Fad diets are the go-to option for some people who wish to lose weight quickly. However, the sustainability of this method is repeatedly questioned and some of these diets may have implications on your long-term health.

Among the diet fads are some that are linked to celestial bodies, such as the sun and the moon.

So, what is the solar diet and is it safe?

Those who swear by it claim that gazing at the sun replaces their need for food and also tends to improve their vision and quality of sleep. A number of women in Hong Kong are subscribing to this fad — usually carried out during times of sunrise or sunset, to avoid damage to the eyes.

“We practice sun gazing as a substitute for eating. Some of us who have finished the therapy now eat less, and others don’t have to eat at all,” a lady told Hong Kong-based Oriental Daily (in Chinese).

Advocates of the diet say it works by absorbing solar energy to fuel their bodies but this claim is widely disputed by healthcare experts.

“Humans lack the ability to photosynthesize — turning the sun’s light into energy — which is why we eat plants who do this for us,” Jo Travers from the London Nutritionist said, calling the diet a “mad idea,” Huffington Post reported.

Charlotte Stirling-Reed from SR Nutrition has called "sun-eating" a potential danger to life, saying: “I’ve seen some terrible faddy diets in my time, but this one definitely makes it up there with the worst and probably one of the most life-threatening ones.”

She added: “Ultimately common sense (as well as science) tells us that our bodies need food. We need calories, macronutrients and micronutrients through diet in order to survive. The food we eat ensures we have the energy needed to survive, to move and to function day to day. ... Nutrition also helps with a whole host of other essential processes such as repairing and replacing cells and tissues, for immunity and to keep our organs such as our brain and heart working. Another word for this fad is starvation.”

And what, then, is the moon diet and does it work?

Also referred to as the lunar or werewolf diet, this fad diet has reportedly been tried by A-list celebrities Madonna and Demi Moore, and is based on the known effects of the moon’s gravitational pull on earth’s oceans and rivers.

According to Christian Dyuraffur, president of the International Federation of Phyto and Aromatherapy, during a new or full moon, there is an effect on our metabolism due to the shift in atmospheric pressure — allowing for faster removal of fat and toxins from the body.

The diet can reportedly help a person shed 6 lbs. in just 24 hours on a full or new moon.

The dieter can opt for the basic moon diet plan, two-moon diet plan or the extended moon diet plan, based on the time period they can devote to the process. The shorter diets require the person to go on a liquid cleanse while the extended one allows solid foods.

The scientific basis of this diet is contested but the effects of the moon have been proven over time. The moon diet results in weight loss possibly because it is based on fasting for several short periods. It leads to a loss in excess body fat, not water, reports say.

However, people going on a lunar diet are advised to consult with their doctor before taking the plunge and looking up the lunar calendar.