Although the link between salt consumption and high blood pressure is well known, the underlying mechanism is not so clear. Now, a new study of rats finds that high salt intake over a long period interferes with brain circuits that normally suppress high blood pressure.
The researchers - led by Charles Bourque, a professor in the Faculty of Medicine at McGill University in Montreal, Canada - report their findings in the journal Neuron.
According to the World Health Organization (WHO), a high intake of sodium - which in our diet mostly comes from salt (sodium chloride) - can raise a person's risk of high blood pressure, which in turn increases the risk of heart attack or stroke. For this reason, WHO recommend that adults limit their salt intake to no more than 5 g of salt a day (2,000 mg of sodium). Low potassium can also raise the risk of high blood pressure.
Many foods already contain sodium, without any being added. For example, milk and cream contain around 50 mg of sodium per 100 g, and eggs contain about 80 mg per 100 g.
Add to this the foods we eat that contain added salt, and the amounts that contribute to daily intake soon add up.
For instance, bread contains 250 mg of sodium per 100 g, processed meats like bacon around 1,500 mg per 100 g, and snacks like cheese puffs and popcorn around 1,500 mg per 100 g, and stock cubes approximately 20,000 mg per 100 g.
Prolonged high intake of salt disables brain circuit that stops blood pressure going up
Prof. Bourque, who also undertakes research at McGill University Health Centre's Research Institute, explains what happened in the rats when their salt intake was raised:
"We found that a period of high dietary salt intake in rats causes a biochemical change in the neurons that release vasopressin (VP) into the systemic circulation. This change, which involves a neurotrophic molecule called BDNF (brain-derived neurotrophic factor), prevents the inhibition of these particular neurons by other cells."
When salt intake is not high, a brain circuit that detects arterial pressure inhibits or stops VP neurons from releasing chemicals that would raise it. When salt intake is high and prolonged, it disables the control circuit, allowing VP neurons to release the chemicals that raise blood pressure unhindered.
Although the finding solves a significant part of the mystery of how salt increases blood pressure, there are still many unanswered questions, note the authors. For instance, is the mechanism the same in humans? And if so, how might it be reversed, and what might the targets be for treatment?
In the meantime, Prof. Bourque says it is still important to limit dietary salt.
Tips for lowering dietary salt
Just paying a little more attention to the food we buy and what we eat in restaurants can help us reduce our salt intake without it being a big deal. Here are some tips:
- Buy fresh or frozen (no sauce) vegetables
- If buying canned vegetables, choose the "no salt added" variety
- Buy fresh poultry, fish, pork and lean meat rather than canned or processed
- When buying pork or fish, check that no saline or salt solution has been added
- Limit sauces, mixes and "instant" food products, including ready-made pasta and flavored rice
- Check the nutrition facts label and choose the ones with lower amounts of salt or sodium
- When eating out, plan ahead and go to restaurants that show salt or sodium levels on the menu
- Check online for nutrition information before you buy food at a chain restaurant or fast-food outlet
- Ask the restaurant not to add salt to your dish
- When eating out, ask for toppings on the side, and be aware that sauces and dressings may have high levels of added salt.
For further information, see the US Centers for Disease Control and Prevention advice on salt consumption.
Funds for the study came from the Canadian Institutes of Health Research, National Institutes of Health, and the Fonds de recherche du Québec - Santé.
Meanwhile, Medical News Today recently reported a placebo-controlled trial that found drinking a daily glass of beetroot juice reduces high blood pressure. Beetroot juice contains high levels of inorganic nitrate that in the body converts to nitric oxide, which relaxes and dilates blood vessels.