Ideal Body Weight (medically speaking)

Factual Questions
1

I recently used a Height & Weight Percentile Chart (Weight and height percentile - Wikipedia), obtained from a physician, to evaluate my weight compared to the average population. I am unclear as to the origins or accuracy of this chart (I think the source is ‘boston’ something?) but I do know the data is quite old. For anyone unfamiliar with what I mean, it is a chart that lets you determine how your height/weight compares to 100 people your age (i.e. are you skinnier than 75 % of ppl your height/age etc).

My questions, in no particular order are:

  • assuming this data is very standard (presumably it is, b/c doctors use it daily), how useful is the chart for determining healthy weight goals?

  • does this type of chart take into account different ‘body types’, i.e. different inherent muscle & bone structures?

  • would the 50th percentile be the medically ideal weight?

  • is there such thing as a medically ideal weight (presumably its more of a ‘weight range’) for a given age/height? if so, how does one figure it out?

  • what do doctors typically use the chart for?

2

No info on the chart, but I’ve been reading a lot about overweight people living longer than their thin counterparts. Interesting, I think:

http://www.nzherald.co.nz/section/2/story.cfm?c_id=2&objectid=10121660

3

I definitely think there’s nothing wrong with being overwieght and not into the “obese” category. In fact, as you point out it may be good for you to be that way.

4

Here’s my take on BMI. I am easily in the best condition I’ve been in since probably high school (I’m now 49). I am very active with working out, hiking, and rock climbing. Yet when I see my doctor (a true BMI follower) he continually tells me I need to lose weight because based on my BMI, I am not just overweight, but in the “obese” category.

Because of this debate, I’ve done some research on BMI, and here’s what I’ve found:

  • BMI is old. It was actually established around 100 years ago. It’s use as an indication of “healthiness”, though, is relatively recent.
  • BMI is statistic. And is meant to be an indicator of “percentage of body fat”. The different rankings of “overweight” and “obese” correspond to various percentages of body fat.
  • BMI is based solely on height and weight. Some charts incorporate gender. BMI does not take into account actual body composition (how muscular, etc.)

I feel my doctor’s use of BMI is flawed because BMI is intended to be an “indicator” of percentage of body fat, but not all that accurate.
To me, BMI should be used as an indicator that you might want to get your true body fat content measured. If your actual percentage of body fat is high, then you should look to losing the weight. However, even if your BMI is high yet your actual body fat is okay, you are not at risk and don’t really need to adjust your weight.

The example that always comes up is Shaq. Based on BMI, Shaq also falls into the “obese” category. Yet he has probably less than 20% if not 10% body fat. However, my doctor would counter that even though his body fat is low, that much mass is still too much for his heart to handle. My doctor interprets BMI not as a measure of excess body fat, but of “healthy mass” period. And I don’t believe this is realistic.

The problem is that I don’t believe there is enough data on “obese but with low percentage of body fat who died of heart disease” mortality. So you can’t break out that minority from the rest of the “obese” statistics. So my doctor plays the conservative approach and concludes that “obese” (by BMI) = “unhealthy” regardless of body composition.

Here was an interesting exercise. Starting from my current weight of 172, according to BMI, I need to lose 40 lbs. to get to 132. Using a “healthy” body fat percentage of 17%, the of the 132 lbs., 22.4 lbs. would be fat.
If I am to lose 40 lbs., then it is reasonable to conclude that that 40 lbs. would all be fat. Working backward, then the 40 I need to lose, plus the target of 22.4 lbs would mean that currently I “have” 62.4 lbs. of fat. This equates to a body fat percentage of 36.2 % !!! No one, looking at me, would conclude that I have 36% body fat !

In a similar vein, using the Shaquille O’Neil example. If his actual body fat is, say, 15% or so. And he decides to believe in the BMI indication. Where does he lose his weight ? Muscle mass ? He therefore needs to allow himself to atrophy in order to become “healthier” ?

5

Renee: That is very interesting, but a little maddening too!

cormac262 I think what you’re saying makes sense. BMI is definitely helpful in quickly obtaining an indication if there are any gross problems, but lacks precision for the reasons discussed (no accounting for muscle/bone structure, and outdated data, my doc’s chart is from 1980).

So if BMI isn’t the answer, how on earth does one determine the healthiest weight goal?

6

IANAD, but I’ve always been told that body fat percentage (measured on an individual basis by a scientific method, not a BMI calculation) is really what matters. Women ideally should be at about 16-20%, while men should be at 6-13%.

If you’re in the 50th percentile weight-wise, that means that 50% of the population is smaller than you. It means you’re average, but if the numbers are calculated using recent measurements of the population, average is probably a bit heavier than you want to be (assuming you’re not an athlete and it’s not muscle weight).

7

Without knowing which chart your doctor gave you, I’d point out that most of these charts are mainly intended for children / teens and may not be appropriate for adults.

Your questions:

If it’s just the statistics, useless. See below.

Probably not. Older calculations of “healthy” weight-for-height did, but the most recent attitude is that even with such corrections you’re still getting only a loose handle on the actual health-related characteristic (percent body fat), so a simpler metric is just as useful.

No. According to the current metrics the average American is mildly/severely overweight, so the ideal body weight is below the 50% percentile for your height.

The BMI gives you an idea, but what you really want to know is the percent body fat (as NinjaChick points out). Some authorities think that being overmuscular can also be a health-problem, but for most people it’s over-fat that you have to worry about.

The most common use for purely statistical charts is tracking growth in children and predicting future height. Being either unusually short or unusually tall for your age can be a symptom of various diseases, especially if the child’s height is disproportionate to the parents’.

JRB