Skip to main content
KBS_Icon_questionmark link-ico

Human biomarkers for measuring sodium intake and status

Human biomarkers for measuring sodium intake and status

Dietary sodium intake is cumulative. Sodium is naturally present in most foods in their native state, and salt is often added during processing, cooking including the use of flavour enhancers and condiments, and before eating. Some sodium may be lost during cooking. Dietary sodium intake is therefore difficult to measure using dietary assessments and food recall questionnaires. Human biomarkers of salt intake such as sodium excretion in urine therefore provide a more efficient and accurate intake assessment.

The main route by which sodium is eliminated from the body is via the urine, and about 90% of total daily sodium intake is excreted in this way (5). Urinary sodium, as opposed to serum/plasma sodium measurements should therefore be used when assessing population sodium consumption. Sodium is also lost in sweat; losses in sweat can increase when exposed to extreme heat conditions or a high sweat production due to intense physical activity, however acclimatisation to these conditions by the body is rapid (6).

Conversion of salt to sodium

The term “salt” (sodium chloride) is not synonymous with the term “sodium”!

To convert mmol to mg of sodium, chloride, or sodium chloride, multiply mmol by (5):
23 (molecular weight of sodium)
35.5 (molecular weight of chloride)
58.5 (molecular weight of sodium chloride).

Sodium in urine is typically measured in mmol. To convert mmol of sodium to salt equivalent:
1 mmol sodium = 23 mg sodium = 0.0575 g salt
104 mmol sodium = 2,400 mg sodium = 6 g salt (~ 1 teaspoon)


Note: the following methods section details recommendations for population biomarker assessment that are not suitable for use to assess individuals or in an individual clinical setting.




Quality control and technical assistance

Quality Control

Standard reference materials

ICP-MS and ISE: Standard Reference Materials (SRM) are available from the National Institute of Standards and Technology (NIST): SRM 2670a Toxic elements in urine (freeze-dried), level 1 (37.2 mmol/L) and level (41.0 mmol/L). See the method link on page 3 for more details.

To order SRM from NIST see this page.

ISE: CLINIQA standards used in the CDC NHANES method are available via this link.

Confounding factors

The analysis of sodium can be biased by day-to-day intra-individual variations in sodium and fluid intake, physical activity, the environment and medication use, including antibiotics and diuretics that can produce artificially high results, and corticosteroids and non-steroidal anti-inflammatory drugs e.g. ibuprofen that can produce artificially low results (7). Sodium is also lost in faeces and sweat, though in temperate climates this factor is negligible (5). In physically active persons or hot and humid climates, loss of sodium via the sweat and faeces may be >10% (14). Seasonal variability should be considered in such countries.

Urine sample stability

Room temperature: Sodium is stable in urine for ≤ 45 days (17), though this is not recommended due to bacterial growth in the urine, and operator comfort during analysis.
Frozen: Sodium is stable indefinitely if stored frozen. Long-term storage for 20-25 years at -70 °C did not affect specimen desiccation (18).
Freeze-thaw cycles: Up to 6 freeze-thaw cycles did not affect sodium concentrations (18).

We advise OpeN-Global users to consult relative SOPs for other biomarker analyses to be conducted in the collected urine samples to ensure all stability restrictions are considered.

Laboratory accreditation

For details on laboratory accreditation, validation or proficiency testing schemes, please contact or see the OpeN-Global page on Laboratory accreditation.

Technical assistance

For technical assistance and advice on sodium assessment write to
Nutritional Biomarker Laboratory, University of Cambridge, UK. Email: