We define junk food as over processed food that is low in nutrients but high in fat, especially the harmful saturated, hydrogenated and trans fats, sugar, salt and artificial colourants, flavourants, sweeteners and preservatives.
Junk food might be popular, not only because it is highly tasty, readily available and convenient, but because it is potentially addictive. Various animal studies have shown that long term exposure to high sugar or high fat diets can alter brain chemistry in much the same way as addictive drugs, although to a much lesser degree.
A high sugar or fat intake appears to be linked to an increased production of natural brain opioids, which are chemically similar to drugs like morphine and heroin. Opioids make us feel good, but they also reduce the feeling of being full. This could therefore have two consequences: an increased craving for the food because of its feel good effect and a tendency to overeat because we never feel full.
And this is exactly what happens when animals are fed high sugar or high fat foods. They become increasingly greedy for such foods and start to compulsively overeat. And, when the sugar, for instance, is suddenly withdrawn from their diet, they experience mild addiction type reactions including anxiety. Could the same be happening to humans?
While it is difficult to extrapolate results from animal studies to humans there is evidence that high fat diets (over 40% fat intake by calories), typical of fast food and processed TV meals, seem to reduce normal appetite control mechanisms and lead to spontaneous overeating in humans as well.
We may therefore have produced a generation of children who are overly dependent on high calorie junk food. And this, combined with a decrease in physical activity, has led to a dramatic increase in the incidence of childhood obesity.
Remember that healthy, well balanced meals involve quantity, not just quality. For example, hard cheese is a high fat food, which is healthy in moderation, but is harmful if eaten in excess.