One of the most enduring physical measurements of health in the last few decades is the body mass index (BMI). Its popularity is global and its presence ubiquitous; it is found everywhere from schools to hospitals to the military.

The appeal of the BMI is in its simplicity: one only needs the weight and height of an individual to do a quick calculation. No fancy equipment is needed; just a weighing scale, a measuring tape, and a calculator. With the advent of smartphones in the last 2 decades, BMI calculator apps have proliferated across various app stores. Another reason for the popularity of the BMI as a health measurement is that it allows patients to self-monitor at home.

The 4 BMI classifications of underweight, normal weight, overweight, and obese are used by the World Health Organization for White, Hispanic, and Black individuals. Most people understand that a BMI of 25 or more is risky and that they should undertake further health measures (such as dieting and exercise) in order to bring down their weight. There are significant health risks to being overweight/obese, and public health campaigns in recent years have effectively driven home the point, both in the US and abroad. 

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The Connection Between BMI and Mortality 

Bhaskaran and colleagues published a study in The Lancet Diabetes & Endocrinology on the association between BMI and overall and all-cause mortality. They used UK primary data obtained from the Clinical Practice Research Datalink (CPRD) linked to national mortality registration data, and were able to track the BMI of individuals aged 16 years or older. This study included 3,632,674 individuals from the UK population.

Bhaskaran et al wrote, “Follow-up began at whichever was the latest of: start of CPRD research-standard follow up, the 5-year anniversary of the first BMI record, or on Jan 1, 1998 (start date for death registration data); follow-up ended at death or on March 8, 2016.” 

The results demonstrated that BMI was associated with all-cause deaths, with the exception of transportation-related accidents. The researchers discovered that most causes of death, such as cardiovascular disease, respiratory disease, and cancer, had a J-shaped association with BMI. However, a lower BMI was associated with an increased risk of mortality for behavioral, mental, neurological, and external causes. 

The statistically significant association between BMI and mortality highlights why BMI remains an important health measurement, both in healthy individuals and individuals with chronic illnesses. 

BMI Considerations in Patients With SMA 

We now turn our attention to a study conducted by Ferrantini and colleagues on the role of BMI in patients with type 2 spinal muscular atrophy (SMA). SMA is caused by mutations in the survival motor neuron 1 (SMN1) gene and is inherited in an autosomal recessive manner. The most common complications of this disease, especially in its later stages, are impaired motor and respiratory function, as well as feeding and nutritional difficulties. 

While a higher BMI is typically more cause for concern than a lower BMI, in patients with SMA type 2, the main worry is patients having very low BMI. This is primarily because patients at severe stages of the disease require tube feeding, which always carries the risk of malnutrition if not done correctly. 

Read more about SMA etiology

Ferrantini et al collected data on patients with SMA type 2 aged less than 20 years from 5 clinical referral centers in Italy. All of the centers carried out anthropometric measurements of their patients, such as weight, height, and length. Infants were weighed on a standard infant scale, and supine length was measured using an infantometer. 

The research team wrote, “Nutritional status of the patients was determined by the calculus of BMI/age z-scores. A child whose BMI/age z-score was < -2[standard deviation (SD)] was considered as underweight, conversely, who had a BMI/age z-score >  +2SD was classified as overweight.” 

The results from 102 patients demonstrated the following: 

  • Baseline BMI/age z-score and gender were significant contributors to eventual changes in BMI/age z-score. 
  • Patients with a low BMI/age z-score were at a greater risk of developing further reduction in those parameters. 
  • The BMI/age z-score for patients aged <5 was significantly different from the BMI/age z-score for patients aged >13. 
  • SMA type 2 patients who lost the ability to sit had a lower percentage of BMI/age z-score ±2SD compared to patients who were able to sit. 
  • SMA type 2 patients on noninvasive ventilation (NIV) had a lower percentage of BMI/age z-score ±2SD compared to patients who were on spontaneous breathing. 
  • Low BMI/age z-score is a common feature in patients with SMA type 2. 

Ferrantini and colleagues concluded their study with a recommendation: “Further studies, including assessments of chewing and swallowing and other methods aimed at assessing lean and fat body mass, will help to better stratify the type 2 cohort and achieve a better understanding of the possible mechanisms underlying nutritional issues.” 

Read more about SMA patient education 

This study, along with many others, highlights why BMI remains so widely used across the medical world: put simply, it is able to reveal important clinical data. Despite some labeling it as oversimplistic, the BMI, both in the abnormally low and the abnormally high range, can invite closer medical evaluation and lead to better patient outcomes. 


Ferrantini G, Coratti G, Onesimo R, et al. Body mass index in type 2 spinal muscular atrophy: a longitudinal studyEur J Pediatr. 2022;10.1007/s00431-021-04325-3. doi:10.1007/s00431-021-04325-3

Bhaskaran K, Dos-Santos-Silva I, Leon DA, Douglas IJ, Smeeth L. Association of BMI with overall and cause-specific mortality: a population-based cohort study of 3.6 million adults in the UKLancet Diabetes Endocrinol. 2018;6(12):944-953. doi:10.1016/S2213-8587(18)30288-2

Weir CB, Jan A. BMI classification percentile and cut off points. In: StatPearls. Treasure Island (FL): StatPearls Publishing; June 29, 2021. Accessed February 27, 2022.