Preventive medicine: 

Dear believing brothers, the Prophet PBUH mentioned in so many Ahadeeth what we call today "preventive medicine", and it is said, "An ounce of prevention is better than a pound of cure." 

The heart: 

The Prophet PBUH said:

(Truly in the body there is a morsel of flesh, which, if it be whole, all the body is whole, and which, if it is diseased, all of [the body] is diseased. Truly, it is the heart.)

[Al-Bukhari, on the authority of an-Nu'man bin al-Basheer]

The heart is the size of the grasped fist size, and this muscle starts pumping in your first month in your mother's womb, and it never stops working till the end of the life journey.

The doctor's knowledge might tell
if people have time left before dying
But when the journey comes to an end
The doctor gets bewildered and his drugs fail

This heart starts pumping since the first month while you are in your mother's womb, and it  keeps working till the journey of life is over; till Allah ends this journey through death. 
If you outspread your fist then grasp it 70 times in one minute, then this is the average rate of heart beats in one minute. During any great effort, that rate might reach 140 beats per minute. The heart is like an engine, but this engine is flexible, so the matter is urgent, it increases up to 70 horses, and when it is not urgent, it gets down to 4 horses. Thus, the power of this heart is changeable. Scientists said that the heart beats 100.000 times in 24 hours, and it beats 3 million times in one month. However, when man reaches his eighties, the average of his heart rate is 2880 million beats according to scientists.  
It is amazing that this small heart is able to pump between 3000 and 5000 gallons of blood per day. Each pump equals half a cup of tea, and so it bumps between 3000 and 5000 gallons of blood daily. 

If an organ in the human body malfunctions, man will lose some of his capabilities, but if the heart stops working, man's life will be over, because it depends on the function of the heart. 

The good health of heart: 

Let me mention the words of wisdom said by one of the most remarkable doctors, "The stomach is the house of diseases and diet is the best of drugs." There is no wonder to read a scientific article that confirms these wise words. The author of the article wrote, "The heart's health, its  regular function, and its long living depend on the amount of food that fills out the stomach and the kind of that food.  This explains why the Prophet PBUH said:

(The human does not fill any container that is worse than his stomach. It is sufficient for the son of Adam to eat what will support his back.) 

[Al-Bukhari, on the authority of Miqdam bin Ma'di Karb]

In another Hadith, the Prophet PBUH said: 

(We are people who do not eat unless we are hungry and if we ate we do not get full up (with food).)

The preventive medicine is taught to the students in one of the faculties in a book that consists of 1000 pages. If we want to summarize all these pages, we will come up with this Prophetic Hadith: 

(We are people who do not eat unless we are hungry…)

If man gets hungry, his cells start to excrete large amounts of digestive secretions, this eases up digesting the food in addition to feeling the nice taste of food. So, if you eat when you feel hungry, you will surely enjoy the food. This fact explains why the Prophet PBUH said:

(Taste of food is best appreciated when man is hungry) 

The best taste of food is appreciated when you are hungry, "The stomach is the house of diseases and diet is the best of drugs." The Prophet PBUH said:

(A human being fills no worse vessel than his stomach. It is sufficient for a human being to eat a few mouthfuls to keep his spine straight. But if he must (fill it), then one third of food, one third for drink and one third for air.) 

[Ibn Majah]

There is one more point referred to in the following Hadith: 

(Truly in the body there is a morsel of flesh, which, if it be whole, all the body is whole, and which, if it is diseased, all of [the body] is diseased. Truly, it is the heart.) 

[Al-Bukhari, on the authority of an-Nu'man bin al-Bashir]

Interpretation and Hadith scholars interpreted this Hadith in a different way referring to the fact that the soul has a heart. The body has a heart, and the soul has a heart too, so he whose heart is clean from deceiving, envy, and all other bitter feelings is healthy. Allah the Almighty says:

(The Day whereon neither wealth nor sons will avail* Except him who brings to Allah a clean heart [clean from Shirk (polytheism) and Nifaq (hypocrisy)].)

 [Ash-Shu'ara', 88-89]

A doctor said, "There are too many things that destroy the heart among which is stress". Allah, Exalted and Sublime be He, says: 

(So invoke not with Allah another ilah (god) lest you be among those who receive punishment.)

[Ash-Shu'ara', 213]

Accordingly, Shirk (polytheism) causes heart's diseases, while faith turns the heart into a healthy one.

(How wonderful is the case of a believer; there is good for him in everything and this applies only to a believer. If prosperity attends him, he expresses gratitude to Allah and that is good for him; and if adversity befalls him, he endures it patiently and that is better for him.)

[Muslim, on the authority of Abu Yahya Suhaib bin Sinan ]

The Hadith indicates that having faith, having a clean heart, having pure soul, trusting in Allah the Almighty, and believing in Allah Alone as the only Deity are reasons for having a healthy heart. That doctor, went on saying, "When doctors dig for the reasons of heart's diseases, they found out that the main reason is stress." Well, the major cause of stress is Shirk, and after it comes overeating, greasy food, and lack of movement. Concerning the last cause, the Prophet PBUH used to serve himself; he used to clean his cloths and broom his house, and he was at the disposal of his family's needs. 

So moving around and making effort, monotheism, moderateness in eating, and watching out for the kind of food are the four elements that can prevent any damage that might  affect the heart.

Dear believing brothers, all the Prophet's sayings match completely the recent scientific facts, because his sayings are but an inspiration that is inspired, and he never spoke of his own desire.  

So if you are keen on having good health, energy, power, and comfort, you should follow the  way of Prophet PBUH, and you should remain a strong believer who never fears human beings, but he fears Allah the Almighty Alone. 

(The Hand of Allah is over their hands.)

[Al-Fath, 10]

(You killed them not, but Allah killed them.) 

 [Al-Anfal, 17]

(…And to Him return all affairs (for decision). So worship Him (O Muhammad PBUH) and put your trust in Him.)

[Hud, 123] 

Verily, herein lies the good health of the heart; it is derived from trusting in Allah the Almighty, believing in Him Alone, depending on Him, submitting  to Allah the Almighty, steadfastness, and good deeds. Furthermore, this heart which has no grudge, envy, deception, bad intention and so on, is healthy, because some of the major causes of heart's diseases are stress and overeating.

(A human being fills no worse vessel than his stomach.) 

[Al-Bukhari, on the authority of Miqdam bin Ma'di Karb]

(We are people who do not eat unless we are hungry and if we ate we do not get full up (with food).

Man should choose the kinds of food carefully. The Noble Prophet PBUH was picky about what he used to eat, so he would eat the healthy kinds of food. 

Dear believing brothers, what we have discussed so far is part of the Prophetic medicine.