Scientists to design heart healthy salt?

Top heart charity launches 'pinch of salt' consumer awareness campaign in the UK to tackle salt consumption while scientists in the US investigating the link between salt and taste forsee new healthy salts in the future.

Findings from a recent survey by the British Heart Foundation (BHF) revealed that 78 per cent of participants add extra salt to food during cooking or at the table, with 29 per cent adding salt both while cooking and at the table.

The BHF is concerned that too many of us are consuming too much salt which can raise blood pressure, leading to a greater risk of coronary heart disease (CHD) and stroke. The daily average intake in the UK is 9g, compared to the recommended daily allowance of 6g for an adult.

Food manufacturers are frequently the first port of call for organisations keen to cut the level of salt intake. This week, Belinda Linden, head of medical information at the British Heart Foundation​ was no exception: "Many processed foods, such as baked beans contain high levels of 'hidden' salt. In fact, up to 85 per cent of a person's dietary salt intake comes from processed foods."​

But new research published this month suggests that the human body could be fully aware when the levels of salt, after all essential for survival, reach a potentially harmful level.

Researchers at the university of Iowa, interested in identifying the receptors that detect small quantities of salt and understanding salt-sensing mechanisms, published their findings in the 3 July issue of Neuron​.

"Given that salt is essential for survival, it is not surprising that animals have developed the ability to detect salt, even at low concentrations. This sense allows them to seek out, and then consume salt,"​ said researcher Michael Welsh.

Previous research suggested a role for a specific type of protein in salt-sensing. Lei Liu, UI postdoctoral researcher and lead author of the study, turned to the fruit fly (Drosophila melanogaster​) to investigate these proteins, known as ion channel proteins.

Fruit flies and humans share the ability to detect salt. Fruit flies also respond to salt in ways that are similar to those seen in humans and other animals. For example, fruit flies are attracted to low salt but are repelled by high salt.

"In humans the taste system is pretty much a puzzle because it is hard to study,"​ Liu said. "But in fruit flies it is very easy to study and you can quickly test ideas. Also, fruit flies are a great genetic model where you can easily screen many different genes to determine what they do and how they interact."​

The ion channel genes studied are called pickpocket (ppk). The UI team discovered that two of these ion channel genes are involved in the detection of low salt concentrations. The study also suggests that these genes play some role in detecting high salt concentrations, but it is likely that other pathways also are involved in high salt detection.

The UI team focused on the two pickpocket genes, ppk11 and ppk19, because they found that these genes are expressed in body parts of the flies that are involved in taste-sensing. In the fruit fly larva (young flies) the genes are expressed in a body part known as the terminal organ, which resembles a human taste bud.

Using a series of behavioural experiments, the UI researchers​ discovered that disrupting the two genes impaired the ability of young flies to detect low salt concentrations.

Normally, young fruit flies prefer water containing low salt concentrations to plain water. However, young flies with disrupted genes were unable to detect low salt. Disrupting either gene also altered how both young and adult flies reacted to high salt concentrations.

"When we disrupt the genes we see defects in behaviour,"​ said Liu. "To see if the receptor cell itself is being affected, we measured the action potential (nerve impulse) of the receptor cell in the terminal organ of the mutant larva. The response of the receptor cell to salt is reduced by the mutation."​

The UI study also showed that the two genes play a role in the ability of flies to distinguish between different types of salt (sodium chloride and potassium chloride). However, disrupting the genes did not seem to affect the fruit flies' responses to other tastes.

Understanding salt-sensing mechanisms and identifying two ion-channel proteins that detect low salt in fruit flies may be directly relevant to the salt-sensing system in humans.

Although salt is an essential nutrient, individuals with certain conditions such as high blood pressure or heart failure are often advised to reduce their salt intake. Liu said that identifying the receptors involved in salt detection may eventually aid in the design of salt substitutes, which enhance flavour but do not hurt our health.