Question

why should we avoid eating junk food

Answer 1

Eating junk food will not keep u healthy . Junk food is just like a momentary happiness. Junk food is only for filling one`s stomach and nothing else. You get any necessary proteins, vitamins, minerals etc from although if u get it it will be in limited amount .
But on other hand if u eat healthy food like chappathi,etc u will get nutrients more than enough and you will feel comfortable and relaxed after having such meals. But if  u opt for junk food it will surely be hazardous. U may not come to know about it`s effects but at the long run u will comparatively be weaker of pale as compared to others. And even junk food are not made in hygienic conditions they are made with rancid oils etc.
And even there is a saying FAST FOOD ARE THE FASTEST WAY FOR ILL HEALTH
note that junk food are only for taste and filling one`s stomach and leading to ill health
so in order to lead a healthy life refrain from Junk food

Answer 2

Because it can cause rancidity. Rancidity is the Arial oxidation of fatty food. It causes loss of concentration and main cause is that they made us chubby which is a house of problems.