Chalk and cheese: the taste of calcium is a warning signal
The so-called “sixth taste” might be a strongly conserved safeguard, fruit fly research suggests. Jeff Glorfeld reports.
Researchers trying to more fully understand the ability to taste calcium have established that it can be detected by fruit flies (Drosophila melanogaster), suggesting that it may function as a warning signal in a wide range of animals, including humans.
Although it has been disputed as a basic taste, calcium is sometimes called the “sixth taste”, along with sweet, sour, salty, bitter and savoury (umami), which the tongue's receptors can identify.
A 2012 study led by American psychobiologist Michael G. Tordoff and published in the journal Scientific Reports found that many animals satisfied their physiological need for calcium by locating and consuming calcium salts, and that this “calcium appetite” was controlled by taste.
The report said concentrated calcium salts were rejected by nutritionally replete animals but avidly ingested by calcium-deprived ones.
Tordoff’s study concluded that “humans can detect calcium by taste”, and identified the gene T1R3 as the responsible receptor.
In this latest research, reported in the journal Neuron, scientists at the University of California at Santa Barbara (UCSB) in the US, and colleagues in Korea, found a unique class of gustatory receptor neurons, necessary for the calcium tasting ability in fruit flies.
Surprisingly, given that some calcium is necessary to sustain life, the flies were indifferent to low levels and averse to high.
“We wanted to understand the underlying mechanisms used to respond to the presence of calcium in food,” explains the report’s senior author, UCSB’s Craig Montell. “We not only identified the taste neurons but also found three receptor proteins that are important in sensing calcium.”
The researchers found that eliminating any one of these receptor neurons allowed them to perform “an interesting survival experiment".
They used petri dishes with one side containing only fructose and the other a mix of fructose and a high level of calcium. Mutant flies, in which any one of the three newly found receptor neurons was removed, were unable to distinguish between the contents of the two halves of the dish. As a result, they consumed too much calcium and eventually died.
Normal flies rejected the high-calcium side, ate only pure fructose, and survived.
“It turns out that fruit flies don't have a mechanism for sensing low calcium, even though it's good for them, but they are trying to guard against consuming too much,” Montell says.
"In humans, high calcium is associated with many diseases and can even be life threatening. Our results suggest that calcium taste might function primarily as a deterrent in a wide range of animals, including humans.”