KosAbility: Staying safe in the heat. Hydration. Electrolytes – Daily Kos

KosAbility: Staying safe in the heat. Hydration. Electrolytes  Daily Kos

This piece was written with attention to the needs of the elderly, but there is plenty of important information here for everyone. For additional advice see also:

Recommendations from the NIH: Hot Weather Safety for Older Adults

Prior diary by Besame KosAbility: Since it’s only going to get hotter, let’s learn how to cope with heat waves

Staying safe in the heat

I hike in 100 degree weather in full sun and like it. How is this even possible? Consider the wet bulb globe temperature:

The Wet Bulb Globe Temperature (WBGT) is an indicator of heat related stress on the human body at work (or play) in direct sunlight. It takes into account multiple atmospheric variables, including:  temperature, humidity, wind speed, sun angle, and cloud cover.

At the “Current WBGT / Calculate” tab at the above link there is a WBGT calculator that takes into account of all these factors. It includes a map that you can use to specify your location. The chart at top uses only temperature and humidity. It uses worst-case for the other variables.

According to the local weather report, the relative humidity for my area was about 10 percent when this was written. If we take a look at the chart at top and find the intersection of 10 percent humidity with 100 degree temperature, the resulting wet bulb globe temperature is 83 degrees Fahrenheit. This puts it in the yellow zone. For the meaning of that we go to this chart:

I spend about an hour hiking at moderate effort, including a rest break. It feels good to me. I have noticed that when it gets above 100 degrees and I try this I do experience symptoms of heat stress – elevated heart rate and fatigue. This puts conditions into the red zone as shown on the chart. So based on my experience this guide is accurate.

For further guidance on safe desert hiking see Ariel’s Checklist, the source of the above tables.


We need to replace what is lost via perspiration – water plus electrolytes. It would be nice to rely upon thirst to guide our need for water but unfortunately that does not work for older folks:

[A] study involving both healthy young and old subjects who were deprived of water for 24 hours and who developed serum hyperosmolality [excess concentration of electrolytes] revealed that the elderly had much lower perceptions of thirst than the young. According to a subjective rating scale, some elderly subjects did not feel thirsty at all, while most of the young subjects were very uncomfortable because of thirst. When these same elderly subjects were presented with water, they did not drink, whereas the young subjects drank enough water to correct whatever alterations in serum osmolality had occurred as a result of the water deprivation.

Note: Osmolarity and Osmolality

Osmolarity and osmolality are frequently confused and incorrectly interchanged. Osmolarity refers to the number of solute particles per 1 L of solvent, whereas osmolality is the number of solute particles in 1 kg of solvent. For dilute solutions, the difference between osmolarity and osmolality is insignificant.

I am in the elderly category and I cannot remember a time in recent history when I was very thirsty. Maybe I am really good at staying hydrated but I doubt it. More likely I am no exception in the impairment of thirst in the elderly. I have observed that it helps to stay hydrated by consuming things that are rich in liquid content — fruit, yogurt, breakfast porridge with plenty of almond milk, etc.

The impairment of thirst perception in the elderly is compounded by age-related loss of kidney efficiency:

[T]he concentrating ability of the aged kidney is decreased. Elderly subjects deprived of water for 12 hours do not significantly decrease their urine flow, and when compared with young subjects, their urine osmolality is substantially lower. Because young subjects both decrease their urine flow and increase their osmolality, there is a considerable difference in the maximum urine osmolalities attained by young and old subjects under conditions of water deprivation.

In plain English, under conditions of water deprivation, elderly kidneys are not very good at conserving water. What may seem like normal urine flow is not an indication of adequate hydration in the elderly.

The hydration recommendations set forth in the table above are based on heat stress as indicated by the wet bulb globe temperature. This is one approach to hydration. On the other hand, how about simply drinking as much as possible? Not a good idea!

Electrolyte Replacement

Here is a table of electrolyte losses in perspiration and the comparable figures in blood plasma. The unit of measure is millimoles per liter which is a measure of the osmolarity of the dissolved electrolytes:

Note that most of the electrolyte loss is sodium chloride. This also makes up more than 90 percent of blood plasma electrolytes. This makes it the dominant factor in blood osmolarity. Chloride has to accompany sodium to maintain electrical neutrality. In the medical literature on hydration mention of chloride is usually omitted and we will do the same here.

The concentration of sodium in sweat is less than the concentration of sodium in the blood. By itself sweating would therefore increase the osmolarity of the blood, but when sweat is replaced with plain water the net effect is to decrease the osmolarity of the blood.

Note further that the concentration of sodium in sweat varies by almost a factor of 10. There is great variation even for a single individual – sodium concentration depends upon the rate of sweating. The sweating process includes a sodium recovery step. The higher the sweat rate, the less sodium recovered, leading to a higher concentration of sodium in the sweat. In addition, the amount of sweat is variable depending upon conditions. So a fixed regimen of electrolyte replacement is not possible.

