Not So Fast. Is Intermittent Fasting Right for You?

You may see more and more fitspiration influencers and MDs posting videos and writing about the health and weight management benefits of intermittent fasting. And research shows positive outcomes for weight loss in subjects who fast intermittently. If you haven’t tried the trend and wonder if it could work for you, read on as we discuss the pros, cons, and what the experts say. 

Fasting has been around for centuries, while “intermittent fasting” is a more recent concept in dieting. As the rate of overweight and obesity in the U.S. has skyrocketed to 71%, Americans struggle to find the perfect weigh to lose weight and maintain the loss. Intermittent fasting could be a viable weight loss strategy for those struggling to drop pounds using other dieting strategies.  

As a Registered Dietitian, I’ve observed intermittent fasting in my clients to be a successful strategy for weight loss and maintenance that has additional health benefits for some. Along with weight loss, intermittent fasting has also shown promise in lowering blood sugar, improving blood lipids, and possibly preventing age-related dementia.

Let’s take a closer look at the various types of intermittent fasting, how and why to use it for weight management, what the experts say, and when intermittent fasting would not be appropriate.

What Is Intermittent Fasting?

Intermittent fasting is a method of limiting food and beverage intake so that the body can use its own fat stores for energy, potentially resulting in weight loss.

Fasting could lead to calorie restriction, which is important for weight loss but only part of the puzzle. The calories we eat are used for energy, stored in the liver and muscle as glycogen, or stored in fat cells. By limiting food and beverage intake to only certain hours of the day, or days of the week, a person may become more mindful of their food choices and consume fewer calories. 

Intermittent fasting impacts the body in a few ways. For starters, it works by changing how your liver breaks down energy. Known as the “metabolic switch,” your body will use glycogen (carbohydrates stored in the liver that break down to glucose) as well as fat-cell-created ketones for energy while you’re fasting. Fasting forces your body to use fat for fuel.

Blood sugar rises after eating or drinking calories, particularly carbohydrates, and, in response, your body’s pancreas secretes the hormone insulin to lower blood sugar. In a fasting state, insulin levels are at their lowest. This is beneficial to weight management because insulin is a fat-storing hormone.

Fasting may also change the bacteria in your bowel, known as thegut microbiome. The timing and composition of meals may impact signals sent from the gut to the brain, which control energy balance and weight.

These signals between the gut and brain are regulated by short-chain fatty acids (created through high-fiber diets), bile acids, amino acids, and other bacterial compounds. When this complicated system malfunctions due to unhealthy diets, frequent overeating, or poor meal timing, weight management becomes more difficult.

Types of Intermittent Fasting

As mentioned above, there are multiple methods of intermittent fasting including 5:2 fast, alternate day fasting, time-restricted feeding, and 24-hour fasting.

The 5:2 Fast

British journalist and physician Michael Mosely developed the 5:2 fast. Mosley’s fast involves two days of “fasting” and five days of normal eating. The two “fast” days restrict calories to 600 per day for men and 500 calories per day for women. The other five days of the week comprise “moderate” calorie intake.

The advantage of the 5:2 fast is that a person may only feel restricted two days out of the week since calorie intake on the other five days isn’t as rigid. You can pick the days you fast. A disadvantage is a low-calorie level that must be adhered to on fasting days; this could lead to feeling lightheaded and dizzy or to binge eating.

Alternate Day Fasting

Alternate day fasting is similar to the 5:2 fast though fasting is repeated every other day. A pro to this strategy is that it could result in more weight loss. A con is that it’s more difficult to follow and could lead to headaches, feeling weak, or nauseous. You’re less likely to enjoy meals with others on fasting days and you may have less oomph to exercise. 

Time-restricted Fasting

Time-restricted intermittent fasting allows a window of time to eat and the remaining time to be in a fasting state. An example of a time-restricted fast is the 16:8 fast. A person has an 8-hour window to “feed” and 16 hours to fast. During the fasting state, water and other calorie-free liquids like unsweetened tea or coffee can be consumed.

One advantage of time-restricted feeding is that it may make you a more mindful eater. It discourages mindless snacking in the evening or between meals. It’s also a viable option for people who aren’t hungry in the morning. The disadvantage is that breakfast and snacks are skipped, which could lead to headaches, excessive hunger, and overeating during the feeding window.

24-hour Fasting

A 24-hour fast is a full day of fasting followed by a regular day of eating. This method is also known as Eat, Stop, Eat. Some people fast for one to two full days per week, but typically alternate this with regular food intake.

The benefit would be a more prolonged fasting period, which may result in faster weight loss. One disadvantage is that this type of diet takes strict discipline. This fasting method would be difficult to follow if you need the energy to work out or enjoy dining with others. Forget the popcorn at the movies.

