Am I Overweight? What does My Leptin Say?June 18, 2012
Am I Overweight? What does My Leptin Say?
Reference: PLOS, April 2 Nirav Shah and Eric Braverman
The BMI, or body mass index, is the common method of telling us if we are overweight or normal weight. Unfortunately, it ends up causing a lot of confusion because we haven’t been able to sort out the variables of muscle mass, fitness, body habitus, and all that. Lots of men, who are quite fit and have lots of muscle get registered as being obese. Lots of women, who are not fit but look small, have lots of fat internally and very little muscle. They register normal but are obese.
There is a better test. The DEXA (dual x-ray absorptiometry) scan is a CT scan that measures your content of total body fat precisely. Problem is: expense and radiation. What’s the problem with fat? We have always thought that fat was just a place to store all the extra calories we ate. We assumed it was a quiet little neighborhood of huge, oversized water balloon shaped cells storing those calories.
Turns out that’s wrong. When you have a normal body habitus, in other words, normal healthy fat content, your fat tissue is very interesting under the microscope. The cells are smaller and with some 10% of the fat tissue being composed of other types of cells. Calories flow easily in and out in response to insulin, activity, and blood glucose levels. As we get bigger, our fat cells change in size, behavior and morph into very active endocrine glands. They secrete lots of inflammatory hormones. Those hormones beacon inflammatory cells to come to hang out.
If you biopsy someone who is obese their fat tissue looks quite different than slender folks. As many as 50% of the cells are now inflammatory cells instead of 10%. That means fat tissue is basically a sack of pus. (Oooooo! Yuck!) What did the study find? “BMI characterized 26% of the subjects as obese, while DXA indicated that 64% of them were obese. 39% of the subjects were classified as non-obese by BMI, but were found to be obese by DXA. BMI misclassified 25% men and 48% women.”
Now, the BMI has been around for about 200 years and is a simple mathematical formula. Your BMI is calculated as your weight in pounds divided by your height in inches squared, times 703. Over 25 is overweight and over 30 is obese. By DEXA scan the American Bariatric Society defines obese for women as being over 30% fat and for men 25% fat.
Now, what makes this study significant is that it also measured leptin. Leptin is your feedback hormone from fat tissue that tells you to stop eating. As we get pudgy, we get leptin insensitive. We lose the feedback message to stop ingesting calories. And leptin correlates strongly with percent body fat, with disease and morbidity, whereas just plain BMI does not.
This study suggests that we need to know our percentage fat, but more importantly, our leptin levels. Levels over 30 ng/mL correlated strongly with higher percentage body fat. It’s leptin that gets you in trouble. Leptin makes us put out C-reactive protein, and CRP starts the inflammatory cascade. This is a huge step forward. It’s not the scales so much as the percent of fat in your body that matters. This is why being fit and overweight can be safer than unfit and skinny. And leptin, or your sensitivity to it, may be one of the keys that unlock the connection and the mystery.
WWW. What will work for me? I step on the scales every day. And groan. But I’m also exercising every day. I have a lovely body fat measuring machine at work that is 99% as accurate as a DEXA scan, without the radiation called an “InBody”. My BMI is 26, so I’m a bit over the top. My percent body fat, by my “InBody” is 18.7%. Normal percent is <20% for men. Whew, that’s not so bad. Maybe I’ll step on that every day instead. And then, when I’m brave, measure my leptin levels.