Dieting will always feel grim as the unpleasant feelings associated with hunger are caused by a certain type of hypothalamic neuron. Being 'hangry' (angry brought on by hunger) is actually an evolutionary response to tell our bodies to go and find food, say researchers.

Researchers report that it is normal to feel down or angry when dieting, as hunger releases neurons which trigger bad moods. They investigated responses to these negative moods in mice, and found that as soon as the neurons were fired in the brain, they actively started looking for food.

The researchers, writing in the report published in Nature, said: "Behavioural responses due to deviation from homeostasis are critical for survival, but motivational processes engaged by physiological need states are incompletely understood."

The study, from the Howard Hughes Medical Institute, used two sets of mice to test the theory surrounding mice and their negative feelings. Half of the mice were treated with the AGRP neuron, which triggers the unsettling sensation of hunger, whilst half were not treated with the neuron.

The results found that for the AGRP neuron group, their choice of meal was almost anything they could find. On the other hand, the mice that did not feel the effects of the AGRP neuron had time to choose their preferred dinner.

"Correspondingly, deep-brain calcium-imaging revealed that AGRP neuron activity rapidly reduced in response to food-related cues," wrote the researchers. Humans, like mice, have ARGP neurons which play a key role in managing hunger, which leads the researchers to believe the results are transferable.

Humans can only lose weight when our bodies burn more energy than it consumes. For that to happen, we have to – inevitably – endure the horrible feelings associated to hunger.