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Verified by Psychology Today

Your Weight on the Scale May Be Misleading

Body weight is more complicated than a number on a scale.

Key points

  • It would be helpful to have a monitoring tool to measure the percent of fat, muscle, and water in the body and determine one's true weight. 
  • The BMI scale does not differentiate between muscle, fat, or water weight.
  • Methods that assess body composition are not readily available for the non-professional, but new technology could change that.

There seem to be electronic gadgets that allow us to monitor just about every aspect of our body’s functions. From sleep quality to heart rhythms, blood oxygen levels and blood glucose levels, as well as calorie intake and energy output, we no longer have to rely on doctor’s visits to monitor our health status. And we can do the monitoring daily.

But it seems that the health device industry has overlooked one essential monitoring tool, one that will measure the percent of fat, muscle, and water in our body that determines our weight.

Given the national obsession with whether we are overweight or obese, it is curious that the only device most of us have to determine this is our scale (or trying on the clothes we wear when we think we have gained weight). To be sure, scales are reliable measures of changes in our weight and will tell us what we weigh today, but can we always translate the weight on the scale into normal, overweight, or obese status?

One standard for normal or obese status is BMI, or body mass index. Charts exist into which the BMI can be plugged to see whether our weight is in the normal or abnormal range (too low as well as too high). BMI is determined by dividing one’s weight in pounds by your height in inches squared, and then multiplying that number by 703. Charts give the normal, overweight, and obese range for men and women.

But there are some obvious problems with relying solely on the scale, and subsequently the BMI, to assess "fat or thinness." Since BMI relates weight to height, shrinking in height as many do in older age may move one from normal to overweight or obese. A medically approved weight for someone 5'10" may be overweight if the individual is now 5'7". (I have a cartoon that shows a woman on stilts standing on a scale, and because she is now 5 inches taller, she tells herself that her BMI is normal.)

Muscle weight can also confound the accuracy of the BMI. The scale does not differentiate between muscle, fat, or water weight. Two people can weigh the same but have different body compositions; one is heavily muscled and one has too many fat cells. A male friend found himself losing weight when his treatment for prostate cancer depleted his body of testosterone. Much of the weight loss was from muscle loss, a side effect of his treatment.

Methods that assess body composition, however, are not readily available for the non-professional, with the exception of skin calipers, and even they require the expertise of someone trained to use them. Skin is pinched with calipers at several spots on the body and the thickness (which represents subcutaneous fat) is analyzed in a mathematical formula. A do-it-yourself skin caliper measurement is possible, but only after much practice and oversight by someone who knows what to do.

Scales are available with an additional body composition measurement device that uses bioelectrical impedance. One stands on or grips electrodes, and tiny electrical impulses are sent through the body. Lean tissue conducts the impulses faster than fatty tissue, so the impulses travel more quickly when the individual is leaner. Again, there are formulas to translate the figures into the relative amounts of fat and lean tissue.

But here too the measurement may be inaccurate. Hydration affects the reading since water conducts electrical impulses, so drinking water or sweating before the measurement will change the results. Eating before the measurements also affects the measurement, as does exercise.

If you have access to a pool, hydrostatic weighing might be an option. After exhaling as much air as possible, you sit underwater and your weight is recorded while fully submerged. It is then compared to your weight on land. The two weights, with the proper mathematical manipulations, reveal body density, and this in turn provides information about which body components contribute to body density. Its only drawback (other than not enjoying holding your breath and sitting underwater) is that the method measures bone density, and bone density that may be altered by exercise or bone loss will affect the accuracy of the measurement. Moreover, it is unlikely that this method will be used frequently to track the effects of a diet, for example.

Dual energy X-Ray absorptiometry, otherwise known as DEXA, is probably the most accurate method of determining body fat content, but it is not something you can do at home. You must go to a special facility, and lie on a table while the arm of the machine passes over your body emitting high- and low-energy X-ray beams. The beams get absorbed into the different components of the body and are translated into readings for bone, lean body mass, and fat density.

The machine can give surprising results. Years ago, when we were studying the eating habits of thin women going through the early weeks of smoking withdrawal, we, with their permission, looked at their body mass using DEXA. To our surprise, the amount of fat in their body was somewhat similar to that of someone who is overweight or even obese. The scale recorded a low weight because their muscle mass was low, not because they had only small amounts of fat in their body.

DEXA measurements require an appointment at a clinic, and justification to one's health insurer for the measurement. At present, the backlog of appointments due to COVID means waiting for permission and an appointment. Clearly this is not an easily accessible method of determining changes in body composition related to weight loss or gain.

So the question stands: Where is the new technology to allow us to make accurate measurements of our body composition at home? We should be able to find out if our weight-loss program is maintaining our muscle mass, or if we are losing weight because we are losing muscle as well as fat. We should be able to learn if the treatment we are taking for bone loss is halting the loss and increasing bone density. We need to know if an elderly member of our family is unable to get from a chair or walk without help because of muscle loss. We need to know if our older teenager’s weight is going up because he is engaged in a vigorous athletic program, or because he eats fast food after school every day.

Given the rapidity with which medical devices for home health care are being developed, one hopes that an easy-to-use, accurate device will be available in the near future. Being able to track our own changes in body composition accurately and often will have benefits beyond learning how rapidly our fat mass is changing.

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