We've all heard the old adage – an apple a day keeps the doctor away. We know fruits are good for us: the USDA suggests working 1-2 cups into your diet daily. But what happens when apples, oranges and pears are all your diet consists of?

Ashton Kutcher found out the hard way when his stint with an all-fruit diet landed him in the hospital for two days. While talking about his work on the new biopic, Jobs, Kutcher told reporters at the Sundance Film Festival, "I was like doubled over in pain. My pancreas levels were completely out of whack."

So what exactly is this all-fruit diet? It consists of eating nothing but fruits and starchless vegetables like spinach, carrots and cucumbers. Also known as the Fruit Flush, this regimen is more of a detox than an actual diet, making it a fad that should not be followed regularly.

Needless to say, eating only fruit leaves a number of essential nutrients out of your diet – but the real danger lies in the loads of natural sugar found in fruits. Your pancreas releases insulin to control your body's sugar levels, but eat too many fruits and the sugar overload can send your pancreas into a serious state of fatigue.

Dietician Beth Wallace, of the Children's Hospital of Philadelphia, shares the big reasons why an all-fruit diet is an all-around no.

"Fruit's main nutritional components are carbohydrates and vitamins. Your body needs two other macronutrients - or big nutrient categories - which are fat and protein. Both are essential for normal metabolic function, and depending on your age, body stores, and your diet, they can be depleted quickly. An all fruit diet also eliminates your fat soluble vitamins, A, D, and E," said Wallace.

"The human body cannot and should not operate with one food group," Wallace concluded.

So follow the doc's orders, but remember every recommendation has a limit – and come to find out, there is such a thing as eating too many fruits!

Read the full story at CNN.com