A small cup of dashi before breakfast and dinner: 20 people with type 2 diabetes in Osaka tried it for a month. Sweets tasted no clearer than with plain hot water

People with type 2 diabetes often need a stronger sugar solution than others before they can tell it's sweet, and many lean towards rich, energy-dense food. Both make a diet harder to stick to. Dashi, the stock behind miso soup, is made from kelp, dried fish and shiitake, and it carries three umami compounds that boost each other. Earlier experiments found that broths with added glutamate could take the edge off people's appetite for savoury snacks. Doctors at the University of Osaka wanted to know whether a little dashi before meals could shift taste and food preferences, and reported a pilot randomised trial in Frontiers in Nutrition on 28 October.

Twenty patients from the university hospital's diabetes clinic took part between 2023 and 2024. Their average age was about 69, and they had lived with diabetes for around 20 years. Half were assigned to drink 2.5 g of powdered dashi stirred into about 50 mL of hot water before breakfast and dinner for four weeks; the other half drank the same amount of plain hot water. The main test was sweetness: participants held sugar solutions of rising strength in their mouths until they could name the taste, and the lowest strength they got right was their threshold.

Eighteen people finished the month, and the dashi drinkers took 97% of their servings. Both groups detected sweetness at weaker solutions by the end, the dashi group more so: their average threshold fell from 30.0 to 13.1 g of sugar per litre, against 25.6 to 17.8 g with hot water. Once starting levels were allowed for, the gap between the groups was 5.2 g per litre, a difference small enough, with so few people, to be down to chance. Of those who started with poor sweet taste, 3 of 5 in the dashi group moved into the normal range and none of 4 in the water group, again too few to read much into.

Food preferences were rated on a picture questionnaire of everyday Japanese foods, scored out of 100. In the dashi group, liking for savoury fatty foods fell from 41 to 30 and for sweet starchy foods from 52 to 41, while the water group barely moved. Compared directly, though, the groups differed by 11 and 8 points with wide margins of error either way, and nothing changed in what people actually ate, their blood tests or their body composition over the month. Nobody reported side effects.

The authors are careful not to oversell this. The trial was meant to have 40 people but recruitment stalled at 20. Everyone knew what they were drinking, the person testing their taste knew too, and plain hot water doesn't smell or taste like stock, so expectation could explain some of the change. Repeating a taste test also tends to improve scores on its own. The dashi maker, Kubara Honke, funded the study alongside a government grant and a second company, supplied the product, employs two of the authors and shares a pending patent on the approach. What the trial shows is that a cup of stock before meals is easy to keep up; whether it changes anyone's palate needs a bigger, blinded trial run by people without a stake in it.