A communist atheist is claiming that fasting is harmful
Question

I live in Russia where I am studying. Most of our professors are communists who do not believe in the existence of the Creator. One of them said to us: “How can this God of yours command you not to eat and drink during the day when this is damaging to the health?” How do you advise us to respond to this communist, may Allaah humiliate him?.
Praise be to Allah.Fasting – as well as being basically an act of worship that is prescribed in Islam and an obligation enjoined by Allaah, is also one of the most beneficial of remedies and most effective means of strengthening one’s body. This is according to the testimony of non-Muslim doctors, let alone Muslim ones. 
Fasting plays an effective role in treating psychological problems and strengthening the will-power of the fasting person, softening his feelings, and making him love to do good and keep away from argumentative and hostile tendencies; it also increases his spiritual feelings. So it strengthens his personality and makes him more able to bear problems and burdens. Undoubtedly this is reflected positively in a person’s health. 
This is one aspect. From another angle, fasting also plays a role in treating many physical disorders, such as diseases of the digestive system like gastritis, irritable bowel syndrome, liver diseases and indigestion, as well as treating obesity, hardening of the arteries, high blood pressure, angina, asthma, etc. 
The Swiss doctor Barcellus wrote: 
The benefit of hunger as a remedy is many times better than using medicines. Doctor Helb used to forbid his patients to eat for several days, then he would offer them light meals. In general, fasting plays a role in destroying the worn-out tissues at the time of hunger, then rebuilds them when food is ingested. This is the reason why some scientists, including Pashutin, regard fasting as a means of restoring youth. 
Tom Burns of the Columbia School of Journalism says: I regard fasting as an experience which is deeply spiritual more than physical. Despite the fact that I started to fast with the aim of losing excess weight, I realized that fasting is very beneficial for the mind. It helps me to see with greater clarity and to develop new thoughts and to focus my feelings. Within only a few days of starting my fast in the spa I began to feel that I was passing through a very spiritual experience. 
Of course fasting may be harmful and extremely difficult for some people in certain situations; Allaah has exempted them from fasting – such as those who are sick and those who are traveling. 
Moreover the benefits of fasting come when one adheres to the etiquette of fasting, such as delaying suhoor, hastening to break the fast, not being extravagant in the manner or amount of food eaten, and avoiding being extravagant in serving a large variety of foods. 
The Encyclopaedia Britannica says: Most religions enjoin fasting and it it is something that people used to do even at times other than religious occasions; some individuals fast in response to human nature.  
In the twentieth century a number of books appeared in America and Europe which spoke of the medical benefits of fasting. There was Treating sickness by fasting by Shelton; Medical Fasting: The Ideal Nutritional System by Alan Coutt; Fasting, the Elixir of Life by Enrique Tanner; Return to a Healthy Life through Medical Fasting by Watzener. 
Fasting is of great benefit in treating some diseases of the heart, because 10% of the blood that the heart pumps to the body goes to the digestive system during the digestive process; this amount is reduced during fasting, when the digestive process does not occur during the day. This means that the heart works less and can rest more. 
Fasting is also used to treat skin diseases, because if the amount of water in the blood is reduced, the amount of water in the skin is also reduced. 
It increases the skin’s impermeability and resistance to bacterial disease. 
It reduces the extent of skin diseases that can spread over a large area of the body such as psoriasis. 
It reduces allergies and other skin problems.  
Fasting reduces toxic secretions in the intestines and reduces the fermentation that can lead to ulcers. 
These are some of the health benefits of fasting, from which you may understand that what that communist atheist says has no sound basis. 
Moreover we fast in obedience to the command of Allaah Who has enjoined us to fast, as He says (interpretation of the meaning): 
“O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (the pious”
[al-Baqarah 2:183]
If Allaah commanded us to kill ourselves we would do it in order to please our Lord, but we believe with certainty of faith that He would never command us to do anything but that which is in our best interests in this world and in the Hereafter. 
And Allaah knows best.