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What is How is the protein Energy Ratio (PER) in diet expressed as – and explain in detail?
Protein energy ratio (Protein energy percentage) 

It is a quantitative measure for assessment of protein.
It measures the percentage of energy that is provided by the protein in the food.

PE percent = Energy from protein / Total energy in diet x 100

If the PE ratio is less than 4%, the subject will be unable to eat enough to satisfy protein requirements.
It is recommended that protein should account for approximately 15 to 20% of the total daily energy intake.