Case study – Limitations of body mass index to evaluate health status

The body mass index, or BMI, is commonly used to detect overweight or obesity quickly, as it is obtained by dividing weight by the squared height, which requires a simple measuring rod and scale. However, although it is relevant to characterise obesity (except in the case of very muscular athletes), its effectiveness to detect overweight caused by an excess body fat remains debatable in healthy individuals (1).

Indeed, its main limitation is that it is unable to differentiate between muscle mass and fat mass, which can lead to errors in the assessment of subject status. For this reason, it is also relevant to assess body composition in addition to BMI.

In this case study, we will present the cases of two people where BMI is limited in assessing their condition and can therefore lead to errors in management.

Case n°1: Normal body mass index

Gender	Woman
Age	32 years
Height	163 cm
Weight	55,20 kg
BMI	20,78 kg/m2

Dans ce premier cas, nous avons une jeune femme qui possède un IMC normal, laissant penser qu’elle est en bonne santé et présente une composition corporelle normale. Il est donc intéressant de vérifier cette hypothèse en analysant sa composition corporelle.

Quick Analysis

Theoretical gap
Fat mass	Skeletal muscle mass	Total body water
+2.28 kg	-2.20 kg	-0.80 L
Phase angle	4.6°
Impedance ratio	0.837
Bone mass index	0.86

By looking at the quick analysis, we can observe quite rapidly that her body composition does not correspond to someone in good health. Indeed, her body fat is 2.98 kg higher than the health reference, i.e. 24%, suggesting that she is overweight, to which must be added a 2.20 kg lower muscle mass than the health reference, i.e. 11%. A deficit in muscle mass may be caused by dehydration, as 75% of the skeletal muscle mass is water and it accounts for around 40% of body mass. However, she does not appear to be particularly dehydrated, as the water deficit is 800 mL, so she does have a significant muscle protein deficit.

In addition, her bone mass index is 0.86, which means that she has 14% less bone mass than the health reference, suggesting a significant deficit in skeletal mass.

It is interesting to note that, graphically, the BMI is in the ‘very good’ zone, while all three parameters are in the light green and orange zones, suggesting an altered body composition.

Considering this woman’s age and BMI, we can see that her phase angle is just above the lower limit of 4.55°, suggesting that she is not in good health. This observation is confirmed by the value of the impedance ratio, which is above the threshold value of 0.82, indicating that this patient has a high level of inflammation.

Skeletal muscle mass and body fat mass

Fat mass at constant hydration	Skeletal muscle mass
Calculated value 27.43 %	18.50 kg
Estimated reference 22.43 %	20.70 kg
Theoretical gap +2.98 kg	-2.20 kg

If we look more specifically at muscle mass, the theoretical gap with the estimated reference puts this woman’s value close to the orange zone, i.e. in the zone where it could become harmful to health over time. However, it is possible to hypothesise, given her weight, that her muscle mass may not necessarily lead to functional limitations in everyday life. In fact, insufficient muscle mass is responsible for a lower capacity to produce strength and consequently an increase in the physical load involved in everyday activities (climbing stairs, carrying groceries, etc). Ultimately, this increases physical perceived fatigue, leading to a reduction in spontaneous physical activity².

This hypothesis is quickly refuted by the fat-free mass over weight ratio of 46.44%, which is far from the normal value in women of 48%, meaning that her muscle mass is insufficient for her weight, and that this may be a limiting factor in the ability to carry out activities of daily living.

For body fat, the extra 2.98 kg corresponds to a body fat percentage of 27.83%, which means that she is overweight, when combined with the high level of inflammation, could induce significant risks to her health particularly the metabolic health. Indeed, the combination of these two events is the cause of numerous clinical complications, including insulin resistance, which is the cause of type 2 diabetes, and cardiovascular complications such as hypertension and stroke³.

Bone mineral content and dry fat-free mass

As indicated by the value of the bone mass index, this woman has a bone mineral content deficit of 310 g, placing her at the limit of the orange zone, which means that she has an 80% chance of suffering from osteopenia, i.e. a bone mass deficit, or even osteoporosis, i.e. a bone density deficit. To confirm this hypothesis, she should perform a DEXA test, which is the reference method for this compartment. From a health point of view, this means that she would be at greater risk of fracture or traumatic injury in the event of a fall or accident.

It is also possible to observe a deficit of 820 g of dry fat-free mass, corresponding to all the proteins and minerals in the body, confirming the reduction in bone and muscle mass observed previously.

Total body water and water balance

	Bone mineral content	Dry fat-free mass
Calculated value	1.84 kg	11.23 kg
Estimated reference	2.15 kg	12.04 kg
Theoretical gap	-0.31 kg	-0.82 kg

Finally, this woman is correctly hydrated with a slight water deficit of 590 mL, giving a fat-free mass hydration rate of 71.8% close to the optimal value of 73%. In addition, she maintains a normal water distribution between the intra- and extracellular compartments, which means that despite all the imbalances observed above, this person maintains good hydration on a daily basis.

