The following story was told to me by the son of one of the prominent Roshei Yeshivah of the last generation.
One morning, shortly after Pesach, the Gadol called his son to say that he had noticed some blood and needed a ride to the doctor’s office. The doctor was, in fact, worried enough about the possibility of intestinal cancer to immediately schedule an appointment for his patient with a well-known gastroenterologist.
Walking back to the car with his father, the son expressed concern over the possibility of receiving a frightening diagnosis from the specialist. The Rosh Yeshivah responded with the following story: An architect once told the Netziv that there was a ﬂaw in the Volozhin yeshivah building; if the defect was not corrected, the expert said, the building would surely collapse.
But the Netziv was not at all alarmed; since the building had been constructed in accordance with structural principles laid out by the Rambam, he told the expert, there was simply no possibility that it would fall in. (Not long ago, I spoke to someone who recently visited Volozhin, and he reports that the building is still standing.)
“It is not possible that I have intestinal cancer,” the Gadol told his son. “The Rambam gave his havtachah that anyone who follows his directives for healthful eating, as I have, will not get sick.”
The Bosh Yeshivah’s trust in the Rambam’s guarantee was well-warranted, as it turned out. When the examination revealed no evidence of bleeding, the specialist questioned the patient about any recent dietary changes. He soon determined that the temporary bleeding had likely been caused by intestinal irritation brought on by the maror the Rosh Yeshivah had eaten at the Sedarim.
I have no idea if either the story about the unnamed Rosh Yeshivah or the story about the Netziv are true. But there are many problems with both of them.
First of all, where does the Rambam write about structural requirements in building a house? I'm not saying that he doesn't - but I consulted a few people and nobody could think of where he writes about such a thing. This leads me to doubt the veracity of the story (but it is true that Volozhin is still standing).
Second, and most importantly, Rambam did not even believe that Chazal any supernatural insights into science. He even believed that prophets were fallible in such things. All the more so would he not have believed himself to possess any supernatural, infallible insights into either architecture or physiology. (See too Menachem Kellner, "Maimonides on the Science of the Mishneh Torah: Provisional or Permanent?")
It is true that Rambam in Hilchos Deyos 4:30 gives an assurance that anyone who follows his dietary and medical advice will never fall sick. But this was no kabbalist's promise (however much they are worth). It was simply a reflection of his belief that he was an excellent physician who had selected the best of Galenic medicine.
Rambam had some excellent advice about healthy eating, exercise, and clean air. But I would not recommend anyone to fully follow his advice; after all, he disapproved of eating fruits. And his medical advice in general was based upon obsolete ideas of "the four humors" and certainly exhibited the standard flaws of medieval medicine, such as recommending a small amount of bloodletting in the spring and fall.
A recent Jewish book about eating a healthy diet purports to be based upon Rambam's principles and claims that "the foundation of this system is perfectly up to date and reliable." I have not read the book, but the evidence given for this claim is rather shaky. The author writes that "Rabbis, doctors and nutritionists have read through this book and verified its principles" - but to what extent does his book accurately reflect Rambam's system? And the fact that "the commentaries on Rambam's works maintain that his advice about health and the prevention of disease remains relevant today" is hardly evidence that it actually is accurate.
I am also wondering to what extent the unnamed Rosh Yeshivah was really following Rambam's advice in such things. Was he engaging in bloodletting? And it is especially interesting that his intestinal bleeding resulted from eating maror. That sounds like he was eating a sizable quantity of horseradish. But according to Rambam, suitable foods for maror are wild lettuce and endives, not horseradish, and a kezayis is the size of an olive. He should have followed Rambam in that area!
Yet I have no doubt that a person who believes himself to be following Rambam's dietary and medical advice, and who believes that Rambam has some kind of divine authority in his assurance, will be extra healthy. My reason for this is not that I believe that Rambam's medical advice competes with that of modern medicine. Rather, it is that the placebo effect is extremely powerful. (Incidentally, Rambam himself was quite ahead of his time in his realization of the power of the placebo effect!)
I'll sign off with the old story about the man who attended a shiur from his Rav about how the Gemara instructs a house to be built. He decided to build his house exactly according to the Gemara's standards, and spent many months constructing it. But as he finished hammering in the final nail, the entire house suddenly collapsed. Furious, the man came to his Rav to complain. The Rav says: "You know, Tosafos asks that question!"
(Thanks to S. for some links)