Does intermittent fasting work?

It seems like everyone’s talking about intermittent fasting, a diet plan known for time-restricted eating and structured meals.

There are a variety of intermittent fasting approaches but the most popular is daily time-restricted eating.

When followed under the supervision of a physician and dietitian, certain fasting regimens can offer benefits like weight loss and related improvements to cardiovascular health.

Intermittent fasting can help restore some order to daily eating habits, too.

But fasting isn’t safe for everyone, and, like most diet trends, there are potential risks associated with intermittent fasting. It’s critical to talk to your health care provider before beginning any restrictive eating plan, says Rachel Sproat, RDN, LDN, a registered dietitian at UPMC Western Maryland.

Here’s what to know about the benefits, risks, and best practices of intermittent fasting.

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What Is Intermittent Fasting?

Intermittent fasting is unconscious calorie restriction, Sproat says. It shifts the focus from traditional calorie-counting — which can be harder to maintain — to a dedicated eating window.

The most popular and safest intermittent fasting approach involves daily time-restricted eating limits.

This means designating an eating window of 8 to 12 hours and fasting for 12 to 16 hours a day to better manage your caloric intake, curb late-night snacking, and stay on track with health goals.

During fasting hours, many people limit their consumption to water and/or other low-calorie beverages.

“You’re often starting your eating later in the morning and stopping earlier in the evening,” says Sproat.

Other fasting approaches include alternate-day fasting, which involves very low-calorie eating on particular days, and modified fasting, which often calls for 25% of calories on fasting days and unrestricted eating on others (such as the 5:2 diet).

Sproat warns that anything less than an eight-hour eating window per day — or regularly eating fewer calories than your body needs to stay healthy and active — can lead to nutrient deficiency and other risks. There is insufficient long-term research on alternate-day fasting.

Caloric and nutritional needs vary based on activity levels, age, height, metabolism, and other factors, but general Dietary Guidelines for Americans recommend women consume a minimum of 1,200 calories per day and men consume a minimum of 1,500 calories per day. These are considered low-calorie diets, and most people require more than these bare minimums. Most adults need to consume anywhere between 1,600 and 3,000 calories per day to maintain a healthy weight, according to guidelines.

What you eat during allotted eating hours is just as important as what you don’t during fasting hours, says Sproat.

Maintaining a balanced diet, staying physically active, and getting enough sleep will yield the best, most sustainable results.

“Make sure those meals you’re eating are healthy and balanced with a variety of fruit, vegetables, whole grains, lean protein, and healthy fats,” Sproat says.

You don’t need to buy expensive meal kits promoted online alongside popular intermittent fasting schemes, either.

Just eating several small, healthy meals a day will most likely do the trick, says Sproat.

What Are the Benefits of Intermittent Fasting?

Researchers are still evaluating the long-term health benefits (and risks) of intermittent fasting, but the benefits of healthy weight loss that may be achieved through intermittent fasting include:

  • Decreased risk for type 2 diabetes.
  • Lower cholesterol levels.
  • Lower risk for cardiovascular disease, cancer, and gallbladder disease.
  • Lower blood sugar and insulin resistance.
  • Reduced inflammation.
  • Reduced blood pressure.
  • Better sleep as issues like sleep apnea improve.

What Are the Risks of Intermittent Fasting

Fasting can be dangerous for people with medical conditions or those taking certain medications.

“There are populations I would not recommend fasting to because it could be dangerous,” says Sproat.

These include people who:

  • Are under 18 years old.
  • Have a history of eating disorders.
  • Have advanced or poorly controlled type 1 or type 2 diabetes.
  • Are on medication for diabetes.
  • Have chronic kidney disease.
  • Are pregnant or breastfeeding.

Sproat emphasizes that any healthy intermittent fasting diet should meet all caloric and nutritional needs. Potential risks of adhering to a very low-calorie diet include:

  • Abnormal heart rate and rhythm.
  • Dehydration and electrolyte imbalances.
  • Fatigue, brain fog, memory problems, and trouble concentrating.
  • Gallstones.
  • Irritability and mood swings.
  • Lethargy.
  • Loss of muscle mass.
  • Low blood sugar.
  • Metabolism changes.
  • Nutritional deficiency, including nutrients essential for heart health.
  • Poor gut health, diarrhea, constipation, and nausea.
  • Potential rebound eating, binge eating, and long-term failure associated with food deprivation.

Takeaways: What to Know About Fasting

Overall, eating a well-balanced diet that supports your lifestyle is key to nutritional wellness, Sproat says. Quality sleep, physical activity, and mental/emotional health play an active role, too.

While some approaches to intermittent fasting can be part of a healthy lifestyle, it’s important to consult a health care professional before starting any type of restrictive eating plan or new diet.


About UPMC Nutrition Services

Nutrition is vital for maintaining your overall health. UPMC Nutrition Services offers comprehensive diet and nutrition counseling on a variety of topics, including eating disorders, weight management, and heart disease. Our team provides medical nutrition therapy for chronic conditions such as celiac disease, cancer, and diabetes. UPMC’s network of registered dietitians is available to help guide all patients toward a healthier life.