As far as I am concerned, food should be whole and fresh. By whole, I mean unprocessed, or at least processed minimally. Fresh organic produce and meat is always best. When buying minimally processed foods such as canned and dried foods, I read ingredient labels. A good rule to follow is if there is more than 1 ingredient that you don't/wouldn't keep in your pantry, don't buy it. It can be very difficult to resist the temptation of convenience. Unlike most people, I do not see any major issue with sugar and fat; moderation is the key here. I indulge in a chocolate bar or bag of chips on a monthly basis, but not weekly. I also will buy hot dogs and kraft dinner for the kids, but these are not things they have regularly.
When buying seeds to plant in our garden we choose organic heirloom seeds which are seeds that were grown during early periods in human history; many of which are seeds from plants that grew wild. They are open pollinators which means you can harvest seeds yourself year after year and sustain your garden. Most common non-heirloom seeds which give us the fruits and vegetables we see at the grocery store have been propagated over the centuries through grafts and cuttings.
Now a days many plants have been genetically engineered and they are typically dosed with pesticides at least twice per growing season. People will say "I wash my produce before I eat it." Frustration! They don't seem to understand that these chemicals are absorbed through the plants skin. Imagine taking pesticides and rubbing them on your children's skin at least twice as they grow, once as infants and again around puberty. Be sure to apply it right after a good local rain and do not wash it off until the next rainfall. This is what is done to our fruits and vegetables which we feed ourselves and our children EVERY DAY! Here is a good link with a list of produce with the highest and lowest levels of pesticide residue (simply what was not washed away or absorbed by the plant). The highest levels are the foods that you should foot the bill for organic. Then think of giving you child large doses of hormones and antibiotics regularly throughout their lives... this is what is done to the animals which supply us with our meat, milk products and eggs which we feed our families EVERY DAY! Small traces of these chemicals are in all of the animal products we consume which means that we are getting small doses of these chemicals every time we eat them.
So, the age old question of "what is healthy food?" is only getting more and more difficult to answer with more complications being thrown in the mix regularly. My hope is that one day people will get back to producing the majority of their own food and what we can't supply to ourselves we will trade and exchange with our neighbours within our community.
One can dream.