I am not sure about the "best" online health magazine but there is a handful of them that are very popular.
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Here are some of the renowned names:
• Muscle and Fitness
• Runner's World
• Shape (.com)
• Psychology Today
• Health (.com)
• WebMD Magazine
• Women's Health
• Men's Health
• Prevention (.com)
• Natural Awakening
All of these are some of the globally renowned print and online health magazines.
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Now, while they are all centric to "health", they have their respective niches, target audience, and overall approach.
So, which one should you purchase largely depends on who you are and what exactly are you looking for.
For example, WebMD Magazine is more about medical studies and researches. Runner's World, as the name suggests, targets the runners. Muscle and Fitness are more for those who are into serious weightlifting or muscle-building programs. Prevention Awakening focuses more on "green living". And Psychology Today talks more about the human mind's health.
(Courtesy: Shape.com)
So, which online health magazine should you buy depends on who you are (female or male), what health topic interests you, and what exactly are you looking for in return from a health magazine.
Know what you need and then decide accordingly.
Hope this answer helps!