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According to the Apollo Program Summary Report, astronauts on the longer 15-17 missions have difficulties maintaining dietary potassium levels. Section 8.4 claims

Negative nitrogen and potassium balances occurred during the Apollo 17 flight and confirmed a loss in total body protein. In addition, a loss of body calcium and phosphorus was demonstrated. This is consistent with previous flights.

According to section 8.2.6, this resulted in cardiac arrhythmias:

A loss of body potassium during flight was considered to be an important factor in the genesis of the Apollo 15 arrhythmias. As a result, several changes were instituted on Apollo 16 to reduce the likelihood of inflight arrhythmias and to further investigate the causes of body potassium loss during space flight.

Section 8.4 discusses the dietary adjustments to address the problem:

In-suit food bars were used by the Apollo 15 and Apollo 17 lunar module crewmen, and in-suit beverage assemblies were used by the Apollo 16 and Apollo 17 lunar module crewmen. The beverage assembly consisted of a drinking device and a 32-ounce bag containing water or potassium-fortified orange drink.

That "orange drink" was Tang. You can read John Young complaining about it in this answer.

Has potassium loss been an issue in other space programs (e.g. Shuttle, ISS)? What causes the loss: inadequate intake, increased elimination, or some other factor?

Related: Is there any research or recommendations for a diet in microgravity?

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  • $\begingroup$ First of all, the body loses calcium in large quantities. Calcium salts in large quantities are harmful to the kidneys. $\endgroup$ – A. Rumlin Sep 10 at 18:25
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The potassium deficiency issues on the Apollo missions were at least to some degree due to exertion during the lunar EVAs. Despite the low gravity, the stiffness of pressurized spacesuits made what would otherwise be moderate activity more strenuous; the astronauts sweated off several pounds each according to Biomedical Results of Apollo:

All Apollo crewmen lost weight ranging from one to twelve pounds with a mean loss of approximately six pounds on a balanced diet providing 2500 calories per man per day. Again, not all the food provided was consumed. Most of the weight loss was attributable to water and electrolyte loss; the remainder of the loss was attributed to lipid (30 percent) and muscle (10 percent) catabolism.

Interestingly, the same document downplays the potassium connection:

It was at first conjectured that a dietary deficiency of potassium might have been a contributory factor. Subsequent careful analysis of the dietary intake and mission simulation studies with potassium restriction failed to substantiate this hypothesis. The etiology remains obscure. Fatigue following strenuous lunar surface activity most certainly was a factor. Other contributory factors are speculative and are likely to remain so.

But it's unclear to me if this just means "it wasn't that they weren't ingesting enough potassium, it's that they were sweating too much of it out".

From Apollo: A Decade of Acheivement, regarding Irwin on 15:

The landing party exhausted themselves and sweated heavily while grappling with the drill on the lunar surface. As any well-trained athlete knows, this can lead to the loss of minerals important to the body like potassium, which is vital to keep the heart beating regularly.

Microgravity inherently seems to affect potassium excretion as well, so this isn't just an Apollo problem. Per Advanced Trauma Life Support for the Injured Astronaut:

Astronauts may also demonstrate marked electrolyte abnormalities. Antidiuretic hormone (ADH) secretion is decreased in microgravity and diuresis of the centrally mobilized intravascular fluid ensues. Serum osmolarity decreases by an average of 4%. Serum sodium falls by an average of 6% resulting in hyponatremia. Aldosterone secretion will increase in response, increasing potassium excretion by 32% and placing the astronaut at risk for hypokalemia. Hypomagnesemia is also common.

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