Feeding and pleasure
We crave to eat those foods that give us pleasure, appreciated both by our conscious mind and our non-conscious mind. We cannot assume we feel pleasure consciously only, as non-conscious experiences of food might explain addictions to foods even though we eat them with automaticity without the sensation of conscious pleasure.
The pleasure center linked to the mouth, whose pleasure receptors (and displeasure receptors, vide infra) include taste buds on the tongue and aroma detectors in the nose, provides a gateway-like service to our brain-located pleasure centers: it rejects foods that taste bad in the mouth. But its gateway-like service has its limitations.
The mouth pleasure center does not perform reliably to serve our craving for the pleasure of sweet foods, however. Mutated mice without genetic support for tasting sweet should find that pure water and sugar-sweetened pure water taste no differently, their mouth pleasure centers genetically proscribed pleasure from sugared water. Yet, given the choice during their development to maturity, they prefer drinking sugared water. 
Sugared water provides energy, however—calories needed for maintaining health in order to have the pleasure of living. Tasteless sugar water must serve the extra-oral pleasure centers, dictating the preference for sugared water. So sugared water would be sweeter, even if we cannot taste the sweetness in our mouth.
Supporting that line of thinking, water spiked with a non-caloric sugar, one the body cannot use for extracting energy, the sweet-mouth-tasteless mice show no preference for it than for pure water. They crave calories, not sweet-mouth-taste, not mouth pleasure.
How do mutated mice know the sugared water contained calories? Can the mouth detect calories? Possibly. Some mouth cells must absorb and metabolize some of the sugar flowing past them during drinking and coating them between drinks. That will stimulate cellular activity, causing a dynamic of chemical interactions that change the profile of the molecules in the cell, exporting molecular information and heat outside itself. Do those exports give pleasure to the brain, do they explain the preference for drinking water with mouth-tasteless calories?
Can the stomach and intestines detect calories? Are they energy detectors? They certainly can detect carbohydrates, protein, and fat—each a source of calories—influence appetite behavior.
- Lehrer J. (2011) Why Do We Like the Taste of Protein?. Wired Blog: The Frontal Cortex, by Jonah Lehrer.
- Araujo IE, Oliveira-Maia AJ, Sotnikova TD, Gainetdinov RR, Caron MG, Nicolelis MAL, Simon SA. (2008) Food Reward in the Absence of Taste Receptor Signaling. Neuron 57(6):930-941.
- “[F]indings suggest that calorie-rich nutrients can directly influence brain reward circuits that control food intake independently of palatability or functional taste transduction.”
- Sclafani A. (2002) Post-Ingestive Positive Controls of Ingested Behavior. Appetite 36(1):79-83. | online article.
- “Accumulating evidence indicates that there are also close ingestive positive controls of ingestion that operate by conditioning flavor preference and acceptance… the available data indicate that food selection and intake during a meal and over longer time periods involve the integration of orosensory stimuli with both positive and negative post-ingestive feedback signals.”
- Ackroff K, Sclafani A. (2011) Flavor preferences conditioned by post-oral infusion of monosodium glutamate in rats. Physiology & Behavior. Article In Press as of 25-Jun-2011.
- (a) Rats learn to prefer a flavor paired with post-oral infusion [infusion into the stomach/intestines] of 60 mM MSG over a flavor paired with water. (b) Both gastric and duodenal infusion sites were effective, suggesting an intestinal origin of the reinforcing signal. (c) Post-oral MSG [monosodium glutamate] reinforcement was not enhanced by adding 2 mM IMP [inosine monophosphate] to the infusion. (d) The post-oral action of MSG could enhance preference for glutamate-rich foods.
- These data show that MSG-based flavor preferences occur reliably with post-oral conditioning, and tend to endure even when MSG is no longer presented.