'Intuitive fasting' promises fewer cravings and more energy. Less restrictive 'intuitive eating' may offer similar benefits.

• 'Intuitive Fasting' offers a flexible form of intermittent fasting.
• Functional medicine practitioner Will Cole says fasting can help address cravings and energy.
• There's limited evidence about some of the diet's claimed benefits.

Intermittent fasting is one of the fastest growing trends in nutrition, lauded for benefits like weight loss, lower risk of chronic illness, and potentially a longer lifespan.

Done right, going without food for 12 hours or more leaves people free from stress, hunger, or moodiness, according to Will Cole, a functional medicine practitioner and health advisor to Gwyneth Paltrow.

Cole's new book "Intuitive Fasting" aims to offer a flexible approach for doing intermittent fasting in a sustainable way. The key, he said, is to gradually use fasting as a tool to reset poor eating habits and heal the symptoms associated with them, like cravings and fatigue.

"It seems like a paradox. How could fasting be intuitive? It's a kind of yoga for your metabolism," Cole told Insider.

There's promising evidence to back some of the health benefits of intermittent fasting, include what Cole recommends in his book.

However, researchers say that certain claims about fasting go beyond what evidence can support. And some dietitians say that the approach doesn't measure up to intuitive eating for helping people develop a better relationship with food.

Fasting may help improve insulin and blood sugar levels

Intermittent fasting can help people break free of poor dietary patterns like stress eating or overeating refined sugars and processed foods, according to Cole.

These foods can all be linked to fluctuations in blood sugar, which Cole says can lead to inexplicable fatigue or moodiness, as well as cravings for the same refined carbs and sugar that cause the cycle to repeat.

"Most people are so metabolically inflexible, they're depending on next hit of sugar for energy. If you have metabolic inflexibility, it's really hard to intuit what your body really needs. Is it hormones, is it cravings?" he said.

He advises gradually using longer periods of fasting to allow people to re-adapt to burning fat for fuel instead of carbohydrates. The ability to do both is known as metabolic flexibility.

There's evidence intermittent fasting can help with metabolic flexibility, and it's linked to benefits like better blood sugar control and insulin sensitivity. In theory, it might also lower risk of chronic illnesses.

Intuitive fasting, as Cole describes it, is unique from other fasting programs in that it encourages people to customize their fast, making it longer or shorter to suit their personal needs. He emphasized that it's never meant to feel punitive or painful, and people who have a history of disordered eating or struggles with body image should seek professional advice before trying a fast.

"The protocols are secondary to someone's relationship with their body and their relationship with their food," Cole said. "It can be a trigger for people. You can't heal a body you hate and you can't shame your way into health. That's the antithesis of intuitive fasting."

We need more research on fasting to know how exactly it works

There are some limitations to fasting research.

There's no clear evidence that intermittent fasting can reduce cravings or hunger in the long term, according to Dr. Krista Varady, a professor of nutrition at the University of Illinois Chicago who has done extensive research on fasting.

It's also not clear how the benefits of fasting for fat burning, blood sugar control and insulin levels apply to humans, particularly if they don't have diabetes or another form of metabolic dysfunction. Lots of research has been done on rodents, with relatively few studies on people.

"What we are finding is that what they find in mice doesn't really relate to humans. Mainly because they're super adherent because they're trapped in a cage and don't have an option," Varady previously told Insider.

Dietitians say intuitive eating offers similar benefits without restriction

Christy Harrison, an anti-diet registered dietitian who hosts the Food Psych podcast, worries that intuitive fasting is co-opting the language of intuitive eating, which does have evidence-backed benefits. Harrison is also the author of "Intuitive Eating," a guide toward helping people develop improve their relationship with food for better psychological and physical health.

Harrison said that even "gentle" intermittent fasting is still a form of restriction, which is counterproductive to curbing cravings and energy fluctuations, and can also trigger binges or emotional eating.

Many of the benefits attributed to fasting, including better blood sugar control, have also been linked to intuitive eating.

Research has found intuitive eating can help people who have overweight become healthier, and sustain those improvements over time, even if they don't lose weight. It's also "the gold standard" for treating eating disorders, Harrison said, and studies show people who eat intuitively have lower risk of disordered eating behaviors.

Cole said that concerns about restrictive fasting are valid, but that his protocol avoids taking a strict attitude toward food.

He said his aim in writing the book is to offer resources for people who otherwise haven't found of way of eating that works for their mental and physical health.

"The world doesn't need another fasting book. It's not just a fasting book," he said. "The goal is for this to be a tool people can use in a sustainable way to improve their lives."