The best and quickest way to see if Grandma lied to you - test her theory for yourself.
She claims that 'covering your kidneys (aka lower back) will help you stay warm' -
Next time you're feeling a bit cold & shivery, grab a scarf and wrap it around your lower back/tummy area...close against your skin will work best, but over clothes if that's easier.
You should feel an instant sense of warmth, which will increase over time.
Now, in an hour, remove the scarf.
You'll definitely notice the body temperature drop and you won't feel snug anymore.
Next time you're feeling HOT, fold your leggings/jeans/shorts/pants down a fold or two so they sit on your hip bones. You should instantly feel a sense of coolness.
It's all in the kidney region.
Kidneys regulate body temp. It's so simple.
Conclusion: Grandma's don't lie.
Ps. The beauty of the belly warmer is that you can fold it down when you're hot and slide it back up when you're cool again. ;)