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[[File:salad 20189577 s.jpg|right|thumb|500px|Fruit and veg are the best sources of potassium.]]
Potassium is a vital, but widely misunderstood nutrient. This is partly because recommended intake of potassium is remarkably high, not because of the potassium itself, but because of the health benefits of the potassium containing foods. Potassium deficiency (hypokalemia) is rare in healthy people, even athletes who train in hot environments. However, care must be taken when training after illnesses that cause vomiting or diarrhea.
=What is potassium?=
Potassium and sodium of the two major positive electrolytes, with potassium concentrated inside our cells and sodium concentrated on the outside. The primary negative electrolyte is chloride, along with some bicarbonate (HCO3) that maintains the pH level (acidity).
=Where do we get potassium from?=
Potassium is an abundant nutrient in most fruit and vegetable. Most of this potassium is in the form of potassium bicarbonate, not potassium chloride. This is important, as the bicarbonate has a number of health benefits, including regulating the blood acidity. If there is not sufficient bicarbonate, calcium will be leached from the bones to maintain the correct acidity level. The fruit and veg that contain potassium also typically contain other nutrients including [[Fiber]].
=How much potassium is recommended?=
For adults the recommended daily intake of potassium is 4700 mg, but this is not set to prevent potassium deficiency (hypokalemia). Instead, "this level of dietary intake (i.e., from foods) should maintain lower blood pressure levels, reduce the adverse effects of sodium chloride intake on blood pressure, reduce the risk of recurrent kidney stones, and possibly decrease bone loss<ref name="RDA"/>." These benefits are largely from the bicarbonate and the other vegetable matter rather than from the potassium.
==Magnesium Deficiency==
Subjects who are deficient in both Potassium and [[Magnesium]] who are given just potassium supplements have normalized serum potassium levels, but not normalized muscle potassium until magnesium supplementation is also given<ref name="WhangWelt1963"/><ref name="Dyckner-1978"/>.
=How much potassium do we need?=
All adults should aim to get sufficient fruit and vegetables to ensure that they get the recommended intake of 4700 mg of potassium. However, as little as 580mg/day can prevent hypokalemia, though might cause insulin resistance<ref name="Norbiato-1984"/>. A study showed that healthy patients could maintain their potassium balance on 1600 mg per day, though they were at the lower end of the acceptable range<ref name="Sebastian-1971"/>.
=Do you need potassium in your sports drink?=
Sweat losses can be as high as 1800mg/day on the initial introduction to hot conditions, but this drops to around 600-800 mg/day with adaptation<ref name="CONSOLAZIO-1963"/>. This is equivalent to around 850 mg/L in the first few days of heat exposure, rapidly dropping to around 250 mg/L. Therefore, if you get close to the RDA of 4700 mg, and need ~1600 mg/day, that would leave ~3000 mg that could be lost in the sweat without creating a deficiency. This would be around 7.5 pints (3.75L) initially, or around 24 pints (12L) after heat adaptation. Therefore, potassium is probably not necessary for sports drinks if you are healthy. It won't do any harm, and you could add a little salt substitute ([http://www.amazon.com/No-Salt-Substitute-11-Ounce-Can/dp/B000H185N6 No Salt]) to your sports drink if you find this is an effective placebo. However, don't count potassium from salt substitute towards your recommended daily intake of potassium from natural sources.
=Running, illness, and potassium deficiency=
While potassium deficiency is rare in healthy people, there are a number of conditions, many that affect runners, which could cause this deficiency<ref name="www.nlm.nih.gov"/>:
* '''Eating disorders'''. There is reason to believe that eating disorders are more common amongst both male and female athletes than the general population<ref name="Sundgot-Borgen-2004"/>. These eating disorders can easily result in potassium deficiency, especially if they involve induced vomiting and laxative abuse.
* '''Diarrhea'''. Severe diarrhea can be a problem on ultramarathons, so additional potassium may be warranted. Be careful returning to training or competition after a serious bout of diarrhea.
* '''Vomiting'''. Many ultrarunners have issues with nausea and vomiting, and this is another situation where additional potassium might be advisable.
* '''Magnesium deficiency'''. Athletes should ensure they have an adequate intake of magnesium, as magnesium has many health benefits in addition to its synergy with potassium.
* '''Antibiotics'''. Antibiotics such as penicillin, nafcillin, carbenicillin, gentamicin, amphotericin B, foscarnet, can all have been associated with potassium excretion.
* '''Kidney disease.''' Diseases that affect the kidneys' ability to retain potassium such as Liddle syndrome, Cushing syndrome, hyperaldosteronism, Bartter syndrome, or Fanconi syndrome, are risk factors for potassium deficiency.
* '''Diuretic medications.''' Medications intended to increase urine output will also flush out potassium.
