Why do you get hungry? Feeling hungry is your body's way of
telling you that your cells need
energy. All cells need energy to live,
grow, and reproduce. Plant cells get
their energy from the sun. Many
animal cells get their energy from
food.
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You may feel hungry frequently if your diet lacks protein, fiber, or fat, all of which promote fullness and reduce appetite. Extreme hunger is also a sign of inadequate sleep and chronic stress. Additionally, certain medications and illnesses are known to cause frequent hunger.

Explanation:

"Hunger hormones" (ghrelin) in your blood and an empty stomach signal the brain when you're hungry. Nerves in the stomach send signals to the brain that you're full, but these signals can take up to 20 minutes to communicate -- and by that time, you may have already eaten too much.

the sum of all protons and neutrons in the atomic nucleus

or

the weighted average of the masses of the isotopes of the element

or

The average mass of all the isotopes of an element