Longevity Briefs: Does Living Near The Coast Increase Cardiovascular Risk?

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem:

Collectively, cardiovascular diseases are the leading cause of death in the world, easily surpassing cancer and neurological diseases combined. A primary determinant of cardiovascular risk is blood pressure, since damage to the lining of the blood vessels is an initiating factor in cardiovascular diseases, and this damage is a lot more likely to occur when blood pressure is higher. Lifestyle factors such as the ‘big three’ (diet, exercise and sleep) can help keep blood pressure normal. Blood pressure also tends to rise with increasing age as blood vessels become stiffer and therefore less able to stretch to accommodate the pulse of blood at each contraction of the heart.

Another lifestyle factor associated with elevated blood pressure is salt intake. Higher salt intake means that the body must retain more water to ensure that the concentration of salt within the blood remains correct. This results in higher blood volume and therefore higher blood pressure. But while we can control the amount of salt we eat, drinking water is often overlooked as a source of salt. The salinity of drinking water is higher in coastal regions, and as sea levels rise, sea salt increasingly infiltrates freshwater supplies. Given that around 3 in 8 people on Earth live near the coast, this is potentially a serious global health concern, but previous studies investigating the relationship between water salinity and blood pressure produced mixed results. In this study, researchers decided to conduct a large-scale analysis of existing studies in an attempt to paint a clearer picture of the importance of drinking water salinity.

The discovery:

Researchers conducted a systematic review and meta-analysis, which means scientists statistically combined data from multiple independent studies. Different studies use different methodologies and look at different populations, meaning their results do not always agree with each other. Combining data from these studies together can produce more reliable conclusions than offered by a single study. The analysis included 27 studies, with over 74,000 participants in total from 7 different countries. These were all observational studies, meaning they compared populations drinking higher salinity water to those drinking lower salinity water (as opposed to randomly assigning people water with different salt contents and measuring the outcome).

Overall, researchers measured an average (mean) increase in blood pressure of 3.22 mm Hg systolic and 2.82 mm Hg diastolic for high salinity water intake vs low salinity. This translated to a 26% increased risk of developing hypertension, which is defined as a systolic blood pressure of more than 130 mm Hg or a diastolic blood pressure of more than 80 mm Hg. To put this into perspective, this is roughly equivalent to being sedentary vs physically active in terms of increased risk of hypertension. In people without existing cardiovascular disease, a 5mm Hg reduction in systolic blood pressure is associated with a roughly 10% reduction in risk of cardiovascular events like heart attack and stroke. However, in this meta-analysis, there was insufficient data to analyse a possible relationship between water salinity and cardiovascular events.

The implications:

This study suggests that there is an association between water salinity and risk of hypertension which, considering the increasing number of people that are exposed to higher salinity water, could be translating to an increase in cardiovascular disease. More research is needed to analyse the relationship between water salinity and cardiovascular disease specifically. It’s also important to note that, as this was a meta-analysis of observational studies, it cannot prove that high salinity caused the increased risk of hypertension, as other factors could have contributed. For example, areas with higher water salinity are often poorer with worse infrastructure (especially in developing countries), and coastal regions are associated with many environmental differences besides water salinity (climate, diet and so on). It was up to individual studies to control for these confounders as best they could.

These caveats aside, is it time to start desalinating your tap water? Well, not really – the vast majority of sodium salt intake comes from food, not drinks, and so the kind of person paying attention to their salt intake probably doesn’t need to worry about how much salt is in their water. The problem highlighted by this study is that, at a population level, higher water salinity may be having a significant impact when added on top of existing drivers of hypertension. If so, initiatives to lower salinity could be a cost-effective way of reducing cardiovascular deaths in a way that doesn’t depend on individual action.