Potassium-Magnesium Aspartate, an Overlooked Endurance Enhancer? Acute 100% Increase in Time to Full Exhaustion

1952, Italian Fausto Coppi is drenched with water by a fan during the golden years of the Tour. Question: Can the topical application of K & Mg do the same magic? Answer: That's very unlikely, ...
What sounds like a supplement producer was trying to sell his product with a sponsored study is, in fact, the gist of a 1968 study from the Departments of Clinical Physiology and Internal Medicine at the venerable Karolinska Institute in Stockholm, Sweden (Ahlborg. 1968).

The authors' conclusion that "[a]fter administration of potassium-magnesium-aspartate [KMgA] the capacity for prolonged exercise increased about 50 per cent" (Ahlborg. 1968) can thus not be discarded as marketing babble. And, before we decide whether it's too good to be true, I'd suggest we take a closer look at the way the data was generated before we either (a) discard it as outdated or (b) get totally excited for nothing.
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Another ten years before Ahlborg et al. published their study, an effect of potassium and magnesium salts of aspartic acid on muscular fatigability has been demonstrated experimentally in animals by several independent research groups. Back in the 1960s, humans studies had yet only subjectively assessed the effects of KMgA on endurance performance in man. Ahlborg et al. were thus right to consider it... "to be of interest to investigate, whether a positive effect on the capacity for prolonged standardized physical exercise after oral administration of potassium-magnesium-aspartate can be objectively demonstrated" (Ahlberg. 1968).
Table 1: Some anthropometric and other data in the test subjects (Ahlborg. 1968).
The scientists recruited 6 out of a group of 300 military recruits who had been examined at the Military Medical Examination Centre at the Karolinska Institute back in the 1960s. All subjects were subjected to the following routine: On 4 consecutive days, which will be called day 1, 2, 3 and 4, prolonged exercise to exhaustion was performed every day beginning at 1 p.m. The subjects were not fasted.

To get the results they wanted and to make sure the subjects' performance was not thwarted, the scientists required all subjects to record all foods they'd been consuming for the 4 days of the test. This practice was meant to avoid interference with of high carbohydrate intakes of which people back in the day still knew and appreciated that they can "increase the capacity for prolonged exercise markedly" (Ahlborg. 1968).
Does this work for strength training as well? While it may help you up your workouts, a study by Consolazio et al. (1964) found  no measurable beneficial effects on muscle strength. This disappointing result was later confirmed by De Haan, et al. (1985). So, I'd venture the guess that - if KMgA is a thing at all - it's an endurance athletes' thing.
To test each subject against itself while still having averages to compare, the authors had all subjects perform the "W170", a bicycle ergometer ride at a pulse rate of 170 beats/min (duration ~90 minutes until physical exhaustion) on days 1, 2, 3 and four. And here's how the supplementation worked:
"Beginning at 6 p.m. on the day before day 1, 5 tablets were administered every sixth hrs. last 5 tablets were given 1 hr before the prolonged exercise test on day 4, see Fig. 1. All subjects were given placebo tablets before the tests on days 1, 2 and 4. Before day 3 active substance was given. The subjects were told that the tests were aimed at elucidating the influence of a vitamin tablet on maximal performance time. No information was given to the test supervisor (the same nurse on all days) or to the subjects as to when placebo or active substance was administered. The placebo and active tablets were identically looking" (Ahlborg. 1968)
This is admittedly not exactly standard procedure, and one could argue that what we are seeing here is a vitamin placebo effect, but the effect (a) appears to be a bit too large (see data in Figure 1) and you could (b) also argue that the previous two 170Ws may have had a negative effect on the subjects' performance during cycling to exhaustion.
Figure 1: Duration of prolonged exercise (y-axis) in the 6 test subjects after administration (x-axis) of placebo (striped area) and after administration of aspartate (black area) and individual data in the table (Ahlborg. 1968).
As you can see in Figure 1 the effect differed from subject to subject but was (a) highly significant in all of the 6 men and (b) doesn't have a residual effect. The latter suggests that the ~100% increase in time to exhaustion is not the result of K or Mg repletion, but an acute response to the KMgA supplement.
Research overview and supplement suggestion: In order to put the results into perspective I've curso-rily searched subsequent studies on aspartate bound minerals with the following results (in random order): [1] Sign. increased ergogenic effects in with 10 g of potassium-magnesium as-partate over a 24 hr in Wesson et al. (1988) in subjects cycling at 70% VO2Max; [2] no benefits in trained indiv. cycling at lower intensities in Hagan, et al. (1982); [3] no benefits were likewise seen, when the supplement was taken in lower amounts chronically, i.e. 5 weeks, only 2g/day (Consolazio. 1964) and / or when the supplement was controlled against equimolar amounts of "regular" (HCL) Mg + K (Maughan. 1983).
Overall, the research, there-fore, appears to suggest that athletes who perform high-intensity endurance exerci-ses could benefit most from the serial administration of a total of 10g/24h of Mg and K - not necessarily bound to aspartate, for which scien-tists have not conclusively proven benefits when it's taken on its own, either (Trudeau. 2008).
So what's triggering these benefits? As you will know I am not happy if I don't understand the cause-and-effect relationships in any field of research that does not belong to quantum sciences. Unfortunately, I have to admit that, in this particular case, where Heisenberg's uncertainty principle obviously doesn't apply, I still cannot explain exactly what the reason for the surprisingly pronounced ergogenic effects is.

