What it takes: Hunger, headaches and low energy
Especially in the first few weeks, long fasts can bring hunger, headaches, irritability and trouble concentrating. Drinking water through the fast helps.
Intermittent fasting
Eating only in a set window, or skipping food on certain days.
Intermittent fasting limits when you eat rather than what. Some people find it a simple way to eat less, and early research suggests benefits for weight and blood sugar, though results are similar to other ways of cutting calories. The common problems come from what happens inside the eating window.
What it takes and what to do
What it takes: Missing nutrients in fewer meals
Fitting a day's food into one or two meals makes it harder to get enough protein, fiber, fruit and vegetables. Breaking a fast with whatever is fastest often means processed food.
What to do about Missing nutrients in fewer meals
Include protein at each mealDailySpread your protein across breakfast, lunch and dinner instead of saving it all for one meal. Hit a daily fiber targetDailyWork up to 25–35 grams of fiber a day from whole grains, beans, fruit, vegetables, nuts and seeds. Eat a range of colorful produceDailyGet at least five servings of vegetables and fruit a day, mixing greens, reds, oranges, citrus and beans. Eat calcium-rich foodsDailyGet two or three calcium sources a day to support your bones, from dairy or from fortified and plant foods.What it takes: Overeating in the window
Large, fast meals after a long fast can bring heartburn and bloating, and can cancel out the calorie savings. Eating slowly and starting with protein and vegetables helps.
What to do about Overeating in the window
Include protein at each mealDailySpread your protein across breakfast, lunch and dinner instead of saving it all for one meal. Limit ultra-processed foodDailyBuild most meals from whole or minimally processed foods, and treat packaged snacks and fast food as extras.What it takes: Low blood sugar risk with some medicines
If you take insulin or certain diabetes pills, skipping meals can drop your blood sugar too low, causing shakiness, sweating, confusion or fainting. Talk to your prescriber before you start, since doses may need to change. If you feel shaky, sweaty or lightheaded, stop the fast and eat or drink something with fast sugar right away. Call 911 if someone is confused or can't safely swallow.
What to do about Low blood sugar risk with some medicines
No daily practice fixes this one. Follow the steps described here.
When to see a professional
Talk to a doctor before fasting if you have diabetes, take blood pressure or heart medicines, have kidney disease, are pregnant or breastfeeding, are under 18, or have a history of an eating disorder. Stop and get help if fasting turns into feeling out of control around food, or for fainting, confusion or a racing heart.