Brooks Glycerin 23 vs Brooks Glycerin Flex:
Which to Choose

Start with the foam and construction and the two look like siblings: the same DNA Tuned nitrogen-infused midsole with dual-sized cells (larger, softer heel cells and smaller, firmer forefoot cells), no plate in either, and a neutral platform on both. From there they diverge in feel. The Glycerin 23 is the plusher, more substantial of the two — it carries more cushioning (5/6 vs 4/6), a taller stack at roughly 37 mm heel / 29 mm forefoot versus about 35 mm / 26 mm on the Flex, and a broader, more planted base that earns it a higher stability rating (4/6 vs 3/6). That extra material comes at a weight cost: the Glycerin 23 is about 303 g in a men's US 9, while the Flex is noticeably lighter at around 264 g, a roughly 40 g gap you feel on the leg over long miles. The Flex's identity is its deep flex grooves, which let the shoe bend with your foot for a more natural, easygoing toe-off, where the Glycerin 23 gives a smoother but more damped, unhurried roll. Drop is close and effectively the same ride geometry — an 8 mm measured offset on the Glycerin 23 and about 9 mm on the Flex (Brooks markets it as 6 mm). Neither is springy: responsiveness is medium (3/6) on both, and each turns vague if you push much past a steady conversational pace, with the Glycerin 23 having a slightly higher effective ceiling. The uppers differ in polish too — the Glycerin 23 uses a premium triple-jacquard warp-knit hold, while the Flex runs a simpler engineered flat-knit. Tellingly, the Glycerin 23 is rated for 10K through the marathon, while the Flex tops out comfortably around the half.