Yes, it is acceptable that 1 in 5 Americans go hungry, because there is always food available for someone who needs it. Anymore, anyone hungry can get food stamps from their state government. On top of that, there are homeless shelters and food pantries that will always feed anyone who is hungry.
It is not acceptable that 1 in 5 Americans are hungry. I don't know how to solve the problem, and I wish I did. However, Americans as a society, restaurants included, waste tons of food each day. Some companies, such as Wegmans and Panera Bread reduce the waste donating foods to food pantries, and other charity groups who will make sure the food gets to those in need. In this country, no one should go hungry.
Morality aside, there is a very practical reason to feed America's hungriest citizens. When food is not available, those who do not have it will be driven to crime. Since food is necessary for life itself, it follows that people will do anything to avoid hunger. In many cases, that means crime. Either people will have to steal food, or they will have to engage in crime to raise money to buy food. If they don't, they die.
I do not believe it is acceptable that 1 in 5 Americans go hungry. I feel like that statistic may be a little high. I live in a poverty stricken town and many of the people in poverty are obese. In fact, I don't know one person that goes hungry on a regular basis. We only have a couple of hundred dollars a month for groceries and we still aren't starving, nor are we hungry most of the time.