One Of Costco's Best-Selling Items Is A Bottled Water Brand You Can't Get In Other Grocery Stores

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The list of top-selling items at Costco doesn't hold too many surprises, but alongside favorites like milk, bananas, paper towels, and rotisserie chickens, there are a few outliers. Tucked between blueberries and protein shakes, the 18-count pack of 1-liter bottles of Kirkland Signature Alkaline Water certainly stands out from the crowd. For those unfamiliar with this private-label product, it is a type of bottled water with a slightly higher pH, a characteristic often described as "alkaline" or "basic."

For decades, the wellness community has made claims about the effects of regularly consuming alkaline water, asserting its ability to help regulate the body's pH levels, slow the aging process, increase bone density, and even prevent cancer or strokes. A few studies claim to have found improvements in aging markers in mice or slightly improved stool samples in humans. As a result of such claims, the product has become quite popular — enough so that Costco has even given out free samples of the water.

Regarding Costco's offering, it appears this popularity is largely dependent on these supposed health benefits, despite the science being unable to back them up. While some shoppers share that the Kirkland water is pretty much all they drink, the majority of online voices are more skeptical. The most common refrain is, as one Redditor put it, "It's water. It does water things." Still, that hasn't stopped other customers from giving it two thumbs up. "I have never liked drinking natural water, but this alkaline water is excellent," noted one reviewer on Amazon, where it boasts a 4.5-star rating.

Before you buy alkaline water, it helps to understand the science behind it. The pH scale ranges from 0 to 14 and is used to measure acidity and alkalinity, with 7 being the neutral center. Anything below is considered acidic, and anything above is labeled alkaline. According to Harvard Medical School, most bottled water measures in right around 7 pH, with typical bottled alkaline waters landing at around 8 or 9 pH. Costco's exclusive offering, however, comes in even higher, at 9.5 pH.

Generally speaking, the effects of this alkalinity are undone as soon as it's swallowed. Stomach fluid is highly acidic, with a 1.5 to 3.5 pH, for the sake of digestion. Any minor change caused by drinking alkaline water will be counterbalanced as the body seeks to return to its natural state. Additionally, minerals like magnesium, calcium, and potassium are introduced into the mix, possibly enhancing the taste. That said, regularly drinking highly alkaline water can lead to hyperkalemia, a condition created by too much potassium in the blood that can especially affect those with kidney disease.

The jury is still out about exactly what alkaline water does for consumers. In moderation, it should be perfectly healthy — and there may even be some potential long-term health benefits. If the concern is just bringing up the pH, however, there's no need to buy that special case of Kirkland Signature Alkaline Water — mixing a bit of baking soda into a glass from the tap will do that as well.