Summer just feels like a healthier season – plenty of daylight after work to play soccer or picnic in the park, beach days, and a free boost of vitamin D from all that extra sunshine. It's like a reset button for body and mind. But when it comes to replenishing your vitamin D, depleted by so much indoor time and skin-covering layers over the winter, that extra time outside might not be enough. A new study from Newcastle University in the UK, published in the European Journal of Clinical Nutrition, found that even during the brightest months of the year, vitamin D insufficiency remained stubbornly common for the majority of people tested."What's striking about these findings is that vitamin D levels didn't improve, even in the summer months when we would usually expect them to recover," says nutrition researcher Bernard Corfe.That extra time outside might not be enough. (kate_sept2004/E+/Getty Images)Between December 2024 and August 2025, Corfe and his colleagues tested 299 people for vitamin D levels using a simple finger-prick blood test. About half of them were over 65, and the other half were people over 18 who had darker skin. Among adults aged 65 and older, almost 55 percent had vitamin D levels below the recommended threshold. The picture was even more striking for adults with darker skin, where more than 72 percent fell below that level. Despite testing beginning in winter and moving into spring and summer, those numbers barely changed.That's a problem because vitamin D plays a key role in keeping bones healthy by helping the body absorb calcium, which is especially important for older adults. It's also involved in muscle function and immune health, and it may even affect mental health.A large meta-analysis published in 2022 in Critical Reviews in Food Science and Nutrition linked higher vitamin D to fewer depressive symptoms in those diagnosed with depression. There's plenty of disagreement about some of the health claims though, with some scientists calling studies linking vitamin D to lower cancer or diabetes risk overblown. Research is ongoing. Unlike most vitamins, vitamin D is something we can both consume in our food and make ourselves.