Why are so many Americans so fat? Let's do some math.
Take a typical fast food meal:
Big Mac: 540 calories
Large Fries: 500 calories
Medium Coke Classic: 240 calories
Total = 1260 calories
The canonical 2000 calorie diet is only a rough estimate, one that is within about 10% accuracy for a male of my age and height, but may be very wrong for other people. In fact, all caloric intake estimates are imperfect, but to even get in the right ballpark you need to take into account factors like age, weight, gender, and rough activity level as this about.com calculator does. (And remember, that calculator is showing what you need to maintain current levels, not slim down.) My wife, for example, should be eating about 400 fewer calories per day than me.
If we don't even account for the problems of empty calories (low nutrition, highly processed food that may impede metabolic reactions needed to properly consume the calories), water retention from high sodium foods, the fact that carbohydrates are burned easily and therefore preempt your body's need to draw from fat reserves -- and other more subtle barriers to weight loss and, more importantly, cardiovascular health and athletic muscle toning -- we can still see the roots of the problem from straight-up calorie math.
1260 calories is more than half the RDA for someone like me (a 200lb, 6 foot, 35 year old male). If all three meals Americans are eating each day are around the 1000 calorie mark, we're talking approx. 3000 calories per day, or about 750-1000 calories more than is recommended for a large male like myself. (And if you're doing the two Big Mac thing, and adding a 250 calorie baked apple pie, that's another 750 calories in one meal.)
I've been watching the readouts on the machines as I've been hitting the gym, and by way of example someone my size, weight, and age doing a moderate (level 5) cardio routine on the StairMaster is burning about 10cal/minute.
750 excess calories / 10cal/minute = 75 minutes
So, if I'm averaging 3000 calories a day, and my break-even estimate is 2250 calories, I'm at least 750 calories over break-even -- which means an hour and fifteen minutes of uninterrupted StairMaster workout just to maintain current weight.
Most people who say that they are "relatively active" and are referring to less than an hour of cumulative strenuous walking, stair climbing, and similar activity during the day are fooling themselves.
"But I walk a lot" perhaps applies to active people in New York City who may average two hours of fast walking per day or San Franciscans who pound the hills at reasonably high speeds an hour a day. But even that's still going to be break even at best if it's not coupled with caloric moderation. And saying "but I walk a lot" doesn't meaningfully apply to anyone who just happens to have to walk during the day because their refrigerator isn't sitting right beside their TV.
The reason so many people are heavy is quite clear: easy access to high calorie foods, combined with the prevalence of sedentary post-industrial workplaces, makes for people who eating far more calories than they'll reasonably burn in a day. This seems like common sense, but as the saying goes: sometimes common sense ain't so common.