Does Intermittent Fasting Work for Weight Loss?

In a word, yes! In a recent meta-analysis of studies reviewing intermittent fasting (IF) and health outcomes, various types of intermittent fasting were found to reduce body weight, body mass index, fat mass, LDL cholesterol, total cholesterol, triglycerides, fasting blood sugar, and insulin. Alternate day fasting for one to two months showed a moderate drop in body mass index in healthy adults as well as those who were overweight or obese when compared to a regular diet.

Intermittent fasting is not necessarily better than traditional calorie restriction. A systematic review of 41 articles describing 77 studies evaluating weight loss in overweight and obese patients saw a weight loss of .8% to 13% of baseline weight without significant adverse outcomes. Twelve studies comparing intermittent fasting to traditional calorie restriction found similar results. In five of the studies including patients with type 2 diabetes, blood sugar control was improved.

A recent systematic review of various fasting methods was compiled. All methods showed strong support for intermittent fasting in improving blood sugar and body composition within 12 to 24 weeks. After 12 to 18 months, IF did not reveal the same results for prolonged weight loss or blood sugar control. 

What Can You Eat and Drink During Your Eating Window?

Calorie levels may vary depending on the type of intermittent fast a person is doing, though food choices are similar. While your goal for fasting may be fat loss, dietitians want people to get “food in their food” and eat nutrient-dense meals that support overall health.

With the 5:2 fasting method, most of my clients eat one meal per day of 500 or 600 calories. This could consist of baked chicken or fish, a large leafy green salad with minimal dressing, and a piece of fruit. Plenty of water, unsweetened tea, (low or no sugar added) electrolyte powders, or calorie-free sports drinks are advised for those eating one meal a day.

One study evaluated the diet composition of overweight subjects following the 5:2 fast. Most subjects' diets were high in protein, moderate in fat, and low in carbohydrates and fiber on fasting days. Lower levels of zinc, calcium, and potassium were observed on fasting days. A daily multivitamin might be needed in those using fasting as a weight loss method.

Alcohol, sugar-sweetened beverages or treats, fried foods, fast food, and high-calorie desserts are discouraged with fasting, as with most weight loss diets. These foods are calorie-dense and highly palatable, making people want to eat more. A “Western-style diet” is associated with weight gain and other health issues.

What Do Dietitians Think of Intermittent Fasting?

"I have had several clients have success with intermittent fasting, specifically in terms of weight loss and prevention of mindless eating. I've found IF to be the most beneficial for those who tend to graze throughout the day with no real meal schedule, or overeat, or snack at night.”

—Melissa Mitri, MS, RDN, of Melissa Mitri Nutrition

“Unlike other popular diets, intermittent fasting has hundreds of published research studies, the majority of which show that intermittent fasting is safe, provides weight loss similar to traditional calorie restriction, can provide additional metabolic benefits like improved blood sugar, insulin levels (HOMA-IR), hemoglobin A1c, blood lipids, and inflammatory markers.”

—Jean LaMantia registered dietitian and author of Complete Intermittent Fasting 

“In working with clients on gut-brain health, I find intermittent fasting helpful with improving focus, clarity, and cognition. Many of my ADHD clients have used intermittent fasting to lose weight, improve their focus, and regain energy. Clients find following a strict eating schedule reduces their temptation to snack in the evenings. Intermittent fasting can improve insulin sensitivity, reduce inflammation, benefit cardiac health, and improve cognitive function including increasing BDNF [brain-derived neurotrophic factor].”

—Amy Archer, RDN, CLT, CHWC, and owner of Wellnessrd 

Is Intermittent Fasting Right for You?

Intermittent fasting is safe overall for most people trying to lose weight, but more research is needed to evaluate long-term effects on weight and overall health. Intermittent fasting should not be used by:

• Pregnant women

• Individuals suffering from eating disorders or those at risk for eating disorders

• Someone taking insulin, as it may cause hypoglycemia (low blood sugar)

Final Thoughts

Intermittent fasting may have health benefits beyond fat loss, although it can be difficult to follow for some and may not be appropriate for others. One study comparing continuous calorie restriction versus intermittent fasting found that weight loss, blood pressure, and other measures of health were improved with both methods, but fasting subjects felt hungrier.

If you’re considering intermittent fasting, seek the help of a registered dietitian for meal ideas and support. Some simple ways to start intermittent fasting include:

• Limit eating times to three meals per day.

• Drink your coffee or tea black or unsweetened.

• Skip snacks between meals and after dinner.

• Reduce intake of fast food, alcohol, foods and beverages with added sugar and excess calories.

• Focus on nutrient-dense foods including lean protein, vegetables, fruit, and limited amounts of whole grains and fats.

• Be mindful of hunger cues. Is it hunger or habit that’s making you eat?