Quickly double tap then hold the forward key (default is W) to sprint.
Alternatively, in version 1.7.2 or later, you can hold Ctrl and W instead of double tapping it.
You cannot sprint if your hunger bar is 6 units (3 drumsticks 
) or lower.
4 of 4
Clarified buttons required to press, the hunger icons aren't "steak bars", and noted that the second solution will not work for the version OP is on
SirBenet
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