Intermittent Fasting — Does It Deliver on Its Promise?

Of all the most recent diet trends, intermittent fasting may be one of the hottest. But does it deliver on its promise? Let’s take a look.

Intermittent fasting is purposely not eating for a predetermined number of hours. While this nutrition philosophy is currently regarded as a “new” diet, fasting — intentionally or not — has been going on as long as humans and their predecessors have walked the earth. Before people learned how to farm and domesticate animals for consumption, they were often forced to survive for several days without food based on the availability of game, nuts, berries, and other edible plant life.

As civilization evolved, fasting became part of certain religious and spiritual practices. While many of those traditions continue, increased numbers of people are beginning to fast for weight loss and health.

The Physiology of Fat Burning

The case for intermittent fasting as a weight loss tool lies in the belief that it places the body in a fat-burning state.

Your body depends on glucose from carbohydrates as its primary and most direct fuel source. After several hours without food, your body depletes its glucose stores and must turn elsewhere for energy. Fat is the next available supply, so your body begins to burn fat to continue functioning.

By limiting the hours you consume food (fuel), intermittent fasting prolongs the period during which your body burns fat.

Benefits of Intermittent Fasting

Proponents of intermittent fasting say it also improves cholesterol levels and reduces the risk of chronic conditions like heart disease and stroke.

Other reported benefits include:

• A boost in working and verbal memory
• Improved blood pressure and resting heart rate
• Reduced symptoms of irritable bowel syndrome

What Science Says

Research shows that calorie restriction increases lifespan in animals. The evidence is less clear for humans. And there is still not enough research on whether intermittent fasting can help manage chronic conditions.

One study of 100 obese people compared intermittent fasting to calorie-restricted diets. After one year, there were no significant differences in weight loss or regain. There were also no appreciable variations in blood pressure, heart rate, total cholesterol and triglycerides.

Another trial found that the 16:8 (see below) method of fasting had no better outcomes than calorie reduction without limited eating time.

Results from other research seem to connect restricting eating periods to wellness. Several studies suggest that eating over long periods (12 to 15 hours) may disrupt your circadian rhythm (the 24-hour cycle of the body’s internal clock) and increase the risk of chronic disease, including heart disease, cancer, and Type 2 diabetes. Based on these findings, one might conclude that a shorter eating window could reduce the risk of developing these conditions.

Intermittent Fasting Methods

There are several approaches to intermittent fasting. Some extend the period you go without food during a given day, while others severely limit intake to a few days a week. All allow you to drink water and other no-calorie beverages like black coffee and tea while fasting. Time spent sleeping counts toward fasting.

• 16:8 – People who choose this fasting method eat normally during an eight-hour window and abstain from eating for 16 hours. Many find this pattern easy to stick to long-term because the eating period is adjustable, and there’s little need to keep track of calorie intake.
• 5:2 – With this method, people eat as usual five days each week and limit themselves to one 500- to 600-calorie meal the other two days.
• Alternate day eating – One eats normally one day and then consumes about 500 calories or 25% of their regular calorie intake the next, repeating the cadence through the week.
• 24-hour fast (also known as eat:stop:eat) – Adherents do not eat at all for 24 hours once or twice weekly. Eat:stop:eat is the most extreme form of intermittent fasting and produces the most side effects, including fatigue, headaches, irritability, hunger and low energy levels.

No matter the eating cadence, it can take two to four weeks for the body to get used to a fasting regimen. During that time, you might feel hungry or cranky. People who make it past this phase often stick with the program because they notice they feel better.

More extended fasting periods are not necessarily better and could have the opposite of the desired effect. During prolonged periods of fasting, your body may adapt to calorie restriction and prevent further weight loss. Going too long without food could put your body in a starvation state where it stores more fat instead of burning it.

What Is “Normal” Eating?

It’s important to note that during the eating window, it’s recommended to consume a wide range of nutrient-dense foods, including whole grains, high-fiber foods, leafy greens, unsaturated fats like olive oil, and lean proteins. Proponents don’t recommend increasing portion sizes of meals and snacks during this eating window.

Getting Started

Check with your health care provider before beginning a practice of intermittent fasting. Any type of food restriction may be dangerous for people on certain medications or with some health conditions. Your health care team can review your health history and advise whether fasting is safe.

Decide why you want to fast and if it is something you plan to do for a short period – for instance, until you reach your desired weight – or view it as a longer-term lifestyle change.

If you are fasting for weight loss, you may still want to focus on eating sensible portions and well-balanced meals during nonfasting times. It’s important to remember you may regain any weight lost if you return to unhealthy eating patterns. A registered dietitian may be able to guide you in developing habits that will help you reach and maintain your weight loss goals.

Talk to your health care professional if you begin fasting and start experiencing unusual anxiety, headaches, nausea or other new symptoms.

It’s Not for Everyone

Beyond health conditions and medications that might make intermittent fasting a potentially dangerous choice, other factors may cause a restricted eating schedule unadvisable.

Intermittent fasting is not recommended if you are:

• Under the age of 18
• Pregnant or nursing
• Being treated for diabetes and/or other blood sugar problems that require you to eat at regular intervals
• Have a history of eating disorders like anorexia or bulimia nervosa that involve unhealthy self-restriction of food