Why maintaining blood plasma osmolarity is important

Under steady-state conditions the osmolarity of the blood plasma, and the extracellular fluid bathing the cells, and the interior of cells comprising bodily tissues will all be about the same. If on the contrary, the osmolarity inside the cell were to be much higher, osmotic pressure would cause it to absorb water and expand, reducing the osmolarity until it is the same as the surrounding fluid. This is exactly what happens if loss of salt and consumption of excess water reduces blood osmolarity. Among the tissues subject to expansion is the brain, which is limited by the interior space of the skull. If blood osmolarity drops too far the result can be fatal.

Exercise-associated hyponatremia in marathon runners: a two-year experience

Exercise-associated hyponatremia [low blood sodium] (EAH) has been described after sustained physical exertion during marathons, triathlons, and long-distance hikes. Although one might expect that hypotonic [Lower than blood osmolarity] fluid losses from sweat would produce a rise in serum sodium (SNa), the development of hyponatremia following such events has been attributed to hypotonic fluid replacement greatly exceeding fluid losses via sweat…

A total of 26 patients from the 1998 and 1999 marathons were hyponatremic [serum sodium (SNa) ≤135 mEq/L] including 15 with severe hyponatremia (SNa ≤ 125 mEq/L). Three developed seizures and required intubation and admission to an intensive care unit. Hyponatremic patients were more likely to be female, use NSAIDS, and have slower finishing times. Hyponatremic runners reported drinking “as much as possible” during and after the race and were less likely to have clinical signs of dehydration. … It is concluded that the development of exercise-associated hyponatremia is associated with excessive fluid consumption during and after extreme athletic events

Exercise-Associated Hyponatremia: 2017 Update

The tragic deaths of two female charity marathon runners in 2003 fully exposed this fatal complication of exercise to sports medicine personnel as well as the lay public…Over the past decade, EAH deaths have been confirmed in the lay press in high school football players following practice, a soldier on the first day of Ranger training, a policeman participating in a 19 km bike ride, a college student performing calisthenics for a fraternity, a bushwalker, an ironman triathlete, and a canoeist during an ultradistance race. Additionally, a highly fit solider died during a 50 km training march with both hyponatremic encephalopathy and exertional heat stroke. The literature also reports symptomatic cases of EAH after long distance swimming, mountain cycling, yoga, 2 h of weightlifting plus tennis,

Overconsumption of Hypotonic Fluids

It is likely that … inappropriate hydration recommendations (that are often taken to the extreme such as recommendations to “drink as much as possible”) account for this excessive fluid intake … For example, only 7.3% of 110 web sites discussed that fluid intake should be based upon thirst, and the potential risk of hyponatremia from overhydration was noted by less than half the websites. Dissemination of more appropriate hydration guidelines is critical…

Drink according to thirst. Because fluid losses through sweat and urine are highly dynamic and variable across individuals participating in a variety of outdoor activities, recommending fixed ranges of fluid intake is not appropriate. The most individualized hydration strategy before, during, and immediately following exercise is to drink fluids when thirsty.

That is great for those who experience thirst. How about for those who do not experience reliable thirst? Here is my personal experience in this matter:

  1. My exposure to heat is an established routine, so I know what works for me. I recommend against any sudden extreme heat exposure if possible.
  2. Although I do not experience much thirst, I have observed that when I do need hydration, drinking fluids feels good to me. So before hiking I am sure to avail myself. I drink as much as I am comfortable to imbibe.
  3. Plain water is okay but not required. I personally use cooled herbal teas among other drinks.
  4. Salt food to taste – I let my body tell me how much sodium I should be taking in over the course of the day.

What about the other electrolytes listed in the table? I rely upon diet to provide adequate calcium and potassium, and take a daily magnesium supplement.


Referring to the above table, note that the glucose lost in perspiration is insignificant, both as a fraction of the other substances in perspiration, and a fraction of the glucose in blood plasma. Glucose is therefore not an electrolyte that needs replacement. High consumption of glucose and other sugars is adverse to cardiovascular health. See KosAbility: Sugar, Fat And Cardiovascular Disease

If you are an athlete in the midst of exerting maximum effort on the playing field, glucose replacement deserves attention. But for the rest of us, consuming sugar in the guise of electrolyte replacement is not a good idea. Here is a quick look at the sugar content of a few popular “electrolyte replacement” powders:

Gatorade endurance formula

serving size 24 grams total carbohydrates 22 grams, sugars 13 grams.

Not clear what the other carbohydrates are.

Vitalyte Electrolyte Powder

serving size 25 grams, 21 grams of added sugar

Skratch Labs Hydration Drink Mix

 serving size 22 grams total sugars 19 grams

In my opinion, sugar laden “electrolyte replacement” schemes are not helpful!