Latest information from the United States. Department of Agriculture suggests that 55 percent of Americans don’t get enough vitamin A. Most people associate A with good eyesight, but it has many more properties, including cancer protection and resistance to infection via better immunity. Also, according to the January ’11 Better Nutrition, a zinc deficiency can interfere with vitamin A activity, and many athletes are zinc deficient. Good insurance is to take around 10,000 international units of vitamin A a day or every other day.