Fatter, in all the wrong places

The fact that the majority of Americans are overweight isn't any news. But here's a worrisome new finding: The extra padding we've added as a population has accumulated in the worst location for our health - our belly - and even people with normal weight seem to be getting wider waistlines.

A new study led by Helen Walls in the research journal Obesity looked at trends of both BMI and waist circumference, comparing the data of two US national surveys, one held in 1988-1994, and the other in 2005-2006. Overall, the study looked at the measurements of almost 20,000 nationally representative adults, and found that bellies expanded disproportionately during the decade and a half studied.

Short intro to BMI and waist circumference:

BMI (body mass index) is a screening measure for overweight, useful for studying populations, and calculated by dividing the weight in kilograms by the square of height in meters. It doesn't differentiate between fat and muscle, but BMI correlates quite well with body fat in most people. A BMI of above 25 categorizes people as overweight, and above 30 as obese.

Waist circumference simply measures the waist at the top of the hip bone. A waist circumference of more than 40 inches for a man and more than 35 inches for a woman is considered as "increased risk" for chronic diseases affected by obesity (lower cut-off numbers are suggested for certain ethnic groups, such as Asians).

Here are the study's main findings:

• Between 1988-1994 and 2005-2006 the mean BMI increased by 1.8 kg/m2 to 28.7 kg/m2 (BMIs above 25 kg/m2 are categorized as overweight)

• Between 1988-1994 and 2005-2006 the mean waist circumference increased by 4.7 cm or 1.9 inches

• During the decade and a half between studies waists widened over and above what would have been expected by BMI

• Waist circumference increased in all weight categories of people younger than 50 years: The waists of people with normal BMI's went up too, and there were substantially more normal weight men and women with "increased risk" waist circumference in the later study.

• Waist circumference went up in all ethnicities, education levels, and for both men and women

BMI does not tell the whole story

The researchers warn that by using BMI as a single tool to monitor the population's obesity we may be underestimating the risk burden of obesity related disease.

Indeed, several studies have shown that belly fat is much more risky than general obesity and that fat distribution patters do matter . Belly fat or "visceral fat" is the fat deposited between the abdominal organs, is especially active metabolically, and plays a part in insulin resistance and diabetes, lipid abnormalities, hypertension and cardiovascular risk. Think of it as a big gland, secreting metabolically active signals.

And perhaps, fitting into your old tight jeans is a better measure of healthy weight than the number on the scale.

Why are bellies getting disproportionately bigger? Some ethnic groups - such as Asians, Indians and Hispanics - have a higher tendency to accumulate abdominal fat, but this study found the trend of widening waists held even after stratifying by ethnicity. Other factors suggested to affect belly fat buildup include lack of exercise, over-intake of sugars, sleep deprivation, aging, imbalanced hormones, being male, stress and certain drugs.

What's clear is that we don't get to choose where our fat is deposited. Unfortunately, no amount of sit-ups will make a fat belly go away (the overlying muscles will get stronger, but the fat inside won't dissolve preferentially in the exercised areas).

So what to do about abdominal fat? Eating a healthy diet and adequate aerobic exercise can prevent the buildup of belly fat; losing weight (easier said than done) will shrivel it. The good news is that visceral fat is lost preferentially with even modest weight loss - belly fat is the first to go when you lose weight!

Dr. Ayala

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