Conditions:

1. Food level must allow the player to eat
2. Player is holding food
3. Player has a shield in their off-hand slot

Actions:

1. Press F5 until the front of the player is visible
2. Hold down whichever button is assigned to consuming food, but do not hold down that key long enough for the player to actually consume the food

As you can see, the shield appears to be raised even though the food is being consumed. In first-person mode, the shield does NOT appear to be raised. It is only in third-person view that the shield appears to be raised.

I am unsure as to whether or not the shield can actually block attacks/projectiles while it appears to be raised.