Poisons Of The Heart

Unnecessary and harmful speech, unrestrained glances, and overindulgence in food, and bad company have the strongest negative impact upon the heart's functioning.

Unnecessary and Harmful Speech
"A slave (of Allah) may utter a word which pleases Allah, without giving it much importance, and because of that Allah will raise his or her ranks (of reward); a slave (of Allah) may utter a word (carelessly) which displeases Allah, without thinking of its gravity, and because of that, he or she will be thrown into the hellfire." (recorded by Bukhari)

 Narrated Sahl bin Sa'd: Allah's Apostle said, "Whoever can guarantee (the chastity of) what is between his two jaw-bones and what is between his two legs (i.e. his tongue and his private parts), I guarantee Paradise for him." 

Guarding the mouth and tongue means protecting it from unlawful speech such as lying, backbiting, slandering, cursing, quarreling and the like. This also includes abstaining from foods and drinks which are unlawful.

Once words are spoken, they can cause extreme harm, which can be difficult to remedy. 

Unrestrained glances

In relation to modesty and protecting the private parts, Allah (swt) mentions the commandment to lower the gaze and avert it from anything that is unlawful.

24:30 Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do.

24:31 And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their headcovers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allah in repentance, all of you, O believers, that you might succeed.

 Gazing at unlawful things may provoke the heart to evil such as attraction and desires, which may then lead a person towards forbidden behavior in order to fulfill those desires.

"Shaitan enters with the glance, for he travels with it, faster than the wind blowing through an empty place. He makes what is seen appear more beautiful than it really is, and transforms it into an idol for the heart to worship. Then he promises it false rewards, lights the fire of desires within it, and fuels it with the wood of forbidden actions, which the servant of Allah would not have committed had it not been for this distorted image."

Falling into these traps distracts the heart and leads one to forget more important matters, such as remembrance and obedience of Allah.
 It can also make the heart become blind due to disobedience, which makes it unable to distinguish truth from falsehood.

Overindulgence of Food

The believer is instructed in both the Qur'an and Sunnah to avoid excess in food and drink. Allah (swt) says, "...Eat and drink, but be not excessive. Indeed, He likes not those who commit excess." (Qur'an 7:31).

Excess in eating leads to laziness in fulfilling obilgatory duties such as praying, as well as other recommended acts.