From the Guidelines
With aging, both total body water and intracellular water decrease, with total body water declining from approximately 60% of body weight in young adults to about 50% in the elderly, as supported by the most recent guideline 1. This reduction occurs gradually over time, with men typically experiencing a greater decline than women. The decrease in total body water is primarily due to the loss of lean body mass (sarcopenia) that occurs with aging, as muscle tissue contains more water than fat tissue. Some key points to consider include:
- The loss of muscle mass and cellular components contributes to the decrease in intracellular water 1.
- Aging kidneys become less efficient at concentrating urine and conserving water, while thirst perception diminishes, further contributing to reduced body water content 1.
- These physiological changes make older adults more susceptible to dehydration and its complications, including electrolyte imbalances, cognitive impairment, and increased risk of falls.
- Maintaining adequate hydration becomes increasingly important with age, even when thirst sensation is diminished, as emphasized in the latest clinical nutrition and hydration guidelines for geriatrics 1. The latest evidence suggests that older adults should be encouraged to consume adequate amounts of drinks to prevent low-intake dehydration, and that assessment of fluid intake is often highly inaccurate in this population 1. Key factors to consider in preventing dehydration in older adults include:
- Reduced thirst and reduced urine concentration by the kidney increase dehydration risk 1.
- Total body water is reduced, and many older adults use medications such as diuretics and laxatives which increase fluid losses 1.
- Memory problems and social isolation can also contribute to dehydration risk in older adults 1.
From the Research
Changes in Total Body Water with Aging
- Total body water (TBW) decreases with age 2, 3, 4
- The decline in TBW is due primarily to a decline in intracellular water (ICW) 4
- TBW is closely correlated with muscle mass in both sexes 2
Changes in Intracellular Water with Aging
- Intracellular water (ICW) declines with age 2, 4
- ICW is associated with muscle strength, functional performance, and frailty in the elderly 2
- Considerable variability in ICW is observed for the same muscle mass, suggesting a protective effect of cell hydration on functional performance and frailty 2
- The ratio of ICW to fat-free mass decreases with increasing BMI and is lower in women than in men 5
Factors Influencing Body Water Compartments
- Body composition, including fat-free mass and body size, influences TBW and ICW 4, 5
- Gender and age also affect body water compartments, with women and older adults tend to have lower TBW and ICW 4, 5
- Hydration disorders are frequent in clinical practice and can be a life-threatening issue in frail patients, highlighting the importance of monitoring body water compartments 5