What appears to be certain (also based on previous studies) is that the 100% increase is not a normal day-to-day performance variation. As Ahlborg et al. point out, the antifatigue effect in previous research in rodents was often interpreted as the result of an ATP and phosphocreatine sparing effect, i.e. a decreased consumption of ATP and phosphocreatine. The authors of the paper at hand, however, believe that it is "more likely that the resynthesis of the energy-rich phosphates ATP and phosphocreatine might be accelerated by potassium-magnesium-aspartate" - quite obviously, the net result will be the same, an increased available amount of energy-rich phosphates in the muscles. This, in turn, was suspected to be due to a glycogen sparing effect of the Mg and K esp. with a focus on aspartate co-administration ('cause Asp is a major source of gluconeogenesis during exercise). Since the latter has been refuted by Trudeau, et al. in 1993, we are thus stuck with hypothesis #2, i.e. a direct effect on the (accelerated) rate of resynthesis of phosphocreatine.

Well, back in the day Ahlborg et al. wrote that "investigations are in progress to evaluate these theories". Unfortunately, the final answer to the question "how does that work" has yet not been found (see box on the right). Increased heart rate, increased lipolysis and glucose oxidation as they have been observed in Wesson et al. who confirmed the benefits on endurance performance in 1988 are probably rather a consequence than the cause of the performance enhancement you may be able to see in the conditions I outlined in the box to the right | Comment!
  • Ahlborg, Björn. Capacity for exercise in man. Forsvarets sjukvardsstyrelse, 1967.
  • Ahlborg, Bjorn, et al. "Human muscle glycogen content and capacity for prolonged exercise after different diets." Forsvarsmedicin 3.Suppl 1 (1967): 85ą89.
  • Ahlborg, Björn, et al. "Muscle glycogen and muscle electrolytes during prolonged physical exercise1." Acta Physiologica Scandinavica 70.2 (1967): 129-142.
  • Ahlborg, Björn, Lars‐Göran Ekelund, and Carl‐Gustaf Nilsson. "Effect of Potassium‐Magnesium‐Aspartate on the Capacity for Prolonged Exercise in Man." Acta Physiologica Scandinavica 74.1‐2 (1968): 238-245.
  • Consolazio, C. Frank, et al. "Effects of aspartic acid salts (Mg and K) on physical performance of men." Journal of applied physiology 19.2 (1964): 257-261.
  • Ekelund, Lars-Göran. "Circulatory and respiratory adaptation during prolonged exercise." Acta physiologica Scandinavica. Supplementum 292 (1967): 1.
  • De Haan, A., J. E. Van Doorn, and H. G. Westra. "Effects of potassium+ magnesium aspartate on muscle metabolism and force development during short intensive static exercise." International journal of sports medicine 6.01 (1985): 44-49.
  • Hagan, R. D., et al. "Absence of effect of potassium-magnesium aspartate on physiologic responses to prolonged work in aerobically trained men." International journal of sports medicine 3.03 (1982): 177-181.
  • Maughan, R. J., and D. J. M. Sadler. "The effects of oral administration of salts of aspartic acid on the metabolic response to prolonged exhausting exercise in man." International journal of sports medicine 4.02 (1983): 119-123.
  • Trudeau, François, and René Murphy. "Effects of potassium-aspartate salt administration on glycogen use in the rat during a swimming stress." Physiology & behavior 54.1 (1993): 7-12.
  • Wesson, Matthew, et al. "Effects of oral administration of aspartic acid salts on the endurance capacity of trained athletes." Research Quarterly for Exercise and Sport 59.3 (1988): 234-239.
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