Measuring salt intake
The biggest pitfall—doing a blood test
Blood tests cannot measure salt intake. Salt is sodium chloride, and the kidneys keep blood sodium constant within very narrow limits, regardless of intake. They do that simply by transferring all the excess salt to the urine.
In the urine it can be collected and measured. All doctors know this, but busy doctors sometimes have to be reminded.
Every doctor can order the 24-hour urine test that the Heart Foundation now recommends [1], but this recommendation is recent, so only a few practices have it in their regular clinical routine.
Doctors can get full particulars from www.salt-matters.org (this website). They reach this page by clicking Measuring Salt Intake and downloading and printing the two PDFs available through these links:
- Patient handout on how to collect a 24-hour urine sample.
- Professional comments on ordering 24-hour urine collections and interpreting the results.
Why the Heart Foundation recommends the 24-hour urine test
Mother Nature is kind to people who do things properly, but will not put up with any nonsense. Better food will stop your blood pressure rising with age, but it has to be much better food (far less salt than most people eat now) [2]. Your salt intake has to be measured accurately.
Food diaries are not accurate enough, even in professional hands. Processed foods supply 75% of the average family’s salt intake and one brand of the same food may have twice as much salt as another.
A urine test is the “gold standard” for measuring salt intake in medical research. Under normal conditions of activity and sweating about 90% of the salt eaten is found in the urine, and the roughly 10% difference between intake and excretion is usually ignored for practical purposes—daily excretion is usually treated as daily “intake” if it is measured by 24-hour urine collections.
References
1. National Heart Foundation of Australia. Salt and hypertension (professional paper); May 2007.
2. Sacks FM, Svetkey LP, Vollmer WM, Appel LJ, Bray GA, Harsha D, et al. Effects on blood pressure of reduced dietary sodium and the Dietary Approaches to Stop Hypertension (DASH) diet. New England Journal of Medicine. 2001;344:3–10.
Page last modified on: Saturday 5 May 2018