Sodium

Sodium is an electrolyte that helps keep the right amount of water inside and outside of your cells. It is central for nerve signals, muscle contraction, and blood pressure control. Your blood (serum) sodium result mainly reflects water balance in the body rather than how salty your last meal was. Keeping sodium in range is a quiet but powerful marker of overall fluid balance, kidney function, and hormonal regulation over time.

Even small, long-lasting shifts outside the normal range can affect how you feel and move. Chronic mild hyponatraemia (low sodium) is linked to problems with balance and attention and to more falls and fractures in older adults. Severe hypernatraemia (high sodium) is associated with confusion, coma and higher mortality, especially when it rises quickly or is very high. Tracking sodium over the long term helps you spot patterns (for example, hot-weather training, fasting, or diarrhoea) that nudge you out of range before symptoms appear.

These habits help keep serum sodium in the healthy zone and are easy to tailor and track over time:

• Match fluids to your day. On normal days, drink to thirst and spread fluids across the day. On hot or high-sweat days, plan fluids around activity so body water stays steady rather than swinging up or down.
  

• Learn your personal sweat pattern. Weigh yourself before and after longer workouts; the difference is mostly sweat. Use that to guide how much you drink next time in similar conditions.
  

• Use electrolytes for long, hot exercise. For multi-hour, hot-weather sessions, a drink with more sodium can help maintain blood sodium and plasma volume compared with typical sports drinks. In a randomised crossover study, a 60 mmol/L sodium drink kept plasma sodium slightly up (+0.8 mmol/L) and prevented plasma-volume drop (0% vs −2%) over 3 hours in the heat, versus ~21 mmol/L sodium. (4)
  

• When you have diarrhoea, switch to an oral rehydration solution (ORS). WHO-style ORS contains 75 mmol/L sodium and 75 mmol/L glucose at low osmolarity to promote absorption and restore balance efficiently. (5)
  

• Avoid “water dumping.” Rapidly chugging large volumes of low-mineral water during prolonged exercise can dilute sodium. Include some electrolytes instead on high-sweat days.
  

• Heat-smart training. Build up heat exposure gradually across days when seasons change. This helps your body adapt its sweating and fluid handling, making sodium balance more stable during hot spells.
  

• Everyday nudges that help: prioritise sleep, moderate alcohol (which can disturb water balance), and include broth or lightly salted foods on very sweaty days. Track how these tweaks shift your sodium result over time

Optimal: 135–145 mmol/L

Low (hyponatremia):

• Mild: 130–134 mmol/L

• Moderate: 120–129 mmol/L

• Severe: <120 mmol/L (1)
  

High (hypernatremia):

• Mild: 146–150 mmol/L

• Moderate: 151–155 mmol/L

• Severe: >155 mmol/L, Extreme: >190 mmol/L 

1. Hyponatraemia—treatment standard 2024. Nephrology Dialysis Transplantation (Oxford Academic), 2024. Available from: https://academic.oup.com/ndt/article/39/10/1583/7713921

2. Yun G, Baek SH, Kim S. Evaluation and management of hypernatremia in adults: clinical perspectives. Korean Journal of Internal Medicine, 2023. Available from: https://www.kjim.org/upload/kjim-2022-346.pdf

3. Corona G, Giuliani C, Parenti G, et al. Prevalence of mild hyponatremia and its association with falls in older adults. BMC Geriatrics, 2019. Available from: https://bmcgeriatr.biomedcentral.com/articles/10.1186/s12877-019-1282-0

4. Rehrer NJ, et al. A randomized cross-over trial assessing effects of beverage sodium concentration on plasma sodium and plasma volume during prolonged exercise in the heat. European Journal of Applied Physiology, 2023. Available from: https://link.springer.com/content/pdf/10.1007/s00421-022-05025-y.pdf

5. World Health Organization. Oral Rehydration Salts: reduced osmolarity formulation. WHO, 2006 (current specification). Available from: https://apps.who.int/iris/bitstream/handle/10665/69227/WHO_FCH_CAH_06.1.pdf%3Bsequence%3D1