Shunning food for religious or spiritual reasons has existed for centuries, as during Ramadan, Lent or Yom Kippur for instance.
"Practised the way it's practised within this framework, it's probably no sillier than anything else and in any case is part of a respectable philosophical approach," said nutritionist Jean-Claude Melchior.
"But to ask people to pay to starve themselves is another matter," he said. "It's ethically scandalous to exploit someone's naivety in order to pocket their cash."
From low-cost fasting in a tiny room in sea-swept western Britanny (France) to an all-liquid diet at Desert Hot Springs (US), or a riverfront in Bad Ems (Germany) or beach spa in Thailand ... offers are all over the web, from several hundred to several thousand euros (dollars) a shot.
In Europe dieting holidays often fall short of a total fast, which involves drinking nothing but water, instead offering fruit juices or vegetable broths that are easier to handle without medical supervision.
But the nutritional intake may be as little as one broth a day or juice diluted with water.
Can this really be good for you?
"Fasting has many positive effects," said Adrien Reygade, who runs a centre aimed at giving guests "a health makeover, a break in routine", an alternative to a stay in a thalassotherapy centre.
The key word for many fasting adepts is "detoxification", or cleansing the body of toxins and impurities. "This also has secondary benefits," added Reygade, citing improved vision and improved body movement.
But nutritionist Melchior downplayed claims that fasting could be good for you "as there is absolutely no proof", and said going without food could even "cause an imbalance in regulating nutritional intake."
As to the current fad for fasting while trekking, Melchior said it was potentially dangerous because there was a major risk of hypoglycaemia."
After fasting for around 12 hours, he said, there is no glycogen left in the liver which means it becomes impossible to feed sugar into the bloodstream "should the muscles, brain or any other organ suddenly increase sugar intake" -- which is the case when the body speeds up or makes an intense effort.
"Trekking improves the body's ability to detox," countered Daniele Cazal, a Frenchwoman who organises "fast and trek" tours overseen by a naturopath.
"There are no risks," she said. "It's not as if you were doing military training."
"You feed on other things, on the country views."
But one of the pioneers of the fasting fad in France, Desire Merien, said organisers of the fasting and trekking tours needed to go softly-softly to avoid alarming the authorities.
"People should avoid excess," he said. "There are extremists too in the world of natural health."
The country's inter-ministry group tracking sects is currently keeping on eye on the fasting/trekking fad because of the potential threat to those in poor health.
And both nutritionists and naturopaths agree that fasting cannot be a cure for a poor daily lifestyle.