This question is difficult to answer as-is, most importantly because we have not identified who "they" are. "They" could be the government, or it could be your parents. It is interesting that the question can be nevertheless understood because we have a cultural idea of a specific but anonymous Other who "puts things in our food", implying that this is a tampering or poisoning of that food. The assumptions in the question are as interesting as the question itself.
Food additives could be addressed in a variety of ways, such as by discussing GMOs and the ethics of genetically altering food that may then genetically alter us, or we may approach it from a healthcare point of view that contrasts the availability of food with the nutrition in that food. For example, you may have seen "enriched flour" in the ingredients of something you buy at a supermarket.
"Enriched" is really just a euphemism. Part of the process of making flour, involving sterilization, also removes some of the nutrients in the flour. Enrichment is simply a matter of adding these nutrients, such as iron, back into the flour again. In this sense we could say that "they", i.e flour companies, "put things in our food", but these things aren't any different from what you would normally eat.
On the other hand, there are products like corn syrup. Corn is a subsidized and therefore profitable crop in the United States, and the source of much controversy regarding sustainability and the involvement of the government in the country's agricultural practices. To add to demand for corn, the United States developed corn syrup as a replacement for sugar, which is largely imported. Corn syrup is not nutritionally identical to sugar and may, due to the way it is metabolically processed, be at least partially responsible for our high obesity rates.
There are also a wide variety of additives that are byproducts of our modern lifestyle; dyes to enhance the appearance of food, preservatives to increase its shelf life, and artificial or synthetic compounds that replace an organic ingredient due to cost or efficiency. However it is difficult to attribute this to a "they" because these practices are so widespread, and the distinction between an additive, with some unhealthy or sinister connotation, and a simple ingredient, is highly dependent upon the context of the argument.
Your question is a bit vague but I will give it a shot in answering it. I learn from your use of 'Our Food' that your are a consumer who most likely buys food from the local grocery or super market nearby. Your use of 'they' must refer to the people who sell the food to the grocery as the super markets are only the middle man in the transaction.
I think your asking about Genetically Modified Food, these are enhanced food so the companies can profit faster as the food grows much faster but is not good for consumers health.
By "they" I assume you mean farmers. You may have heard about the trend of eating organic food. People want organic food because what is put in our normal food is bad for our bodies. In order to make the food we grow grow faster, bigger, and last longer, farmers are pressured by the government/food industry to use pesticides, preservatives, and hormones. This creates genetically modified (GMO) foods that have completely changed their genetics to meet our needs as a society. So, what is in our food? Whatever it is is not natural. Some people say this stuff is over exaggerated, but there are scientific facts that back up all this information.