How Much Should You Weigh?

You’d think that the question “How much should you weigh?” would be an easy one to answer. But scientifically, it’s not that easy.

The first reason for this is that we usually forget that when losing weight, body fat is what we are trying to get rid of. But body weight is not the same as body fat. The body also contains a large amount of non-fat tissue and the proportion of this non-fat tissue to the total varies among people. Therefore measuring weight is not necessarily measuring fat. (Scientists hate ambiguities like this.)

Of course, there has always been one extremely accurate way to directly measure the amount of fat in the body. You simply render (boil down) the body at high heat and weigh the amount of fat that melts off. But, though accurate, there is an obvious problem with this measurement method—it kills you.  Therefore, to avoid such problems, other—less accurate—measurement methods have been developed.

These other methods include underwater weighing, bioelectrical impedance, ultrasound, and so forth. All these methods make various assumptions in order to work and many of the assumptions can introduce measurement errors under certain circumstances (thus keeping lots of Ph.D.s gainfully employed ). But from the practical point of view of the dieter the biggest problem with all of these methods is that they require expensive special equipment that makes them effectively unavailable to most of us—compared to the bathroom scale.

Fortunately, scientists have been able to use these derivative methods to in turn create and/or validate height-weight tables and formulas that give reasonably accurate approximations of body fat in relation to total body weight under “normal” circumstances. We can use these tables and formulas to help you set goals and measure progress. “Ideal Body Weight” is among the most useful of the tables.

Ideal Body Weight

“Ideal Body Weight” (IBW) is a term that stands for the weight at which you can statistically be expected to live the longest and be in the best health while you are alive (other things being equal). Intuitively, this is a pretty satisfying definition of the term “ideal”.  The IBW (Ideal Body Weight) is also used quite extensively in obesity research. It is, for example, the method most used by researchers to determine such things as (e.g.) how much protein you need when you are dieting.

We can use the ideal weight method of determining what you should weigh because it fits so well with the threshold theory. “Ideal” weight is defined as “healthiest weight”. According to threshold theory, the mechanism that makes us get fat is really just trying to push us toward ideal health—so we have a satisfying synchronicity of terms. (See more on… the threshold theory.)

A Short “Reality Check”

I am aware that many if not most overweight people simply cannot believe that the “ideal” healthiest weights will ever be relevant to them. Most will say things like “I’d be happy just to get within 20 pounds of that weight”. I must respectfully disagree.

I think that you can get to these weights. I also think it will be easy to do. By “easy” I mean that the process will have none of the horrendous diet suffering which is what you really fear and are trying to avoid.

But you should not doubt that those last few pounds will take time to get rid of. This is due to the unavoidable phenomenon of “diet slowdown” which can be successfully managed but not eliminated. We discuss this further below.

The idea that good health and weight are related is not seriously questioned by anyone (although specialists do still squabble over many of the details of the relationship.) The three largest and statistically most powerful recent studies of the weight-health relationship all demonstrate two things:

Death rates and poor health increase with increasing weight, and this increase is both large and significant.

The greatest longevity occurs at weights lower than the average for the population.

The three studies referred to above are the 1959 Build and Blood Pressure Study (4.9 million people studied), the 1979 Build Study (4.2 million people studied), and a 1979 American Cancer Society study (750,000 people studied). These studies have great “statistical power” due to the huge numbers of people in them. Other studies using small numbers of people have often been shown to lack the statistical strength required to detect true associations.

You may have heard vague rumors that “the studies” have “all been proven wrong”. As usual, this rumor is itself “a ‘fragment of fact’ in ‘a bushel of blather’ creating ‘a carload of confusion’”. Herewith, some facts.

In 1987, Walter Willett and a highly respected group of researchers analyzed the 25 major prospective studies which had been done up to that time on weight and longevity. They reached the following three conclusions:

Each of the 25 studies had at least one of three major biases.

Each of these biases would cause the study to conclude that your “ideal” weight is higher than it really is.

The least biased (and therefore most reliable) of the 25 studies was probably the 1959 Build and Blood Pressure Study. (This was biggest study and the basis for the well-known 1959 Metropolitan Life Insurance Company Desirable Weight Tables).

For this reason, we use data from the 1959 Metropolitan Life Tables as the best available (though still not perfect) approximation of “Ideal Body Weight”.

Look up your ideal weight in the table below. Remember it, but don’t worry about it. We use it to calculate other things. I promise we won’t worship it or even fall in love with it.

To find your Ideal Body Weight in Table 1below, first find your height in the first or last column, as appropriate. Then find the corresponding weight range for your sex. If you are above that weight range, it would be worthwhile for you to lose weight.

BODYWEIGHTS ASSOCIATED WITH LONGEST LIFE SPAN

Ideal Body Weights -- English Measurements

Ideal Body Weights -- Metric Measurements

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