Women should do shorter fasts than men to avoid disrupting hormones, a new book claims - but the reality is more complicated

• Advocates of intermittent fasting are claiming that women should do it differently than men.
• Bulletproof CEO Dave Asprey says women can be more attuned to the effects, and should do shorter fasts.
• Experts say there's isn't enough evidence yet that women are more or less susceptible to side effects.

As intermittent fasting has skyrocketed in popularity, more people are trying to maximize potential benefits like weight loss and energy, while avoiding side effects like hunger and muscle loss.

But women may need to fast differently to get the most of the practice, according to some fasting enthusiasts. They theorize that longer periods of not eating could be more disruptive to women's hormones, and that women are better suited to shorter, less frequent fasts as a result.

Among them is Dave Asprey, CEO of Bulletproof and author of a new book about intermittent fasting called "Fast This Way."

In his book, Asprey recommends that women start with "less aggressive" fasting schedules of 12 or 14 hours instead of 16, or fast every other day instead of seven days a week.

Experts say we don't yet have the evidence to support these claims. For now, we do know intermittent fasting is relatively safe to try, but you should stop fasting immediately if you feel extremely fatigued, nauseous, or dizzy, and if your menstrual cycle is affected.

Women may be more sensitive to fasting, which can be an advantage

While men can experience side effects of fasting, women tend to be more sensitive to changes in their body, Asprey told Insider.

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That means they might react more strongly to hormone changes and the physical stress of going without food, he said.

There is one small study suggesting that longer fasts can lead to worse blood sugar control for women. However, a lot of the available data on sex hormones and fasting comes from research on rodents, with studies showing that fasting can disrupt the reproductive cycle of female rats.

Asprey acknowledged that recommendations are based on preliminary data, and more research is needed. He also said there's anecdotal evidence that women might benefit from a different kind of fasting, and one of the best ways to find out is to try it. The CEO is a proponent of "biohacking," or changing the environment around and inside your body to better control your performance and health.

"Permission to experiment on yourself is a hallmark of biohacking," said Asprey. "And I'm impatient. Part of my job is to say, given what we know, here's the best we can do, but we'll know more tomorrow."

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Experts say it's there's not enough evidence for specific recommendations

Despite evidence that fasting can have different effects on male versus female hormones in rodents, those studies don't tell us much about how humans will respond, according to Dr. Krista Varady, a professor of nutrition at the University of Illinois Chicago who has done extensive research on fasting.

"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," she said.

So far, there's been very little research on hormonal effects of fasting, and what evidence does exist suggests that it doesn't significant affect women's hormones, Varady said.

Intermittent fasting is a growing field of research. It's much too early to have good evidence in support of unique fasting protocols for different people, she added.

"There's not any information on whether it works differently for men and women, because we're not there yet. We hardly know if it works at all," she said. "Hopefully, we'll have some answers to your questions in a few years."

Intermittent fasting is mostly safe, but you should stop if you lose your period or experience major fatigue, nausea, or dizziness

Intermittent fasting is safe to try for most people. It should be avoided if you're pregnant (or trying to be), breastfeeding, have a compromised immune system, or if you have a history of disordered eating.

Those conditions aside, experts have found that fasting usually doesn't have adverse effects. There are some warning signs to be aware of, though, that it might be causing more harm than good.

Read more: Some fasting advocates say you can sneak food into the diet without sacrificing the health benefits, but others aren't so sure

Losing your period is a major sign that your body is trying to conserve energy and you should stop fasting, according to Dr. Caroline Apovian, co-director of the Center for Weight Management and Wellness at Brigham and Womens Hospital.

"For men and women, if you're in the middle of a fast and get dizzy or nauseous, or feel excessively fatigued, these are signs that you've got beyond the effectiveness of fasting, you've lost too much weight, or something is wrong," Apovian told Insider.

If you're not sure about whether fasting is safe for you, consult a qualified medical professional before you start.

"Many people overexert themselves trying to lose weight. Intermittent fasting and other methods of weight loss are really treatments and it's always a good idea to seek medical counseling," she said.