Traditionally those of a mystical persuasion (including the authors of ancient texts, modern sages, and me) refer to all action that takes place in the Universe as the result of interplay of the 3 qualities, or gunas, and the 5 elements (earth, air, water, fire, and ether).
Depending on the combination of these elements, and their interplay, specific actions arise. It is easy to witness this at the chemical level. Combine oxygen and hydrogen and they will invariably form water. If you or I could have a conversation with these elements - that is, if they could talk - and they were to tell us that it wasn't necessary at all but rather they had decided, on their own and entirely of their own accord, to combine to form water, we would laugh. They had no choice in the matter. It is in the very nature for water to be formed from these atmospheric elements. Tell that to the rainy day!
But at the human level, an individual is not so easily convinced. Imagine telling someone that they do not have full responsibility for their actions, which are caused by the interplay of the elements and qualities present within their body, many of these determined by their birth and physical location, although to some extent by personal choices such as the food they consume. You are liable to get smacked. The ego is a sensitive thing, and each of us likes to be in charge, even if it's the illusion of control. But I have had first-hand experience that everything happens by itself, and that an individual doer is totally a figment of the imagination.
First a bit about the three gunas. These are sattva, rajas and tamas. Sattva is the harmonious and tranquil; rajas the passionate and restless; tamas the slothful and inert. These qualities are present in food and then in the organism consuming the food. Sattvic foods are fresh fruits and vegetables; rajasic foods are spicy foods and fried foods, and foods eaten to excess; and tamasic foods are processed foods as well as meat, fish, chicken, eggs and alcohol.
When I was in college, I decided to experiment with animal foods. I was lifting weights at the time and interested in packing on a lot of muscle, and the bodybuilding magazines I devoured said animal protein was the way to go, so I went. Up until then I had been a vegetarian. But almost overnight, it became eggs for breakfast, canned tuna for lunch, and beef or grilled chicken for dinner. Snacks were whey protein shakes. In the span of about 3 months I went from my high school graduation weight of 165 to about 185 pounds. The added 20 pounds were mostly muscle. But there were other physical changes. I broke out in raging acne, for instance. And the crippling fatigue. I slept like a log and in the morning could hardly get out of bed, even after a whole night's sleep! And the mood changes. I was restless (from eating too much) and lethargic (the influence of tamas). It was as if I had become a different person.
With time I gave up the animal products and returned to a plant-based diet, ultimately transitioning from vegetarianism to veganism. The change my body and actions underwent as I returned to a diet mainly sattvic in nature was to be expected. Harmony and tranquility once again prevailed. Although I do still eat spicy food. You can't sit around and meditate forever. Some action is, after all, required.
My point is that though we are responsible for what we put in our bodies, we are not the doer of our actions, which are the result of the interplay of the elements and qualities we choose to include and emphasize. After all, the body is just a conglomeration of chemicals.
Try it for yourself. Change what you eat (hopefully for the better) and sit back and observe how your behavior is affected.
To quote the Srimad Bhagavatam: "Heaven is the domination of Sattva in the mind."