The leaner you are—meaning, the less subcutaneous fat you have covering your muscles—the more pronounced your veins will look, says Dr. Nadolsy.
But it’s not just about being lean: Having low body fat along with upped muscle mass is the magic combination for veins that pop, even when you’re at rest. So in some ways, pronounced veins are an indirect sign of fitness.
That’s because muscle has something called “residual tension at rest,” which provides some resistance against venous return, says Dr. McGuff. Your veins carry blood to the heart, and when you exercise, the increase in blood flow creates a sort of blood backup in your veins, creating higher blood pressure. That compresses the returning veins and causes the blood to dam up and engorge the veins at the skin surface. So the more muscle you have, the more residual tension at rest you have, which means your veins are become more dilated.
“Combine that with a low level of body fat and the veins will really ‘pop’,” he says.
So yes, vascularity while you’re at rest can be a marker of fitness, but context is everything, Dr. McGuff says.
For example, sometimes vascularity can be a marker of excess stress, with increased secretion of the stress-hormone cortisol, or over-production of the hormone aldosterone, which causes your body to hang on to sodium. As a result, your body retains water and makes your veins swell, he says.
The veins can also be engorged due to damage, or because of varicose veins or hemorrhoids, Dr. McGuff says (Here’s everything you ever needed to know about hemorrhoids).
But your vascularity also depends on your genetics, too.
“Some may be very lean but with little vascularity and some can have more fat and still have some vascularity in cases,” says Dr. Nadolsky.
Bottom line, it most likely is a good indicator that you’re fit, but it’s not a given.