Hi SELF Family,

When a lab report shows potassium creeping up, people with kidney disease are usually told to clear out the fruit bowl and the salad. Researchers in Tokyo checked what people with kidney disease actually ate. Fruit wasn't the problem. One food was.

Potassium is a mineral your heart needs to keep a steady beat. Healthy kidneys get rid of whatever you don't need. When kidney function slows, potassium can build up, and too much can throw your heartbeat out of rhythm. Normal blood potassium is 3.5 to 5.0. Your lab report may print it as mmol/L or mEq/L, and for potassium they are the same number.

The total wasn't linked to high potassium. The source was.

The study looked at 245 people with kidney disease at three hospitals in Tokyo, none of them on dialysis or on medicine to lower potassium. Researchers used a detailed diet questionnaire to work out where each person's potassium came from, then matched it against their blood tests. The total amount of potassium people ate was not linked to high potassium, which this study set at 5.0 or above. Potatoes were. The people who got the most potassium from potatoes had about 4 times the odds of high potassium compared with the people who got the least. Beans and green and yellow vegetables were tied to higher blood potassium too, so their portions matter. Fruit wasn't on the list.

So if your potassium runs high, start with potatoes. You don't have to give them up, but you do have to cook them differently. In lab tests, boiling potato cubes removed half the potassium, and boiling shredded potato removed three quarters. A potato that is baked, roasted, microwaved or boiled whole keeps most of its potassium.

Soaking a raw potato, the old leaching trick, made no real difference on its own. Boiling did the work.

If you're on dialysis, your kidney team sets your potassium limit. Ask them before you change anything, and ask whether potatoes boiled in small pieces can stay on your plate.

If your potassium runs above 5.0 and you're not on dialysis, the first food to change is potatoes. Cut them small, boil them in plenty of water, and pour that water down the sink. Set your portions of beans and greens with your kidney team, and ask your doctor to review your medicines, since some raise potassium. And check the label on your salt substitute. Many are made with potassium chloride, and those are the ones to skip. If your result is 6.0 or higher, call your doctor the same day. Palpitations, chest pain or sudden muscle weakness with high potassium mean you should go to the emergency room.

If your potassium is normal and you're not on dialysis, keep the fruit and vegetables. In a five-year trial in Texas, researchers took 108 people with kidney disease, heavy protein leaking into their urine and acid building up in their blood, none of them with diabetes, and gave them fruits and vegetables, sodium bicarbonate pills (the standard prescription for that acid), or usual care. The fruit and vegetable group lost about half as much kidney function as usual care, matched the pills, and ended with lower blood pressure (the top number) and lower LDL cholesterol than both. If you use a salt substitute, check the label for potassium chloride, and skip it if you have advanced kidney disease, take a potassium pill, or take a potassium-sparing water pill like spironolactone.

That trial is number 1 in this week's video. Several of the other six are on the usual kidney avoid list. The video gives the amount of each, the drink people are told to chug that didn't slow kidney decline in a year-long trial, and the two lab numbers to recheck in 90 days. Watch The 7 BEST Foods to Protect Your Kidneys (Ranked by a Kidney Doctor)

With gratitude,

Dr. Sean

The research behind this letter

1. Morimoto N, Shioji S, Akagi Y, et al. Associations between dietary potassium intake from different food sources and hyperkalemia in patients with chronic kidney disease. Journal of Renal Nutrition. 2024;34(6):519-529.

2. Bethke PC, Jansky SH. The effects of boiling and leaching on the content of potassium and other minerals in potatoes. Journal of Food Science. 2008;73(5):H80-H85.

3. Goraya N, Munoz-Maldonado Y, Simoni J, Wesson DE. Fruit and vegetable treatment of chronic kidney disease-related metabolic acidosis reduces cardiovascular risk better than sodium bicarbonate. American Journal of Nephrology. 2019;49(6):438-448.

4. Clark WF, Sontrop JM, Huang SH, et al. Effect of coaching to increase water intake on kidney function decline in adults with chronic kidney disease: the CKD WIT randomized clinical trial. JAMA. 2018;319(18):1870-1879.

5. Xu X, Zeng L, Jha V, et al. Potassium-enriched salt substitutes: a review of recommendations in clinical management guidelines. Hypertension. 2024;81(3):400-414.

This content is for educational purposes only and is not medical advice. Always consult your healthcare provider for individual care. The views expressed are Dr. Hashmi's own and do not represent his employer.