That’s an interesting observation and I applaud you for reflecting on what makes your stutter better or worse. For me it’s caffeine and refined sugars. I’ve heard of other stutterers becoming more fluent whilst supplementing their vitamin stack with vitamin D3. Now, since you say you’re a lot more fluent in summer months, I’m assuming you’re probably one of those people who would benefit from taking vitamin D supplements on a daily basis. It’s possible you’re borderline vitamin D deficient, which in the winter months exacerbates your stuttering. Vitamin D is often underestimated in its brain effects, but it’s actually a neurosteroid that directly affects dopamine, serotonin, GABA, and even myelin health — all of which are central to speech fluency and motor timing. If I were you, I would get my vitamin D levels checked and if you’re deficient, definitely consider taking supplements.