r's koso drink on a long table

Could a 100-Year-Old Drink from Japan Be the Ultimate Gut Health Hack?

We tried it for a month to find out.

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Nowadays, everyone’s looking for the product to bring their gut health into a state of  perfect harmony. From superfood powders to prebiotic sodas (and probiotic ones, too), many products claim to “fix” your digestive health. Sometimes, though, it’s better to stick with the old-school tactics when you want to improve your health. That’s the pitch behind R’s KOSO, a  traditional Japanese prebiotic-probiotic-postbiotic combo drink that’s quickly gaining popularity in the U.S. I tried it for a month—and spoke to a pair of registered dietician nutritionists—to see what the hype is about. 

Why You Should Trust Me

I spent a month drinking R’s KOSO and speaking with RDNs on prebiotics, probiotics, and postbiotics. I’ve been a health and wellness writer, editor, and public health professional for years and hold a Master’s degree in Public Health.

What Is R’s KOSO?

According to company lore, Ryu Okada started R’s KOSO brand after noticing improvements in his gut health after his wife suggested he try the famous Japanese fermented beverage, Koso. 

“Koso drinks are Japanese traditional fermented drink products typically made from vegetables, fruits, and plants,” says Lena Bakovic, MS, RDN, CNSC, a registered dietitian nutritionist at Top Nutrition Coaching. “They have been used in Japanese tradition as part of fasting or cleansing programs.”

R’s KOSO ingredients

The dietary supplement contains more than 100 ingredients and while we won’t list out every single ingredient, but here are a few of them to take note of:

  • Vitamins A, B1, B2, B6, C, D, E, and K
  • Minerals: including calcium, phosphorus, potassium, sodium, chloride, magnesium, iodine, iron, zinc, copper, manganese, and iodine
  • 17+ amino acids
  • Short-chain fatty acids
  • Acetic acid
  • Polyphenol 

R’s KOSO is non-GMO, vegan, and dairy-free. It is made in a factory with over 70 years of experience making Koso. This drink is also fermented for up to one year for prebiotic, probiotic, and postbiotic benefits.

R’s KOSO potential benefits

According to Yelena Wheeler, MPH, RDN, a registered dietitian nutritionist at the National Coalition on Health Care (NCHC), drinking R’s KOSO may “improve the immune system, reduce inflammation, and may improve symptoms of inflammatory bowel diseases. It may also benefit glucose management and weight management.”

The brand also claims drinking it could improve skin health, boost mental clarity, and get better sleep.

The brand recommends mixing the fermented drink with sparkling or regular water and drinking it one to five times daily to improve gut health.

r's koso drink poured into glass
The bottle and branding looks a bit more like high-end booze than it does a gut health drink. We think that's probably a good thing.

What’s Good About R’s KOSO?

You can feel it working

If you’re looking for a great cleanse—you’ve found it. After just one drink, I could tell R’s KOSO was working quite well in my system. I only took one tablespoon mixed with water and felt results within a few hours. Without providing too much in the way of detail, it will definitely flush you out. 

For folks with digestive problems, be cautious when taking R’s KOSO, as it might be too strong for your stomach. If you’re not sure, it’s always best to discuss any new health supplement with your doctor before trying.

Mixes well with other drinks

As I’ll mention later, it was really hard to drink this by itself. I found it went down much smoother with a cup of water. The brand also recommends sparkling water, but I’m not the biggest fan, so I went with good ol’ tap.

R’s KOSO has a thick, syrup-like consistency but still blends nicely into other liquids, so adding it to regular or sparkling water helps to both thin it out and get used to the flavor and affects.

Genuinely natural ingredient base

Some “gut-friendly” products claim to be natural but contain many artificial sweeteners and flavors that might actually harm your health. I appreciate that R’s KOSO only uses natural ingredients in its formula.

And it does contain more than 100 ingredients naturally sourced from Japan, so I believe it’s a healthier choice than most detox tea syou might come across, for example.

Hours after drinking R’s KOSO, I didn’t feel icky or tired, which was a plus for me.

What’s Not Good About R’s KOSO?

Flavor is hit or miss

I have really strong tastebuds—which can have its downsides. To give a fair review, I wanted to sample the drink in its original, syrupy form. But the moment that drink touched my tongue, I felt instant regret. 

It comes in the flavor Sweet Plum Juice. I’ve never encountered this flavor before, so I initially had difficulty understanding it. I’ll say it has a sweet and herbal taste—and I wasn’t a fan of it. Thankfully, the taste improved once I diluted it in filtered water.

Many reviews rave about the flavor, but it’s such an odd taste to me. As always with flavor and taste, your mileage may vary!

The smell is quite powerful as well, and it smells exactly how it tastes. If you have a strong nose, prepare yourself for the scent. It took a few minutes for me to recover from the smell before tasting it.

High in sugar

Though pitched as a healthy drink, R’s KOSO sure does have a lot of sugar. In case you missed the nutrition info, one tablespoon of R’s KOSO contains over 9 grams of sugar. This high sugar content is likely why the drink is so darn sweet.

Considering the brand recommends that you drink this one to five times a day, you’re looking at a daily sugar intake ranging from totally reasonable to an additional can of coke and then some at the highest recommended consumption level. While I wouldn’t say the sugar level is a dealbreaker—many gut-focused yogurts contain far more sugar, for example—it’s worth considering. 

The Bottom Line

R’s KOSO is a legitimate product. We like that it contains many natural ingredients sourced from Japan that can, in time, improve your gut health. However, the flavor can be off-putting, and the sugar level per serving is higher than we’d like it to be.