Hydration after 40: what the research shows about fluid intake and ageing.

Staying adequately hydrated is a basic requirement for health at any age. After 40, however, the body's ability to signal and self-regulate hydration changes in ways that make adequate fluid intake a more deliberate consideration. This article covers what those changes are, what Australian guidelines recommend, and what the research shows about the consequences of chronic under-hydration in this age group.

How the body's relationship with water changes after 40

Declining thirst sensation

A consistent finding in the research literature is that thirst sensation diminishes with age. A 2025 review published in Nutrients notes that in older adult populations, thirst is triggered only during substantial fluid deficits, meaning the sensation of thirst becomes a less reliable early indicator of hydration status.[2] Younger adults tend to experience thirst earlier in the dehydration process, making self-regulation more automatic.

The proposed mechanism involves a higher osmotic threshold in older adults, meaning the body tolerates a greater rise in blood solute concentration before signalling the need for fluid.[2] As a result, an older adult may be meaningfully underhydrated before any subjective thirst is experienced.

Changes in kidney function

The NHMRC Nutrient Reference Values note that kidney function declines with age, reducing the kidneys' ability to concentrate urine efficiently.[1] This means older adults excrete more water relative to waste products, increasing baseline fluid requirements compared to younger adults with the same lifestyle and environment.

Total body water decreases with age

Total body water as a proportion of body weight decreases with age, partly because muscle tissue has a higher water content than fat tissue, and lean mass tends to decline while fat mass increases from around age 40 onwards.[3] This means older adults have a smaller reserve of body water and are more vulnerable to the effects of even modest fluid deficits.

Australian fluid intake guidelines

The Australian Dietary Guidelines recommend that adults drink plenty of water, noting that water is the best choice for hydration without the energy, stimulant or diuretic effects associated with many other beverages.[4]

The NHMRC Nutrient Reference Values set Adequate Intake figures for total water, which includes fluids from all beverages and approximately 20% from food. For adult women the figure is 2.1 litres per day. For adult men it is 2.6 litres per day. These figures are baseline estimates for temperate conditions and increase with physical activity, high ambient temperatures and other factors that increase fluid losses.[1]

Healthdirect, the Australian Government health information service, confirms these figures: approximately 8 cups or 2.1 litres of fluids per day for women and around 10 cups or 2.6 litres for men, from all sources including food.[5]

What the research shows about under-hydration in adults over 40

The NHMRC Nutrient Reference Values summarise the consequences of dehydration in older adults as ranging from constipation through to cognitive impairment, functional decline, falls and stroke.[1]

A 2025 review of hydration strategies in older adults, published in Nutrients, found that 25 to 33% of adults in the United States and Europe consumed less than 1.5 litres of fluid per day, well below recommended levels. In a multi-country analysis of more than 16,000 participants, approximately 50% of older adults reported inadequate fluid intake.[2]

Hydration and cognitive function

Research has identified an association between hydration status and cognitive performance. The NHMRC Nutrient Reference Values note that dehydration has been shown to be a reliable predictor of impaired mental performance.[1] A 2022 prospective analysis from the PREDIMED-Plus study, involving 1,957 adults aged 55 to 75, examined associations between hydration status and changes in cognitive function over two years. Participants classified as dehydrated or in a state of impending dehydration showed different cognitive trajectories than those in the hydrated category, though the researchers noted the causal direction of the relationship requires further investigation.[6]

Heat and hydration in the Australian context

Australia's climate makes hydration particularly relevant. The NHMRC notes that water losses from the lungs and skin increase substantially at high temperatures, at altitude and in low humidity.[1] Fluid requirements on hot days in Australian conditions may be considerably higher than the baseline adequate intake figures, particularly for adults who are physically active outdoors.

The 2025 Nutrients review also notes that cold weather presents a separate dehydration risk, as reduced thirst sensation in cold environments can compound the age-related decline in thirst already observed in older adults.[2]

Fluids from food

The NHMRC estimates that solid foods contribute approximately 20% of total water intake, or around 700 to 800 millilitres per day for adults eating a typical mixed diet.[1] Foods with high water content include fruits, vegetables, soups, dairy foods and cooked grains. The remaining 80% of fluid needs must come from beverages.

The NHMRC notes that while tea and coffee contribute to total fluid intake, caffeine has a mild diuretic effect that should be considered in the context of overall hydration.[5]