Conclusion

In conclusion, we can see that despite her normal BMI, this woman has a high deficit in muscle mass, similar to sarcopenia, and in bone mass, which are associated with an excess of fat mass corresponding to overweight. In addition, the phase angle value and the impedance ratio show a deteriorated state of health despite her age and BMI being within the norm. If the analysis had stopped at BMI, all these imbalances would not have been detected, leading to inappropriate treatment.

From a health point of view, she has all the characteristics of someone at risk of serious complications such as type 2 diabetes, cardiovascular complications (heart attack, stroke, etc.) or osteoporosis. Therefore, it is necessary to correct these imbalances through appropriate management (physical activity, nutrition and/or medication) to avoid these future complications.

Case n°2: Body mass index corresponding to overweight

	Bone mineral content	Dry fat-free mass
Calculated value	1.84 kg	11.23 kg
Estimated reference	29.20 L	42.00%/58.00%
Theoretical gap	-0.59 L	0.33%

Gender	Man
Age	30 years
Height	176 cm
Weight	82 kg
BMI	26,47 kg/m2

In this second example, we have a 30 years-old young man weighing 82 kg for 1.76 m, resulting in a BMI of 26.47 kg/m², which makes him overweight and, consequently, he could be advised during a medical consultation to lose weight and particularly body fat. However, as before, it is necessary to monitor his body composition before jumping to conclusion.

Quick Analysis

Theoretical gap
Fat mass	Skeletal muscle mass	Total body water
-0.34 kg	+4.52 kg	+0.21 L
Phase angle	7.0°
Impedance ratio	0.756
Bone mass index	1.17

Firstly, the quick analysis showed that this man’s excess weight was not related to fat mass but to a higher muscle mass. Indeed, he has 300 grams less body fat and 4.52 kg more muscle mass than the health references. In addition, his hydration status was good, with a slight total water surplus of 200ml. Finally, the bone mass index showed a bone mineral content that was 17% higher than the health reference.

Taken together, these data suggest a good body composition and health status, and, unlike the previous case, we can see that these compartments are in the green or blue zones, while the BMI is the only one to be in the orange zone. Taken together, these data also suggest that this man is athletic

.

Additionally, we can see that his phase angle is within normal values, which is also true for the impedance ratio, highlighting that his state of health is good, which is in line with his body composition.

Skeletal muscle mass and body fat mass

Regarding muscle mass more specifically, we can see that it is elevated and represents a normal percentage of fat-free mass (50.83% of the fat-free mass) and, therefore, that this man will not be limited in his activities of daily living by his muscle mass. As for his body fat, his body fat percentage is equal to 16.82%, slightly below the health reference, which is a normal value for a man of his age.

Bone mineral content and dry fat-free mass

	Fat mass at constant hydration	Skeletal muscle mass
Calculated value	16.82 %	34.69 kg
Estimated reference	17.19 %	30.17 kg
Theoretical gap	-0.34 kg	+4.52 kg

As highlighted by the quick analysis, his bone mineral content is elevated, with a value of 430 grams above the health reference, suggesting that he possesses a good bone mass. Therefore, he seems less likely to suffer from fractures or bone damage caused by a fall or traumatic shock.

Similarly, his dry fat-free mass is 750 grams higher than the reference, confirming that his muscle mass and bone mass are higher than the health references.

Total body water and water balance

	Bone mineral content	Dry fat-free mass
Calculated value	2.96 kg	18.19 kg
Estimated reference	2.53 kg	17.44 kg
Theoretical gap	+0.43 kg	+0.75 kg

From a hydration point of view, we can see that he was perfectly hydrated at the time of the measurement, with a difference of 30 mL in total water volume associated with a maintained water balance.

Conclusion

In conclusion, these data show that this man has a good body composition, even though the BMI considers him as overweight, which can be explained by his elevated muscle mass. As a result, focusing on BMI would lead to an erroneous diagnosis and potentially to poor health advice and/or management. In fact, people with a BMI over 25 kg/m² are usually advised to lose weight and body fat, which is not appropriate in this case. In this case, if he tries to lose weight, he will lose muscle mass over fat mass, which would reduce his physical performance.

References

1. Dybala MP, Brady MJ, Hara M. Disparity in Adiposity among Adults with Normal Body Mass Index and Waist-to-Height Ratio. iScience. 31 oct 2019;21:612.
2. Twomey R, Aboodarda SJ, Kruger R, Culos-Reed SN, Temesi J, Millet GY. Neuromuscular fatigue during exercise: Methodological considerations, etiology and potential role in chronic fatigue. Neurophysiol Clin. avr 2017;47(2):95‑110.
3. Heymsfield SB, Wadden TA. Mechanisms, Pathophysiology, and Management of Obesity. N Engl J Med. 19 janv 2017;376(3):254‑66.

	Total body water	Water balance (ECW/ICW)
Calculated value	50.06 L	39.74%/60.26%
Estimated reference	50.03 L	40.00%/60.00%
Theoretical gap	0.03 L	0.26%