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Commentary on Sodium Supplementation during Prolonged Exercise

Sodium (na) and salt (NaCl) are two different things in terms of weight (milligrams). Sodium is 39.3% by weight of NaCl. Hence 1000 mg (1 gram) of table salt contains 393 mg of sodium. Other electrolytes matter also and need to be in balance, such as potassium (K), Calcium (Ca), and Magnesium (Mg).

See prior posts such as Sodium and Electrolyte Losses During Prolonged Exertion — an Experiment.

This posts comments on selected excerpts from Sodium supplementation is not required to maintain serum sodium concentrations during an Ironman triathlon. I chose that paper because all the others I’ve found seem to have no relation to my reality over a decade of ultra endurance riding, whereas this paper seems to dovetail with my self-observation conclusions while avoiding any contradictions with my own self-observations.

Emphasis added in places.

Core summary

I would say with confidence that *no* supplementation is unwise, particularly under very hot conditions where fluid losses exceed 2L/hour and hyponatremia becomes a risk with too much fluid ingestion. Plus, pure water is absorbed more slowly than something approaching an isotonic solution. For years I used Hammer HEED as my sports drink, and it is very low in sodium (only 40mg per 100-calorie serving, which is 303mg per serving)*. I had no evidence of sodium deficiency when using HEED (e.g., cravings for salt).

* I stopped using Hammer HEED because I am pretty sure that its Xylitol and/or Stevia additives might be responsible for gastric distress, particularly at mile 140 and beyond.

Results

Subjects in the sodium supplementation group ingested an additional 3.6 (2.0) g (156 (88) mmol) sodium during the race (all values are mean (SD)). There were no significant differences between the sodium, placebo, and no supplementation groups with regard to age, finishing time, serum sodium concentration before and after the race, weight before the race, weight change during the race, and rectal temperature, systolic and diastolic blood pressure after the race. The sodium supplementation group consumed 14.7 (8.3) tablets, and the placebo group took 15.8 (10.1) tablets (p  =  0.55; NS).

Conclusions

Ad libitum sodium supplementation was not necessary to preserve serum sodium concentrations in athletes competing for about 12 hours in an Ironman triathlon. The Institute of Medicine's recommended daily adequate intake of sodium (1.5 g/65 mmol) seems sufficient for a healthy person without further need to supplement during athletic activity.

As I read it, the study is not saying that zero intake is advisable, only that additional supplementation over and above sports drinks is unnecessary.

Sodium stasis

What the study is saying here dovetails with my own finding using the simple “lick test”: there is just not that much salt on my skin after finishing a double century. There is notably more if it was very hot (very high sweat/fluid losses), but I don’t look like a floured dumpling that 9 grams of sodium X 12 hours would imply (the idea that over 100 grams of NaCl would be excreted is absurd).

Moreover, highly trained athletes surely well be mal-adapted if after years of training the body did not adapt its electrolyte balance to ultra endurance events.

Triathletes competing in the 2001 South African Ironman triathlon maintained their serum [Na+] within the normal range whether they drank the Na+‐poor drinks (water or a sports drink with [Na+]  =  18 mmol/l) or supplemented with ∼156 mmol Na+.

... Athletes in the placebo, sodium supplementation, and “no” supplementation groups did not differ in their finishing time nor in subjective measures of exercise intensity and mental wellbeing, nor in the prevalence of medical diagnoses after the race.

Clinical measures of fluid status, rectal temperature, blood pressure, absolute and percentage weight loss were also not different between groups

Therefore predictions of the expected consequences of “large” Na+ losses during prolonged exercise are inaccurate either because athletes sweat less or have lower sweat [Na+] than are currently believed. Alternatively, during acute states of Na+ loss, additional Na+ may be released either from intracellular body stores—for example, bone, skin—or by contraction of the extracellular fluid volume, in order to buffer acute Na+ losses until these are replenished by Na+ ingestion during the next meal.

Bottom line: extra sodium is of zero benefit.

Flawed studies

Other studies are clearly flawed and I don’t need this study to tell me that: glycogenolysis has long been part of my own planning for fluid-replacement. During the first five hours of a double century, considerable water is released as glycogen stores are largely burned off.

