This MIGHT have been true many years ago, but nowadays many doctors are now privy to the science of nutrition.
Many medical doctors, naturopaths, chiropractors, and functional medicine docs are recommending the need for regular cleansing.
And now many of them are starting to offer various detoxes in their office.
To be fair, It's no secret that most medical doctors don't receive much nutritional training.
Most medical students are still getting less than 20 hours of nutrition education over 4 years, and even most of that has limited clinical relevance. There are no requirements for nutrition before medical school either.
Instead, med students take courses like calculus, organic chemistry, and physics. Most of these pre-med requirements are really not useful to the practice of medicine and mostly used to “weed out” students.
Unfortunately for them to learn much about diet and nutrition, they must spend the time and money going through a postgraduate programs to learn about natural methods for health.
So be sure you're not relying on information from a doctor not up on the latest research surrounding food and nutrition.