Not consuming the right nutrients is bad for the body both physically and mentally also known as mal nutrition. If a person were to just consume junk food and soda for an entire week their body would not be getting the right nutrients and might become weak or more fatal problems. Eating a while balanced diet is important for the body to perform right. For example look at major athletes, they eat the right foods and drink the right liquids. But also exercise is important especially when consuming carbohydrates.
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