 diet: you eat normally five days a week and eat very little (about a quarter of your usual needs) on two days.
• Alternate-day fasting: you alternate a normal eating day with a day of very little or no food.

16:8 is a common schedule: many of the fasting hours fall overnight, while you sleep.

In a 12-month randomized trial published in the New England Journal of Medicine in 2022, 139 adults with obesity followed a calorie-restricted diet, and half of them also ate only between 8 a.m. and 4 p.m. That group lost an average of 8.0 kg (about 18 pounds) and the other 6.3 kg (about 14 pounds), but the difference was not significant: the eating window added nothing measurable to cutting calories every day.

A systematic review of 40 studies summarized by Harvard found that intermittent fasting helps with weight loss; in the 12 trials that compared it with steady calorie restriction, there was no significant difference in weight loss or body composition. In a 12-month trial of alternate-day fasting, blood pressure, fasting glucose and fasting insulin were no different from daily calorie restriction, and LDL ("bad") cholesterol rose.

What is missing is time. The 40 studies in that review ran from 2 weeks to 2 years, and the National Institute on Aging notes that trials have typically enrolled adults 60 or younger, so the results don’t necessarily apply to older adults or children. An umbrella review in JAMA Network Open concludes that longer trials are needed to know whether fasting changes heart attacks or death rates. For some people the real advantage is practical: counting hours can be easier than counting calories.

According to Johns Hopkins Medicine, during the fasting hours you can have water and zero-calorie drinks such as black coffee and plain tea. Unsweetened sparkling water counts too.

Does coffee break a fast? Black coffee, with no sugar, milk or cream, has almost no calories, so it doesn’t. What breaks the fast is what you add: sugar, milk, cream, syrup or creamer. Go easy on coffee on an empty stomach if it gives you heartburn or palpitations.

Juice, sugary sodas, sports drinks, smoothies and alcohol all break a fast.

According to a New England Journal of Medicine review, hunger, irritability and trouble concentrating are common in the first month and usually ease as the body adjusts.

Headaches can happen too. Drink enough water and, if you want less coffee, cut back gradually.

Stop fasting and eat something if you feel dizzy, shaky, sweaty or your heart is racing. If you take diabetes medicines, these can be signs of low blood sugar: follow the plan your doctor gave you. Also stop if you find yourself preoccupied with food, overeating when you break the fast, or feeling worse over time.

With confusion, fainting or seizures, call 911.

If you are in Puerto Rico, you can book an online doctor visit to check whether intermittent fasting is safe for you. You see Dr. Diana Nieves Castro, a Puerto Rico–licensed general practice physician. We review your conditions and your medicines; if needed, we give you a lab order for a lab near you. You need to be physically in Puerto Rico during the visit.

If you already have diabetes or high blood pressure, chronic care helps you adjust your medicines to your eating plan. If you were told your blood sugar is a little high, read about prediabetes. And if you want a weight plan with follow-up, see weight care with the doctor.