There are Three types of Guna
1. Sattva (the quality of goodness): Sattva is Calm, Saintly. Its brings up quality that allows spiritual holiness, forgiveness and submission to God. Foods dear to those in the mode of goodness increase the duration of life, purify one’s existence and give strength, health, happiness and satisfaction. Such foods are juicy, fatty, wholesome, and pleasing to the heart. BG reference
2. Raja (the quality of passion): Foods that are too bitter, too sour, salty, hot, pungent, dry and burning are dear to those in the mode of passion. Such foods cause distress, misery and disease. Onions and garlic fall into this category.BG reference
3. Tamas (the quality of ignorance): Food prepared more than three hours before being eaten, food that is tasteless, decomposed and putrid, and food consisting of remnants and untouchable things is dear to those in the mode of darkness.BG reference
We are what we eat, so you see. When you see the whole picture in Gunas perspective it will start making sense altogether. Why we can't eat certain types of foods, why we do not use some types of food in our Pujas (rituals or worship).
In order to be a saint/God's servant, humans must suppress raja and tamas guna, and eating sattvic food is a large part of that.