Does My Body Weight Affect My Bones?

For most of your life you have probably heard a strong message. Thinner is better, both for looks and for health. This may have come from your friends, your family, the media, and even your doctor.

Now you are thinking about keeping your bones strong. Is thinner still healthier when it comes to your bones?

You might be surprised at the answer. Let’s find out how body weight affects your bones, and your overall health.

The BMI As A Weight Measurement Tool

You’ve probably heard of the BMI, or Body Mass Index. BMI is measured in kg/m². It is often used to categorize people as underweight, normal weight, overweight or obese. 

History of the BMI

The BMI, which is used so often in healthcare, was not developed by anyone in the healthcare field. It is the work of a Belgian statistician named Quetelet. Back around 1832, he was working on a concept of “social averages” and identifying characteristics of the “average man.” His “Quetelet Index” was an attempt to quantify the average size for people at that time.

One thing to note is that his calculations only included a narrow subset of Western European men living at that time. This is important, because the measurement might not apply to a diverse population of people nearly 200 years later.

About a hundred years later, Ancel Keys, a physiologist and epidemiologist, used the BMI as a part of his famous Seven Countries Study. He renamed this measurement the “Body Mass Index.” Fast forward to today. Most of the health care industry is using BMI to quantify the health status of individuals (1).

How BMI is Used Today

The BMI is not the best measurement to determine how healthy you are. It doesn’t take into account how much body fat you have compared to muscle. It cannot differentiate between subcutaneous fat (under your skin) or visceral fat (around your organs). 

However, this is the measurement used in most doctor offices. It is also used by researchers, including the research we will look at in this post. A BMI between 18.5 and 24.9 kg/m² is considered to be normal weight. When your BMI creeps to 25 and above you may learn that you are overweight. A BMI of 30 and above falls into the obese range.

Curious about your BMI? You can calculate it here.  

Later on in this article I’ll talk about other ways to measure your body size.

What’s the best BMI for overall health as you get older?

Research is showing that for older adults, being in the overweight range can lower your risk for several chronic health conditions.

CVD, Cancer and Respiratory Conditions

A study published in the Journal of the American Geriatric Society assessed over 9000 individuals for risk of dying from cardiovascular disease, cancer or chronic respiratory conditions, based on their BMI. People in the overweight range, which is a BMI between 25 and 30 kg/m², actually had the lowest risk of death for those conditions (2).

To narrow it down even further, the best BMI range was between 26 and 27 kg/m². This is just above the threshold of being considered overweight. For study participants in the normal weight range, a lower BMI carried a higher risk of dying from any of those conditions.

Risk Related to Muscle Mass and Grip Strength

A study in the American Journal of Clinical Nutrition examined changes in muscle mass and grip strength as we age. As grip strength declined the risk of death increased for everyone. However, there was a significantly greater risk for those who lost weight at the same time as their grip strength declined (3).  

Grip strength is often used as a proxy to determine a person’s muscular health. It makes sense that as you lose muscle your health will decline. However, this study illustrates that it is more pronounced when you lose muscle along with, or because of, weight loss.

Take Away

The research suggests that being slightly overweight can be protective against chronic diseases like heart disease, respiratory conditions and cancer. It’s important to maintain your muscle as you get older, especially if you are losing weight. 

What’s the best BMI for bone health?

The evidence is pretty clear. Underweight people have a higher risk of hip and major bone fractures due to osteoporosis.

BMI and Fracture Risk

A large meta-analysis compared the effects of BMI on fracture risk. The study included over a million participants from 32 different countries (4).  

They found that even after accounting for bone density, people with low BMI (underweight people) still had a higher fracture risk. So, low BMI appears to increase fracture risk through mechanisms beyond just having lower bone density.

This could be due to less cushioning during falls, greater overall frailty because of poor nutrition, and lower muscle mass.

Higher BMI (in the obese range) appeared to be protective against fractures when you looked at the raw data. However, when they compared people in the underweight, normal weight, overweight and obese range with the same bone density, a higher BMI was actually associated with more fractures.

So, a body weight in the obese range is not protective against fractures if you have low bone density. This might be because of poor balance leading to more falls, and more force exerted when there is a fall.

During the follow-up period in the included studies, being in the underweight or obese range were both associated with a greater risk of death. Being in the overweight range was protective for all participants.

Incidentally, the increased risk for higher or lower fracture rate was based on a starting point of a BMI of 25 kg/m². This is at the top of the normal weight range and just borders on the overweight range.

Best BMI for Minimizing Osteoporosis

A Korean study published in 2019 looked at NHANES data from that country between the years of 2008 and 2011. The first study in 2019 found that for Korean men aged  > 50 years and postmenopausal women, a BMI of 23.0 to 24.9 kg/m2 was the optimal range for minimizing osteoporosis (5).  

Again, this is at the upper end of the normal weight range and just borders on being overweight.

Take Away

The sweet spot seems to be at the upper half of normal range or in the overweight range, but not obese.

Other Ways To Assess Your Size

There are more factors to consider with body size than just your weight and height. Body composition, or what actually makes up that weight, is important when it comes to supporting your bones. One reason the BMI is not a great measurement is because it doesn’t distinguish between fat stores and muscle. 

Let’s look at some other ways to assess whether you are a healthy size.

Waist circumference

People who carry a greater proportion of their body weight around in their abdomen are at an increased risk for numerous health conditions. These can include insulin resistance, diabetes, hypertension, hyperlipidemia and stroke. Waist circumference is a good indicator of abdominal fat, and an easy and low-cost way to assess this risk.

It is easiest to get an accurate waist circumference measurement if you work with another person.

Here is how to measure your waist circumference:

1. Measure on bare skin or light clothing that doesn’t constrict the tape measure.
2. Stand erect, arms at sides, feet together and abdominal muscles relaxed.
3. Place the tape measure in a straight line around the torso at the point just above the iliac crest. This is the solid part of the hip bone. It should be at the smallest part of your waist.

High Risk Waist Circumference:

Female     >35 inches

Male         >40 inches

Waist circumference is not a useful or predictive measurement for people whose BMI is >35.

Body composition scale or DEXA

Bioelectric impedance scales measure the amount of water in your tissues. They use this to calculate your lean and fat body mass. These scales have become more and more sophisticated. They can give you metrics like fat mass and percentage, lean mass and percentage, subcutaneous and visceral fat mass, and even skeletal mass and bone mineral content. NOTE: This does not measure bone mineral density in the same way as a DEXA scan. These scales cannot tell you whether you have osteoporosis!)

These scales may not be 100% accurate, but they are good for measuring trends. They are a great tool for measuring your progress with an exercise or weight loss program. You can see if you are gaining, or at least not losing, muscle mass.

Many gyms and doctor’s offices offer access to higher end body scans. If you are trying to build muscle, or if you are on a GLP-1 and losing weight quickly, I recommend seeking out a way to measure your body composition.

Final Thoughts

Underweight is the riskiest weight range for fractures.

The higher end of normal weight range seems to be more protective than the lower end. So getting closer to the overweight range, and even being slightly overweight, may reduce your risk of fracture.

As you go higher up into the obese BMI range, your risk of fracture increases if you have low bone density. So, being in the obese range may be protective for a person with adequate bone density, but is more risky if your bone density is low.

Building muscle is a good strategy both for overall health and longevity, and to protect your bones. You can get a good idea of your risk and progress by tracking your waist circumference, or taking advantage of body composition scan technology.

Have you measured your body composition? Were you surprised at what you found? Let us know in the comments.