Balancing Act

It is a straightforward equation: E_in = E_out
Energy output must equal energy input in order to maintain balance. Energy input (E_in), which in nutrition is represented in Calories (1 food Calorie = 1 kilocalorie), is the amount of one's "fuel" intake. A kilocalorie is the amount of energy required to raise the temperature of a liter of water 1ºC at sea level.
Energy output (E_out) is the total energy expenditure, which is a combination of a person's basal metabolic rate (BMR), the amount of energy used for physical activity, and the thermic effect of food. While the only E_out factor one can reasonably affect is one's level of physical activity, it is the BMR that represents the lion's share of E_out. BMR accounts for 60% - 70% of energy expended. The thermic effect of food (what you burn by eating and digesting food) is nominal, at about 10%.

Behind the curve or curve the behind?

Ahem. And why are women behind the curve in these weighty matters? It largely rests in the BMR calculations.

BMR (males) = (W x 10) + (W x 2)
BMR (females) = (W x 10) + (W x 1)
    where W = weight

An average, 170-pound man burns more than 2000 Calories each day without added exercise.

170-pound male, BMR = 1,700 + (2 × 170) = 2040 

A 170-pound woman burns fewer than 1900 Calories each day without added exercise.

170-pound female, BMR = 1,700 + (1 × 170) = 1870 

Men's bodies have a 9% higher BMR. The fact that men also tend to be denser (speaking strictly in terms of body mass here) and larger than women compounds the problem. So let's look instead at a 120-pound woman.
For a 120-pound woman, BMR = 1,200 + 120 = 1320. Therefore, an average, 120-pound woman can consume only 1,320 Calories without gaining weight.
So women are jilted on BMR.
What other factors affect BMR? First, BMR lessens as we age, and it lessens as muscle mass decreases (and increases as muscle mass increases), which generally occurs as we age. Again, men have an advantage here, as men tend to have larger muscle mass. Then there are some other factors that stack the deck. As if they need it, people who are tall and thin tend to have BMRs that are higher than the average. (Randy Newman may have been right in decrying the plight of "short people.") Stress eating? That is a double whammy, as while more Calories are consumed, mental or emotional stress simultaneously lowers BMR. Fad or temporary diets? They work for awhile, then have the opposite effect, as lower food intake and fasting decrease BMR.

"Let's Get Physical"

Although it may seem like the best way to control weight, exercise and physical activity contributes only 20 to 30 percent to E_out. Furthermore, like BMR, the net effect of physical activity hinges on other factors. Age, sex, height, and weight all figure into how many Calories are burned. A younger, taller, heavier man burns more Calories than a shorter, older, lighter woman. The good news here is that increasing the frequency of activity can have a positive effect. That is, the more active one is, the more Calories one burns for the same amount of activity. Going from inactive to active can increase the daily Calories burned by 21 percent; from moderately active to very active adds another 15 percent.

What is average weight?

There's no such thing. Realistic averages depend on many factors, including age and sex. However, the mean (half more, half less) weight, and, to a lesser degree, the mean height of all women and men is increasing. In the U.S., mean height of adult males increased over 4 decades from 5'8" (1960) to 5'9½" (2002), and mean weight increased from 166 to 191 pounds. For the same time period, female mean height increased from 5'3" to 5'4", and weight rose from 140 to 164 pounds. For women in my age category (I'm not telling, but you can figure it out from the sources), the mean weight in 2002 was more than 169 pounds, a 23 pound (15.5%) increase from 1960. The smaller overall percentage increase in women's weight gain should not be construed as weight gain slowing. Women's cumulative mean weight in all categories leapt more than 6 percent since the 1984-94 survery. Men's increase in the same period was signicantly lower, at 4 percent. This surely is not a good trend for either.

Overweight?

All that good news about men's advantage in weight maintenance? Men in the same age category were 16.5% heavier than in 1960. And while my category topped the charts with a mean body mass indicator (BMI) of 29.2, representing a 10.6% increase over 4 decades, the same age category of men weighed in at 28.7 BMI, a 12.1% increase. Body mass index is a measure of weight with regard to height. A normal BMI is between 18.5 and 24.9. The mean BMIs of 28.7 and 29.2 are considered "overweight," with over 30 being considered obese and over 40, morbidly obese. Because it factors only height and weight, and not lean muscle mass versus fat mass, BMI is not a good indicator for elite athletes, bodybuilders, or children. As I am not currently running daily due to an injury (from a car crash, not from running) and, thus, cannot consider myself an "athlete," BMI is a decent indicator for me. (Again, I'm not telling.) My BMI is on the high side of "normal," which is higher than I would prefer it to be.

And?

For me, reducing E_in simply does not work. I already fast (for religious reasons) several times a year. The only way to keep my BMR up is to not fast or dramatically reduce my caloric intake outside of those fasts. Neither does a flurry of activity at gym provide much benefit. The key for me is to include physical activity throughout my daily routine: take the stairs instead of the elevator, park in the farthest spot rather than the closest one; mow my own lawn. This is a big one, particularly in the heat of a Texas summer. Heat and cold extremes can increase BMR. And sleep. Never underestimate the power of a good night of sleep. Maybe when I advance to PhD candidacy that last one will be achievable. Sources: National Institutes of Health. Teacher's Guide: Information about Energy Balance. http://science.education.nih.gov/supplements/nih4/energy/guide/info-energy-balance.htm. Ogden CL, Fryar CD, Carroll MD, Flegal KM. 2004. Mean Body Weight, Height, and Body Mass Index (BMI) 1960-2002: United States. Advance data from vital and health statistics; no 347. Hyattsville, Maryland: National Center for Health Statistics.