* '''Glycyrrhetinic acid'''. Eating large amounts of licorice or products that contain licorice made with glycyrrhetinic acid can cause potassium deficiency. Note that glycyrrhetinic acid is no longer used in licorice made in the United States.
The symptoms of potassium deficiency include:
* Abnormal heart rhythms (dysrhythmias), especially in people with heart disease.
* Constipation.
* Fatigue.
* Muscle damage (rhabdomyolysis).
* Muscle weakness or spasms (but not [[Cramps]]).
* Paralysis (which can include the lungs).
I have seen an ultrarunner need urgent medical attention because they attempted to complete 100 mile race soon after a serious digestive tract upset. The nausea and vomiting the previous week had left them potassium depleted, which resulted in their collapse at an aid station.
=Sources of potassium=
Most fruits and vegetables are sources of potassium, but it's worth making sure you include a few foods that are particularly high in potassium, mostly for the associated bicarbonate. Here are some that I think are particularly noteworthy:
* Spinach has a number of health benefits, and has even been associated with improved muscle growth. It is a great source of potassium and fiber, and 100g spinach contains 558 mg of potassium.
* The average avocado (200g) contains 975 mg (16% DV) of potassium as well is plenty of fiber and healthy oils.
* A 3 ounce fillet of salmon contains 534 mg (15% DV) of potassium along with healthy oil and protein. Other fish that are good sources of potassium include Halibut (13% DV), Yellowfin Tuna (13% DV), Anchovies (12% DV), and other fish are about 10% DV.
* While people tend to think that bananas are the best source of potassium, the average banana contains 422mg (12% DV). This is pretty good, but far from the best.
=Summary and recommendations=
My personal recommendations based on my study of the literature are:
* You should aim to eat fruit and vegetables that provide you with 4700 mg or more of potassium each day. 
* Don't worry about potassium in your sports drink. It's far more important to get plenty of sodium, but a little potassium won't do any harm.
* Be careful when recovering from a bout of nausea or diarrhea as this can cause life-threatening potassium deficiency. 
* If you or someone you know is suffering from an eating disorder, please seek professional help.
=References=
<references>
<ref name="Sundgot-Borgen-2004"> J. Sundgot-Borgen, MK. Torstveit, Prevalence of eating disorders in elite athletes is higher than in the general population., Clin J Sport Med, volume 14, issue 1, pages 25-32, Jan 2004, PMID [http://www.ncbi.nlm.nih.gov/pubmed/14712163 14712163]</ref>
<ref name="www.nlm.nih.gov">Hypokalemia: MedlinePlus Medical Encyclopedia, http://www.nlm.nih.gov/medlineplus/ency/article/000479.htm, Accessed on 25 August 2013</ref> 
<ref name="Sebastian-1971"> A. Sebastian, E. McSherry, RC. Morris, Renal potassium wasting in renal tubular acidosis (RTA): its occurrence in types 1 and 2 RTA despite sustained correction of systemic acidosis., J Clin Invest, volume 50, issue 3, pages 667-78, Mar 1971, doi [http://dx.doi.org/10.1172/JCI106537 10.1172/JCI106537], PMID [http://www.ncbi.nlm.nih.gov/pubmed/5101785 5101785]</ref>
<ref name="CONSOLAZIO-1963"> CF. CONSOLAZIO, LO. MATOUSH, RA. NELSON, RS. HARDING, JE. CANHAM, Excretion of sodium, potassium, magnesium and iron in human sweat and the relation of each to balance and requirements., J Nutr, volume 79, pages 407-15, Apr 1963, PMID [http://www.ncbi.nlm.nih.gov/pubmed/14022653 14022653]</ref>
<ref name="RDA">Accessed on 25 August 2013</ref> 
<ref name="Norbiato-1984"> G. Norbiato, M. Bevilacqua, R. Meroni, U. Raggi, R. Dagani, D. Scorza, G. Frigeni, T. Vago, Effects of potassium supplementation on insulin binding and insulin action in human obesity: protein-modified fast and refeeding., Eur J Clin Invest, volume 14, issue 6, pages 414-9, Dec 1984, PMID [http://www.ncbi.nlm.nih.gov/pubmed/6441716 6441716]</ref>
<ref name="WhangWelt1963">R. Whang, L. G. Welt, Observations In Experimental Magnesium Depletion*, Journal of Clinical Investigation, volume 42, issue 3, 1963, pages 305–313, ISSN [http://www.worldcat.org/issn/0021-9738 0021-9738], doi [http://dx.doi.org/10.1172/JCI104717 10.1172/JCI104717]</ref>
<ref name="Dyckner-1978">T. Dyckner, PO. Wester, Ventricular extrasystoles and intracellular electrolytes in hypokalemic patients before and after correction of the hypokalemia., Acta Med Scand, volume 204, issue 5, pages 375-9,  1978, PMID [http://www.ncbi.nlm.nih.gov/pubmed/82374 82374]</ref>
</references>