The answer is nearly 172 million people.
National Center for Health Statistics reported in 2005 that the American Adult Population had a proportion of:
BMI = Body Mass Index
If we add all of them we obtain 72,9%
The total population in 2005 in USA was 295.500.000 people
The percentage of adult people (>15 years old) was 80%.
If we multiply: 0,729 x 0,80 x 295.500.000 people
we obtain a resulting value of 172.335.600 people.