London: Ever wondered why people usually have a craving for sugary or fatty snacks rather than healthy options like fruits and vegetables? Well, it has nothing to do with one having a ‘sweet tooth’, say British experts.

According to them, food craving is not simply hunger -- in fact, it occurs for a variety of reasons like evolution; psychological factors such as stress, anxiety and unhappiness; and sometimes a genuine need for certain foods.

"There are a number of chemicals in the brain that are associated with this. First, there is dopamine, a brain chemical that's involved in learning and concentration. When we experience something new, dopamine is released in brain.

This works in cycle with other brain chemicals called opioids, which give feelings of enjoyment and pleasure. The combination of these two factors mean the brain associates certain activities with pleasure, and it teaches us to do them again and again," the 'Daily Mail' quoted Dr Leigh Gibson of Roehampton University as saying.

Then, according to the experts, from an evolutionary point of view, junk food cravings are linked to prehistoric times when the brain's opioids and dopamine reacted to the benefit of high-calorie food as a survival mechanism.

"We are programmed to enjoy eating fatty and sugary substances, and our brains tell us to seek them out. Today, we still have the same chemical reactions to these so-called hyper-palatable foods, causing an unignorable desire despite there being less of a nutritional need for them," he said.

Another factor in longing for sugary or fatty foods is stress, say the experts.

"The body produces a hormone called cortisol in response to stress. Its primary functions are to increase sugar in the blood to be used up as energy by the body's cells, suppress the immune system and aid in fat, protein and carbohydrate metabolism.

"It also blocks the release of leptin and insulin, increasing hunger. This is why studies have shown that when we're stressed, we're more likely drawn towards high-energy foods, such as cakes and sweets," said Dr Gibson.

Moreover, there are the psychological components to cravings. "Mood is unquestionably a potent context especially negative mood. We crave reward foods. The pattern for this is partially set in childhood when parents give us sweet food to show love or reward," Prof Andrew Hill, Head of Psychiatry and Behavioural Sciences at Leeds University, said.