Not losing weight in my experience has always been associated with over-hydration and performance problems: I expect to be about 8 pounds lighter (e.g. 175 lb => 168 lb) after a double due to the loss of fuel and its associated water, and a moderation dehydration. I have seen such losses even on only 4 hour rides when ingesting 3L of water!

...two studies are often quoted to support the notion that Na+ ingestion during exercise is essential if a progressive fall in serum [Na+] is to be prevented. Both studies contain important logical flaws, not least because they encouraged trial subjects to drink to excess during exercise.

The goal of the study of Vrijens and Rehrer24 was for athletes to drink sufficiently to ensure that they did not lose weight during two hours of laboratory exercise. Normal fluid balance during exercise requires that some weight must be lost due to (a) the release of stored water consequent to glycogenolysis, and (b) irreversible loss of fuel through substrate oxidation.... athletes in any trial who do not lose weight during exercise must complete the trial in a mild state of over-hydration. That study therefore evaluated the effect of Na+ supplementation on serum [Na+] in subjects encouraged to overdrink during prolonged exercise. The data show that the response of the serum [Na+] to overdrinking was determined by the renal response to exercise, so that those athletes who passed the most urine during exercise were best able to maintain their serum [Na+].

This is compatible with the conclusion that serum [Na+] is far more sensitive to changes in total body water than to Na+ balance during prolonged exercise28 and with the explanation that acute hyponatraemia is always due to altered renal function in which the rate of free water clearance fails to match the rate of free water ingestion, whether at rest or during exercise.29 Finally only four of 10 subjects completed all trials in that study, further limiting the validity of these findings.

Similarly Twerenbold et al1 studied athletes who drank to excess while running ∼40 km in four hours. As sweat rates were only ∼500 ml/h whereas rates of fluid ingestion were ∼1000 ml/h, subjects gained an average of 2 kg weight during the run. In the presence of this large weight gain, the ingestion of additional [Na+] predictably lessened the fall in serum [Na+] by about 2–3 mmol/l. Yet, despite the ingestion of an additional 118 mmol Na+, the group that ingested the most Na+ still developed hyponatraemia during exercise (mean serum [Na+] after the run  =  134 mmol/l). The authors' suggestion that their data prove that all athletes should ingest additional Na+ during exercise is incorrect, as fully argued elsewhere.12 Rather the correct conclusion is that, as the single best predictor of post‐exercise serum Na+ is the change in body mass during exercise (fig 1​1),), avoidance of overhydration is the most important intervention necessary to prevent the development of symptomatic exercise associated hyponatraemia.14,30,31,32

Conclusion

My take away here is that the rate of ingestion of water should target finishing in a mild to moderate state of dehydration. A goal of mild to moderate dehydration seems best, because overhydrating is associated with stomach issues (bloating, very uncomfortable) and an increased risk of hyponatremia whether or not there is supplemental sodium/electrolytes, as per the comments above. I know that my very best performances have *always* left me depleted by at least 2 liters of body fluid, as determined by my fluid intake over the following 3-4 hours.

Put another way: the body has an easier time maintaining statis in a state of mild dehydration than it does trying to correct an electrolyte balance from excess fluid and/or excess sodium intake. Excess sodium intake is particularly problematic.

Tentative plan of attack

I conclude that I need to focus on the rate of fluid intake along with the caloric content. Typically I end up ingesting less than 1L/hour of fluid, as my stomach cannot handle it.

Possibilities:

  • Ingest small amounts of fluid for the first two hours (assuming going out fully hydrated and with fluid in stomach). This implies a higher concentration of calories in the fluid in order to take in adequate calories (so as not to prematurely burn off body stores), e.g., 4 scoops of Tailwind per liter of fluid.
  • Being ingesting 1L/hour of fluid by the end of hour 3, depending on intensity and beginning hydration status. Reduce caloric content of the fluid to 300 calories per liter.
  • At hour 4 and beyond, shoot for 1L of fluid intake per hour, 200 to 300 calories per liter. Experiment with 200 vs 300.

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