Some claim to have 'a sweet tooth', or 'a salt tooth'. And many believe cravings are the body's way of telling them what they need. But how true is that really?

Experts believe that cravings occur for a variety of reasons. They attribute them to evolution, psychological factors such as stress and unhappiness, and  -  sometimes  -  a genuine need for certain foods.

'It's crucial to remember that a food craving is not simply hunger,' says Professor Andrew Hill, Head of the Academic Unit of Psychiatry and Behavioural Sciences at Leeds University.

Hunger is the body's way of making sure it is provided with energy, in the form of nutrients from food. When the stomach is empty, it releases the hormone ghrelin, which communicates with the brain's command centre, the hypothalamus. This creates the feeling of hunger and is how we know when to eat.

Satiation is signalled by the release of the hormones leptin by fat cells, and insulin by the pancreas, in response to increased blood sugar.

Cravings, however, are much more complex.

'Those who are starving will eat literally anything  -  even foods they do not enjoy  -  to stay alive,' says psychologist Dr Leigh Gibson, Reader in Biopsychology at Roehampton University.

'Cravings, on the other hand, are an overwhelming sensation of desire for a certain food. There are a number of chemicals in the brain that are associated with this.

'First, there is dopamine, a brain chemical that is involved in learning and concentration. When we see or experience something new, dopamine is released in the brain.

'This works in tandem with other brain chemicals called opioids, which give us feelings of enjoyment and pleasure. The combination of these two factors mean that the brain associates certain activities with pleasure, and it teaches us to do them again and again.

'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.

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