Healthy kidneys are very efficient at excreting any excess potassium. Also, if a large amount of potassium is ingested acutely, a significant % will be taken out of the blood and moved into the cells. From WholeHealthMD,
Potassium:
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Most people can safely absorb up to 18 g (18,000 mg) of potassium a day. In fact, potassium toxicity--a condition called hyperkalemia--is very unlikely to occur unless you take potassium supplements inappropriately or your kidneys don't function properly. That's because the kidneys carefully monitor the balance of potassium in the body and excrete any excess.
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Other contraindications are if you're taking ACE inhibitors, beta blockers or certain potassium-sparing diuretics for blood pressure.
The problem is that the kidneys aren't as efficient at recognising when blood levels of potassium are getting low, and they will continue to excrete potassium in the urine until levels are very, very low. As a temporary measure, potassium is released from inside the cells to make up for the depleted levels in the blood, but eventually this wears out. Many people are chronically borderline-low in potassium .. not low enough to have acute symptoms such as extreme weakness, and heart rhythm disturbance, but low enough that symptoms of fatigue, mild weakness and muscle aches are evident.
As an ICU nurse, I'm sure you're very familiar with using insulin and glucose to treat high potassium levels ... insulin forces glucose out of the blood and into the cell, and takes potassium into the cell along with it. In persons consuming a sugary, high-carb diet, their insulin levels are high. When they begin a low-carb diet, circulating levels of insulin and glucose go down .. and potassium is released from the cells. There is a transient rise in blood potassium levels .. but this is flushed out by the increased water output (diuresis from ketone production and breakdown of body fat). Within a few days to a week, the person is experiencing the symptoms of lowered potassium levels.
Another factor leading to decreased potassium levels in the blood is a high sodium intake. High sodium level in the blood will force potassium out of the blood and into the cell. The natural ratio of potassium to sodium in the body is 5:1 .. but experts estimate the average North American consumes almost twice as much sodium as potassium. Processed foods, soda pop, condiments, fast foods .. are all high in sodium and low in potassium.
For more information, check out the WholeHealthMD article I linked to above, and also
Potassium Metabolism from the Merck Manual, 2002 edition.
The recommendation to take a nominal amount of potassium either as a supplement or by using a potassium salt-substitute is prudent for low-carbers in the weight-loss stages of their program. As noted above, the Drs. Eades of Protein Power recommend taking 4 x 99 mg pills a day, in divided doses. Low-carbers who are working out heavily, or live in hot climates and sweating a lot should definitely be taking extra potassium, since a significant amount can be lost in the sweat.
Doreen