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In this cross-section of the spine (left), there's a bit of tissue called the meniscus that acts as a shock absorber between the pieces of the spine. If you're overweight, every time you take a step, you take seven times that weight on your spine. If you don't stay limber or lose weight, over time that piece of tissue can get crushed down. In addition, according to Dr. Oz, you recreate your bones every ten years. If you don't do some sort of weight-bearing exercise, your bones will become fragile from osteoporosis and break more easily.
The knee (right), also has the meniscus to protect your joint. In an overweight person, it can get fractured and the bones begin to wear. This can lead to knee pain, back pain, and a number of orthopedic ailments.