What someone eats is often implicated in the cause of these diseases. However, there's an area of research we should pay closer attention to - involving the immune system. Just because an illness manifests as some reaction to a particular food does not mean that food caused the illness. For example, diabetes manifests as elevated blood sugar, but it is not caused by eating sugar or starch. A common form of heart disease involves the accumulation of fat along arteries, but heart disease is not caused by eating fat. (However, once an individual has developed these conditions, what they eat can accelerate disease progression.)
Both diabetes and heart disease are diseases of inflammation. They, as do most chronic diseases, involve immune system cells and processes. It is the immune system which is primarily responsible for the characteristics of inflammation. So, you might say, it is the immune system which is responsible for diseases of inflammation.
The immune system's job is to protect the body from foreign substances, things like bacteria, viruses, and other proteins and molecules that could damage cells. It keeps track of what our normal cells and molecules look like, and when it can't recognize something as belonging to us, it initiates an attack. Example - Urushiol is an oil in poison ivy that elicits an immune response in humans, resulting in the classic signs of inflammation - redness, swelling, heat, pain, and loss of function. Urushiol does this by chemically changing the shape of proteins on skin cells it contacts. The immune system then fails to recognize these proteins as our own.1
We already know that exposure to cigarette smoke and environmental pollutants such as pesticides increase - substantially - the incidence and severity of many chronic diseases, including diabetes, heart disease, and many cancers. We need to pay closer attention to the role of the immune system in the development of these diseases, especially given the rise in pollution that accompanies an increasingly urbanized world.
By the way, this is one reason I feel genetically engineered foods need more study, since they introduce foreign proteins to our bodies which have been shown to elicit an immune response.
I was looking for a photo to accompany this post and saw this. It is the Citarum River near Jakarta in Indonesia, "choked by the domestic waste of nine million people and thick with the cast-off from hundreds of factories."
The water is still used for drinking and to irrigate local crops.