Back in December, the New England Journal of Medicine published an article on intermittent fasting. “Fasting” refers to time-restricted feeding. This article was one of the first high-profile scientific studies of this way of eating, and the effects it could have on people who eat this way.

Intermittent fasting isn’t new, though. This is how our ancestors ate – they had to deal with times of feast and famine. After all, those hunter-gatherers didn’t always know where the next meal would come from. They certainly didn’t have pantries or refrigerators to store food.

Jennifer Kirby, MD-PhD, is an endocrinologist at UVA. Many of her patients have diabetes. Weight- and hormone-management are very important for those patients. A common question they ask Kirby is, “what should I be eating?”

The answer varies, but intermittent fasting can be an option for many of those patients. So what is intermittent fasting?

Variations of Intermittent Fasting

Usually, our bodies burn glucose (sugar) for energy. The idea with intermittent fasting is to push the body to burn ketones (from our fat stores) for energy instead. You can think of glucose as short-term energy storage. It is the fuel from the meals we’ve recently eaten.

When glucose is circulating in our blood, that’s the only fuel that’s available to our bodies. If that glucose isn’t used, it’s stored as fat. But if it is used up, our bodies can switch over to burning ketones (from that previously stored fat) for energy. Intermittent fasting allows that switch. You don’t necessarily change what you eat, but when you eat.

There are two most common variations of intermittent fasting:

Time-Restricted Eating

This is when you spend part of each day in a fed state, and part of each day in a fasted state. For example, eating for 8 hours out of every day, and fasting for the other 16 hours. Usually, people work up to 14 to 18 hours of fasting each day.

5:2 Ratio

Is how we eat and how we exercise important? Absolutely. There are lots of different eating patterns that can be used for weight-loss. Intermittent fasting is a great option for some patients, but it is not the only option.

Jennifer Kirby, MD-PhD

This version involves eating normally five days a week and either fasting completely, or eating a very low-calorie diet (about 500 calories) the other two days a week.

Is Intermittent Fasting “Just Another” Calorie-Restricted Diet?

So, does intermittent fasting work by reducing calorie intake? The answer isn’t entirely clear, but there is mounting evidence that it’s more than just calorie consumption that makes intermittent fasting effective.

Potential Health Benefits of Intermittent Fasting

Science is beginning to support many more benefits of intermittent fasting than just weight-loss. According to Kirby, the research is still in its infancy, but there’s potential for intermittent fasting to have health benefits across a number of chronic diseases, even beyond weight-loss. Some of the additional benefits appear to include:

• Increase longevity (very controversial at this point)
• Decreased rates of cancer and obesity
• Reduction in glucose
• Reduction in cholesterol

These health benefits seem to come from the metabolic changes associated with fasting, not from calorie reduction.

It’s Not the Only Option

Kirby stresses that obesity is an incredibly complex disease, and there is no one-size-fits-all solution. “Is how we eat and how we exercise important? Absolutely. There are lots of different eating patterns that can be used for weight-loss. Intermittent fasting is a great option for some patients, but it is not the only option. I offer it up as an option, but not the only option. We have to take each individual patient and their needs into consideration.”

According to Kirby, it’s essential to find something that is going to work for the patient for the long term. “There are a lot of patients who drop out of intermittent fasting after giving it a try, because it’s just not a sustainable mode of eating for them. If a patient tries it for six weeks and it’s working for them and is sustainable, awesome. If it’s torture, it’s probably not something that’s going to be sustainable for the rest of their life.”

It’s Not for Everyone

There are some groups of people who should not practice intermittent fasting. That includes children, pregnant women, the elderly, and people with a history of disordered eating.