Why Americans are fatter now

It is common knowledge that the average American now is heavier and has a higher body fat percentage than 50 years ago. There are three possible interrelated reasons for this:

1) We need fewer calories due to decreased physical activity.

2) We eat more calories than we need.

3) We both move less and eat more.

Question: What is the evidence that Americans are less active than decades ago?

Answer: A 1955 survey indicated that the average housewife caring for a two- or three-bedroom house walked 9 to 11 miles a day. Depending on a woman’s body weight, this distance translates into 500 to 1,000 calories per day — just for household chores. Additionally, since the 1960s, estimates of physical activity of mothers have declined 175 to 225 calories per day according to analysis of data from the American Heritage Time Use Study. This calorie change might not seem significant, but if calories consumed stayed the same, weight gain could be substantial. For example, consuming 200 calories more than you use daily for a year would contribute to a predicted weight gain of about 20 pounds.

Q: Is there evidence that Americans are eating more calories?

A: Measurements of calories consumed have lots of problems. Researchers evaluating what people eat complain that people tend to underreport the number of foods and amounts of foods they eat. Consequently, it is difficult to come to a confident conclusion about changes in calorie intake over the years.

Although it is human nature to underestimate how much we eat and to overestimate how much exercise we do, it’s also important to keep in mind that chronic dieting can substantially reduce the calories that the body needs at rest — especially when weight lost is not just fat.

When weight loss includes muscle loss, a person’s calorie needs, even at rest, decline substantially. Muscle loss can reduce calorie needs much lower than would typically be predicted based on a person’s body weight.

The bottom line, it seems clear that there is a decline in “calories out” due to reduced physical activity over the decades, but how much calorie intake has changed is not as clear.

Q: If overweight, how important is it to lose weight?

A: The answer to this question is not as clear as commonly thought. A recent study conducted in Denmark found that the BMI (height to weight ratio) with the lowest risk of death has increased since the late 1970s. In fact, the BMI with the lowest mortality risk in Denmark is now 27, near the middle of what is commonly considered to be the “overweight” range of 25 to 30. In the late 1970s the BMI with the lowest mortality in Denmark was around 24.

If you have a BMI lower than 27, that doesn’t necessarily mean you should gain weight unless your BMI is below 20. Also, different racial groups most likely have different mortality risk patterns related to BMI.

Losing weight too quickly usually reduces muscle substantially. This can compromise immune function, so maintaining muscle mass with age might be more important than reducing body fat. The Denmark study reinforces the common advice to avoid quick weight loss that decreases muscle and aim to eat well, stay active and let your body find its “sweet spot.”

Alan Titchenal, Ph.D., C.N.S., and Joannie Dobbs, Ph.D., C.N.S., are nutritionists in the Department of Human Nutrition, Food and Animal Sciences, College of Tropical Agriculture and Human Resources, University of Hawaii-Manoa. Dobbs also works with University Health Services.