Risk of Dehydration in Older Adults During Heatwaves

This morning, I posted on social media about the extreme heat wave that the Vancouver-area will be having over the weekend, with temperatures hitting as high as 40° C or higher, which is almost 105°F. One of the people that follows me on social media mentioned about the risk of leaving clear water bottles in a car on a hot sunny day, and which I had read about this in previous years, but never thought much about it, as I always parked in a garage.

Since my car was currently out on the street and I remembered that I had a partly filled water bottle in it,  I went out to remove it. On my way back inside I thought of writing a social media post about the risk of leaving a partially- or completely-filled water bottle in a vehicle, which results from sunshine passing through the windshield, and then through the water in the bottle — acting like a magnifying glass.

While there is a risk of burn marks to the interior of a vehicle and smouldering as a result of this type of light magnification through water bottles, reports of full-blown vehicle fires resulting from this are unheard of, although theoretically possible. What is well-established, however is that there are over 600 deaths per year in the US as a result of extremely hot weather [1].

forecast from The Weather Network

Extreme heat are summer temperatures that are much hotter and/or humid than average [1] — such as the 40° C temperatures that are expected for the Vancouver-area this coming weekend. While extreme heat makes people of all ages more prone to getting dehydrated, infants under the age of 4 and adults over the age of 65 are especially at risk [1].

As people age, the amount of available water in their body decreases largely as a result of having decreased lean body mass (muscle), compared to younger people. I’ve mentioned decreased lean body mass as people age in previous articles about sarcopenia — which is the loss of muscle as people age, and have highlighted the importance of older adults being eating sufficient protein to reduce the risks of falls, and about how much protein older adults should eat but since muscle holds more water than fat, retaining muscle mass as people age also has the added benefit of helping older people to stay adequately hydrated.

Women are at higher risk of becoming dehydrated in the heat than men of the same age, because men at any age have a higher amount of muscle and therefore a higher percentage of body water than women. This means that older women (compared to older men) are often at greater risk of dehydration.

The following table lists the average percentage of water in the body, according to age and gender, as well as their ranges [2].

Age 12—18 years Age 19—50 years Age 51 years and older
Male Average: 59%
Range: 52—66%
Average: 59%
Range: 43—73%
Average: 56%
Range: 47—67%
Female Average: 56%
Range: 49—63%
Average: 50%
Range: 41—60%
Average: 47%
Range: 39—57%

As can be seen from the above table, adult men and older men have, on average almost 10% more body water than women of the same age, so it can take a less time for a woman to get dehydrated in the heat, than for a man.

While we remind children and young adults to be sure to drink when they are thirsty, older adults also are less aware that they are dehydrated because the feeling of thirst decreases as people age [3]. For this reason, it is important that older adults drink more especially when they don’t feel thirsty.

Another reason that older people get dehydrated easier is that kidney function decreases with age, and the hormonal response to dehydration (which is handled in part by the kidney) may be impaired [4]. Other factors that contribute to older people becoming dehydrated easier include conditions such uncontrolled (or undiagnosed) diabetes which can cause more urination, or as the result of the effect of various medications that they may be taking, such as diuretics for high blood pressure. Older adults with osteoarthritis in the knees or hips may find it more difficult to go get water, while others may have some memory impairment that keeps them from remembering when they last drank some water.

A heat wave is an excellent time to check in more frequently on an aging parent, or even on elderly neighbours that you are friendly with.

But what to look for?

Symptoms of dehydration may include dry mouth,  or feeling overly tired, but dry mouth is a common side-effect of many medications that older people may be taking, and feeling fatigued may not  be that unexpected. Finding out if they are feeling dizzy, or light-headed may be helpful and if is comfortable to ask, find out if the person is urinating less, or if their urine is darker in colour. The can provide helpful clues that they are already dehydrated.

Serious symptoms may include confusion or disorientation, feeling faint, or having diarrhea or vomiting, so be sure to seek medical treatment if these symptoms are present. Severe dehydration can result in the person going into hypovolemic shock as a result of low blood volume, or having seizures if they have lost too much sodium and potassium in the urine.

If you are checking on older parents or on elderly neighbours, consider bringing along a few bottles of sparkling water, or a non-sweetened flavoured soda such as Bubbly® which can encourage them to drink more when it’s hot outside, and seeing if they have a fan that is safely set up may also help them stay cooler. These are not “big” things to do, but to older adults can make a great deal of difference in extreme heat.

…and when you are out and about, remember not to leave partially- or completely-filled water bottles in your car, as sunlight passing through the windshield and then the water bottle can cause burn marks and even a smouldering fire.

To your good health!


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  1. Centres for Disease Control and Prevention (CDC), About Extreme Heat, https://www.cdc.gov/disasters/extremeheat/heat_guide.html
  2. National Academies Press, Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate, Chapter 6: Water, 2005. https://www.nap.edu/read/10925/chapter/6, pg 73
  3. Picetti D, Foster S, Pangle AK, et al. Hydration health literacy in the elderly. Nutr Healthy Aging. 2017;4(3):227-237. Published 2017 Dec 7. doi:10.3233/NHA-170026
  4. British Nutrition Foundation, Dehydration in the Elderly, https://www.nutrition.org.uk/nutritionscience/life/dehydrationelderly.html

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