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Research finds Northern latitude and winter linked to adverse outcomes after bariatric surgery

Posted on: October 31, 2016   by  Amber Tovey


A study of nearly one million patients who underwent bariatric surgery found that both seasonality and latitude were associated with health outcomes following surgery.

Bariatric surgery provides a weight loss option for individuals with severe obesity who have been unable to lose weight using other methods. The surgery causes weight loss by limiting the amount of food the stomach can hold.

Side effects of bariatric surgery may include infection, diarrhea, nutritional deficiencies, gallstones, hernias and dehiscence. Dehiscence refers to a ruptured wound along the surgical incision. These side effects may occur right after surgery, causing a prolonged length of stay (LOS) in the hospital, or the side effects may occur later.

As the obesity epidemic continues to rise, bariatric surgery will likely become more common. Thus, it is critical to optimize the results of these surgeries by managing modifiable risk factors.

In a past study, researchers discovered that 80% of patients who underwent gastric bypass were considered vitamin D deficient a year after surgery, despite all patients receiving vitamin D supplementation after surgery.

Vitamin D strengthens the immune system by increasing the production of antimicrobial proteins, which aid in fighting off infections and disease. Furthermore, research has found low vitamin D status is associated with increased LOS and increased incidence of hospital mortality. Because of these findings, researchers recently became interested in the relationship between vitamin D status and adverse outcomes following bariatric surgery.

In a recent study, researchers assessed proxies of vitamin D status and outcomes after bariatric surgery of 932,091 patients. The researchers used both seasonality and latitude to estimate vitamin D status. Those who live near the equator have more opportunity to synthesize vitamin D from the sun, and therefore, patients who attended hospitals located at or above 37° North were considered to have lower vitamin D levels on average compared to those located below 37° North.

In a separate analysis, the researchers compared seasonality of the surgery to adverse health outcomes. During the summer (July to September), vitamin D status is highest; whereas, during the winter (January to March) vitamin D status is the lowest. Vitamin D status is considered moderate during both fall and spring.

Here is what the researchers found:

  • All adverse outcomes, including wound infection, non-healing wounds, prolonged LOS, dehiscence and delayed healing were all significantly different by season.
  • All adverse outcomes except non-healing wound decreased in frequency with increasing vitamin D availability during the season (p < 0.05), meaning the highest frequency of adverse outcomes occurred during winter and the lowest frequency occurred during summer.
  • Prolonged LOS was significantly more common in the North compared to the South (p < 0.001).
  • All significant relationships remained after adjusting for factors that may influence bariatric surgery success and vitamin D status, including comorbidities, age, sex, race and socioeconomic status.
  • Patients in the North experienced more complications than those in the South, but this did not reach statistical significance (p = 0.108).
  • The cost per patient was $680.63 higher during the winter compared to the summer.
  • The cost per patient was $8,961.84 higher in the North compared to the South.

The researchers concluded,

“Our results support the potential role of VitD in preventing complications following bariatric surgery, especially as it pertains to wound healing and infection as well as prolonged LOS in the hospital.”

They went on to state,

“VitD supplementation, an easy and inexpensive treatment may…prevent adverse outcomes following bariatric surgery.”

The study presents promising findings for the role of vitamin D in bariatric surgery. The large sample size increased the strength of the findings. Furthermore, the significant relationships between proxies of vitamin D status and adverse outcomes of bariatric surgery remained after adjusting for several confounding factors.

On the other hand, the study did not directly measure vitamin D status. Instead, the researchers looked at latitude and seasonality to represent vitamin D status. The vitamin D status of the patients could be significantly higher than expected based on latitude or season if they supplemented with vitamin D. Lastly, due to the observational design, the study does not prove that Northern latitude or surgery conducted during winter causes adverse outcomes for bariatric surgery patients, but rather, these proxies for low vitamin D status are linked to adverse outcomes.


Tovey, A. & Cannell, JJ. Recent research finds Northern latitude and winter linked to adverse outcomes after bariatric surgery. The Vitamin D Council Blog & Newsletter, 2016.


Peterson, L. et al. Proxy measures of vitamin D status – season and latitude –correlate with adverse outcomes after bariatric surgery in the Nationwide Inpatient Sample, 2001–2010: a retrospective cohort study. Obesity Science & Practice, 2015.

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