Miso Soup Is Salty. The Blood Pressure Studies Say Something Odd

You've been told two things about miso soup. It's good for your gut. And it's loaded with salt.

Only one is plainly true. It's the salt.

But the salt story has an odd twist. The gut story has a hole. And the seaweed carries a warning that rarely makes the label...

The broth has a famous past. In 1907, Kikunae Ikeda, a chemist at Tokyo Imperial University, got hooked on the taste of kombu. That's the thick dried kelp behind dashi, the clear stock under miso soup.

The broth had a taste that wasn't sweet, sour, salty or bitter. By 1908 he had pulled the cause out of the kelp. It was glutamate. He named its taste umami.

So your broth is where a fifth taste was found. Here's the pot...

The Salt, Straight Off A Label

Miso is cooked soybeans mixed with salt and koji, a mold grown on rice. Then it ferments for months. The salt keeps it safe.

Hikari's organic white miso lists 540 mg of sodium in 12 grams. That's from its US Department of Agriculture filing.

This pot uses 60 grams. That's about 2,700 mg, or about 675 mg a bowl.

The US label daily value is 2,300 mg. So one bowl is close to 30 percent of your day.

The Blood Pressure Puzzle

Here's the odd part. In Japan, miso soup is a top source of salt. You'd expect heavy eaters to have higher blood pressure.

In 2017, Koji Ito's team at Kyushu Hospital in Kitakyushu studied 527 adults aged 50 to 81. How often they ate miso soup had no link to blood pressure. Frequent eaters also had slower heart rates.

There's one small trial. In 2019, a Japanese team gave 40 adults miso or a plain soy food for 8 weeks. All had high-normal or mildly high blood pressure. Daytime readings didn't change. Night readings fell in the miso group, even with 3.8 g of salt a day against 0.2 g in the other group.

Here's the catch. Staff from Marukome, the miso's maker, were authors. And it used two special miso blends, not the tub in your fridge.

Rat studies point the same way. Still, I found no trial of ordinary miso soup against the same salt in people for years. Nobody has tested it. On a low-salt plan from your doctor? The label wins.

What The Fermented Soy Brings

Soy carries isoflavones, plant compounds shaped a bit like the hormone estrogen. Call them soy compounds. Miso keeps them.

In 1990, Japan's National Cancer Center began to track 21,852 women aged 40 to 59. Seiichiro Yamamoto's team reported on them in 2003. Women eating the most soy compounds had about half the breast cancer risk of those eating the least. More miso soup showed the same link.

That's observational. Only 179 of the women got breast cancer. Big miso eaters may also differ in many ways. It's a lead, not proof.

The Gut Claim, Checked

Unpasteurized miso holds live microbes. But a living food isn't a probiotic.

In 2021, a panel led by Maria Marco at the University of California, Davis set the bar. To earn the word probiotic, a fermented food's microbes must be named, alive when you eat them and shown to help.

Miso hasn't cleared that bar. I found no human study showing its microbes survive your stomach and do you good. A hot bowl may kill many of them first. Nobody seems to have measured that either...

In 2021, Hannah Wastyk and Justin Sonnenburg at Stanford had 18 adults eat fermented foods daily for 10 weeks. Think yogurt, kefir and kimchi. Their gut microbes grew more varied. And 19 markers of inflammation fell. That was a mix of foods, not miso.

The Seaweed Needs A Light Hand

Kombu is rich in iodine, which your thyroid needs. The National Institutes of Health sets the adult need at 150 micrograms a day. Its safe upper limit is 1,100.

In 2004, Jane Teas at the University of South Carolina tested 12 kinds of seaweed. Iodine ran from 16 micrograms per gram in nori to over 8,000 in one kelp product.

In 2015, Ryoko Katagiri's team at the University of Tokyo tracked 390 Japanese adults. The middle intake was 632 micrograms a day for men and 462 for women. Kelp and kelp stock made up nearly 90 percent of it.

Too much can backfire. In December 2009, Bonsoy, a soy milk made with kombu, was pulled from Australian shelves. Sydney doctors linked it to thyroid trouble in adults and newborns. One sample held 25,000 micrograms per liter.

That's why this pot uses 5 grams and pulls the kombu before the boil. Nobody has measured how much iodine that leaves in your bowl.

The Shiitake, Without The Hype

Dried shiitake add more savory depth. They also bring copper. Federal food data puts four dried caps, about 15 grams, at 86 percent of the copper you need in a day.

You'll see shiitake sold as an immune cure. It isn't one. The human studies are small. They measured blood markers, not fewer colds.

One real caution. Raw or undercooked shiitake can cause an itchy rash like whip marks. A 2016 review from Rutgers blames lentinan, a fiber that heat breaks down. Five min at a simmer fixes it.

Three Things Worth Knowing

Ikeda tasted something in kelp broth and wouldn't let it go. He was right about the taste.

The other claims deserve that stubborn look. Salt is real. The blood pressure puzzle is real but thin. And the gut promise is untested.

Make the pot this week. Stir the miso in off the boil. Read the label before you reach for a second bowl.

Stay well,
The Healing Plate