'Hunger is the Best Sauce in the World'
as Miguel de Cervantes put it

Things really do taste better when you're hungry, according to researchers at the University of Malawi. The team, led by Professor Yuriy Zverev, found that their test subjects' ability to detect sweet and salty tastes was significantly increased after fourteen to sixteen hours without food, while sensitivity to bitter tastes remained unaffected.

Previous studies have shown a correlation between taste sensitivity and hunger in obese humans and other animals, but this was the first test on fit and healthy subjects to discriminate between sensitivity to several different taste elements.

The researchers asked a group of sixteen fit, non-smoking, non-drinking male university students to eat a full lunch and supper, and then sip solutions containing either sugar, salt or quinine in order to determine the threshold level at which they could detect their taste. The subjects were then deprived of any food until between ten and eleven the next morning, and the taste tests repeated. This time the subjects were able to detect the sweetness of a sugar solution just two thirds as concentrated as the weakest they had been able to detect in their satiated state. They were able to taste the salt in solutions almost twice as dilute as they could previously distinguish.

The mechanism behind the selectively increased taste sensitivity in hungry subjects is not yet clear; Zverev suggests three possibilities. Firstly, it may be that systemic activation of the brain may affect the central brain structures involved in the perception of taste. The second possibility is that the taste receptors themselves have their sensitivity reduced by satiety. Alternatively, it may be changes in the autonomic nervous system which make the difference.

Zverev argues that the contrasting biological roles played by sweet and salt on the one hand, and bitter on the other, may explain why sensitivity to the former is enhanced by a state of hunger, while the latter remains unaffected. Sweet and salty tastes indicate the presence of nutrients such as carbohydrates and sodium, and tend to result in a positive reaction in the consumer. Bitter tastes, on the other hand, usually trigger an aversive response, probably because our sense of bitterness evolved to discourage us from eating foods containing poisonous alkaloids. There should be no further need to stock up on calories and salts after a good meal, while it remains as important as ever to avoid toxins - so the Malawi findings make good evolutionary sense.

See Zverev's paper here: http://www.biomedcentral.com/1471-2202/5/5