How can you prevent varicose veins?

Varicose veins are caused by the blood pooling in the veins in the lower extremeties. Your veins have valves that normally keep this from happening, but the effects of gravity can make them weak. The contractions of your leg muscles help pump the blood back to your heart. So basically anything you can do to relieve the effects of gravity and help the blood circulate is going to help prevent varicose veins. Exercise, elevating your legs when you can, wearing support stockings, etc.

One big thing is to only cross your legs at the ankles, and not thigh over thigh.

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Smoking can cause varicose veins. Just get lots of cardio exercise and don't smoke.

Exercise regularly to improve your leg strength, circulation, and vein strength. Focus on exercises that work your legs, such as walking or running.
Control your weight to avoid placing too much pressure on your legs.
Do not cross your legs when sitting.
Elevate your legs when resting as much as possible.
Do not stand or sit for long periods of time. If you must stand for a long time, shift your weight from one leg to the other every few minutes. If you must sit for long periods of time, stand up and move around or take a short walk every 30 minutes.
Wear elastic support stockings and avoid tight clothing that constricts your waist, groin, or legs.
Eat a low-salt diet rich in high-fiber foods. Eating fiber reduces the chances of constipation which can contribute to varicose veins. High fiber foods include fresh fruits and vegetables and whole grains, like bran. Eating too much salt can cause you to retain water or swell.