Intermittent Fasting for Weight Loss

Alternating between eating and fasting may benefit your waistline — and health

Many eating plans focus on what you eat. Intermittent fasting is different: It focuses on when you eat. The eating pattern alternates between periods of eating and fasting — a stretch of time with little-to-no calories.

Some people try it to lose weight. Others hope it will boost their health. But is it a smart long-term strategy or a passing fad?

“Research suggests intermittent fasting may help with weight management and improve health markers like blood sugar, cholesterol and blood pressure,” says Kimarie Segiel, RDN, a registered dietitian nutritionist at Geisinger. “Although the science is still evolving, particularly in the area of long-term studies.”

Intermittent fasting for weight loss can be a healthy lifestyle change, but it’s not right for everyone. Here’s what to know before you start.

What is intermittent fasting?

Intermittent fasting is an eating pattern that sets defined windows for eating and fasting.

During fasting windows, you avoid calorie intake, which encourages your body to shift from using incoming food for energy to drawing on stored energy. During eating windows, you focus on consuming balanced meals. While intermittent fasting doesn’t include a specific diet, you’ll get better results if you fill your plate with nutrient-dense foods.

“It can take 2 to 4 weeks for your body to become accustomed to your fasting schedule,” says Ms. Segiel. “You may have hunger, headaches, fatigue and irritability as you adjust to this new eating pattern.”

Intermittent fasting schedules

There are several effective intermittent fasting approaches, but you must go at least 12 hours for it to be considered a fast.

Common intermittent fasting schedules include:

Time-restricted fasting. You have a window of eating and then fast for the remainder of the day. Common approaches include 16/8: Fast for 16 hours and eat within an 8-hour window; 14/10: Fast for 14 hours and have a 10-hour eating window; and 12/12: Equal fasting and eating windows.
The 5:2 fasting plan. You have 2 low-calorie days (about 500 to 600 calories) and 5 regular eating days each week.
Alternate-day fasting. You restrict calories to about 25% of your usual intake every other day and eat normally on your non-fasting days.

The best intermittent fasting schedule depends on your goals, health and lifestyle.

If you’re using intermittent fasting for weight loss, sticking to a consistent schedule like 16/8 can help naturally cut back on calories. If you’re aiming to improve insulin sensitivity, shorter fasting windows like 12/12 or 14/10 can be effective. Just know that extreme fasting plans, such as not eating for 24 hours, are not recommended. Sometimes, they can lead to nutritional deficiencies and other health concerns.

The key is picking something you can actually stick with while still meeting your nutrition and energy needs. Pay attention to how you feel — your energy, mood, sleep and hunger — and adjust your fasting window if needed.

“Many people start with a 12/12 or 14/10 time-restricted schedule and gradually extend their fasting window if it feels doable,” says Ms. Segiel. “The great thing about this eating pattern is that you can choose a plan that fits your daily routine and lifestyle.”

How does intermittent fasting work?

When you fast for at least 12 hours, your body progresses through several physiological changes. Glycogen (stored carbohydrates) is used first for energy. But after several hours without food, your body exhausts its glycogen stores and starts burning fat.

This “metabolic switch” is the key part of intermittent fasting.

“When you eat throughout the day — 3 meals a day, plus snacking — and don’t exercise, your body is running on those calories you’re consuming and not burning your fat stores,” says Ms. Segiel. “But intermittent fasting may help you make this metabolic switch more often and burn more fat.”

Timed eating also may push cells to focus on repair and energy use to improve cellular health and reduce inflammation.

What are the benefits of intermittent fasting?

Intermittent fasting does more than just burn fat. There are many potential benefits of intermittent fasting for your overall health, including:

Weight management: Intermittent fasting can help you naturally decrease total calorie intake without having to consciously restrict calories.
Improved insulin sensitivity: It can reduce insulin resistance and lower blood sugar levels. Decreasing body weight and improving insulin sensitivity can help prevent — or even reverse — Type 2 diabetes.
Better heart health: It may improve lipid profiles (higher HDL, lower triglycerides and LDL), blood pressure and resting heart rates to reduce heart disease risk factors.
Reduced inflammation: It can trigger autophagy, the body’s cellular cleanup process, which can lower chronic inflammation — a key driver of many chronic diseases.

“Intermittent fasting isn’t a quick fix — and it’s not an excuse to eat whatever you want,” says Ms. Segiel. “It has to be combined with healthy food choices and an active lifestyle to positively impact your health.”

What should I eat in the window?

Even though intermittent fasting is about when you eat, you should consider what you eat for better results.

Use your eating windows to focus on whole, nutrient-dense food that helps you stay full between meals and offers prolonged energy. Ms. Segiel suggests the Mediterranean diet, which is built around lean proteins, fruits, vegetables, whole grains and healthy fats.

“If you’re packing your eating window with ultra-processed foods and sugary treats, you’re not likely to lose weight and meet your health goals,” she says.

During fasting windows, water and no-calorie drinks like black coffee and tea are permitted. Just be sure to avoid beverages that have calories like juice, alcohol or creamers, since these can break the fast.

Who shouldn’t try intermittent fasting?

Talk to your doctor before making any significant changes to your eating pattern. Intermittent fasting isn’t for everyone, especially if:

You’re under age 18 or over 65
You’re pregnant or nursing
You have a chronic disease like diabetes
You have a history of eating disorders

If you do get the green light from your doctor, start small and experiment with different schedules. Ms. Segiel suggests starting with the 12/12 time-restricted plan and slowly lengthening the fast from there as you get more comfortable.

“There’s no one size fits all, but once people find a plan that safely works for them, they tend to continue the routine indefinitely,” says Ms. Segiel. “It can be a healthy lifestyle change — and a powerful tool to kickstart weight loss and improve your metabolic health.”