(Geoffrey Woo, cofounder and CEO of Nootrobox, breaks fast at a WeFast breakfast in San Francisco.Melia Robinson)

The hottest way to lose weight these days involves eating whatever you want — on some days.

On others, you don't eat at all.

As strange as it sounds, the diet — known as intermittent fasting — has a lot of scientific backing. Large studies have found it to be just as reliable for weight loss as traditional diets; other studies in animals have suggested it could have other benefits as well, such as reducing the risk for certain cancers and even prolonging life.

Silicon Valley loves it. One Bay Area group of enthusiasts called WeFast meets weekly to collectively break their fasts with a hearty morning meal. Facebook executive Dan Zigmond confines his eating to the narrow time slot of 9 a.m. to 5:30 p.m., and many other CEOs and tech pioneers are sworn "IF" devotees.

But the diet has one easy-to-miss pitfall, according to University of Illinois nutrition professor Krista Varady.

Varady is one of the first researchers to study intermittent fasting in humans, and wrote a book about it called "The Every-Other-Day Diet" in 2013.

"Many people who try the diet complain of things like headaches," Varady told Business Insider. "But the problem is a lot of them aren't drinking enough water."

After going hours without eating, it can be tough to remember to go the kitchen and fill a glass of water. Plus, since many people on the diet notice a drop in energy during the first three or four days, they can wind up drinking more coffee than usual, which dehydrates them further.

The real problem, though, is that a good portion of our daily water consumption comes from food. According to the Mayo Clinic, roughly 20% of our daily fluid intake comes from what we eat; the rest comes from drinks like water and tea.

(Beefsteak/Facebook)

Many of the vegetables we eat are mostly water — cauliflower, eggplant, peppers, and spinach are all 92% water. Carrots, green peas, and even white potatoes are more than 79%. Abstaining from these foods for hours at a time, then, can put you at risk of dehydration.

Like vegetables, our bodies are also made up of a lot of water — roughly 60% of our weight. Every cell, tissue, and organ relies on that fluid to function. When you don't get enough, it's no surprise that you can feel tired.

"Even mild dehydration can drain your energy," according to the Mayo Clinic.

So drink up! The Mayo Clinic recommends that most adults consume about 14 cups a day. If you're fasting, you might want to up that intake. And keep in mind that just like with any diet, the beginning is typically the most challenging.