Obesity is a global pandemic related to excessive adiposity and mediated by inflammation, with long-term damage that impairs the quality of life of obese people. ^{1 , 2} It may affect all age groups and ethnicities in many countries. ^{3 – 5}

Adopted life-style and genetic factors are related to the genesis of obesity and associated with an increase in cancer, cardiovascular diseases, blood hypertension, type 2 diabetes mellitus (DM) and chronic kidney disease (CKD). ^{6 – 10}

The causes of obesity-related renal impairment are multifactorial, ranging from the deleterious mechanisms of obesity-associated comorbidities in the kidneys, as well as the production of adiponectin by adipose tissue. In addition to weight gain itself with deleterious renal action leading to the onset or worsening of CKD. ^{11 , 12}

Other mechanisms for the onset of kidney injury in obesity involve oxidative stress, activation of the renin-angiotensin-aldosterone system and insulin resistance. ^{13 – 17}

Bariatric surgery has become the main method for controlling morbidities associated with obesity, with adequate glycemic and blood pressure control, in addition to promoting sustained weight loss. ^{18 – 22}

The effect of bariatric surgery on the renal function is a subject of intensive research. In most studies, one can observe, after bariatric surgery, an improvement of glomerular filtration rate (GFR) and in preventing the onset of CKD and its progression. But in other papers, the improvement of kidney function is not so clear. ^{23 – 25}

The aim of this study was to evaluate the impact of bariatric surgery on patients' renal function and associated factors through a bivariate analysis, corroborating the findings of improvement in GFR in most studies.

Predominantly, female (71.4%) and white (77.1%) patients were observed in this study. All had high school education and health insurance. The mean time of follow-up was 16.2 ± 2.6 months. Ages ranged from 24.0 to 57.0 years. Regarding the type of bariatric surgery, most underwent the sleeve type (65.7%).

In the preoperative bariatric surgery, it was observed that 31.4% were diabetic patients and 40.0% were hypertensive patients. In the postoperative surgery, among the patients with hypertension and diabetes, the majority of obese patients obtained control. Of the total of 11 pre-surgery diabetic patients, seven were no longer diabetic, and of the total of 14 pre-surgery hypertensive patients, 12 became non-hypertensive post-surgery ( Table 1).

In the post bariatric surgery there was a significant reduction in the mean abdominal circumference (p < 0.0001) and in the mean BMI in postoperative period of bariatric surgery (p < 0.0001) ( Table 2). No significant difference was observed between the mean levels of cystatin C pre and post bariatric surgery (p = 0.094) ( Table 2). There was a significant improvement in glomerular filtration rates in obese patients undergoing bariatric surgery ( Table 2) ( Figure 1).

On analyzing the association between the outcome of the variables: type of bariatric surgery, diabetes outcome and hypertension outcome, no association was observed with the outcome improvement in GFR, under the effect of bariatric surgery, without statistical significance, respectively (p = 0.312; p = 0.217; p = 0.476) ( Table 3).

There is an indication that bypass surgery tends to lead a greater number of patients to an improvement in the GFR (91.7%), although this result is not statistically significant (p = 0.312) ( Table 3).

One-Way ANOVA results demonstrated that there was no difference between the mean BMI losses associated with the GFR outcome groups (p = 0.904). The type of GFR outcome under the effect of surgery did not affect the result of BMI expressed by the loss between pre and postoperative periods ( Figure 2).

According to the literature, in this cohort study one can observe a prevalence of white, females with an average age of 41 years, having high school education and health insurance, as women tend to take more care of themselves. They had a good socioeconomic level, given the difficulties of access to bariatric surgery in public services. ³²

Bariatric surgery has shown encouraging results in the control of comorbidities in obese individuals, proving to be effective in sustained weight loss, corroborating the findings of the present study. ^{33 , 34}

With regard to comorbidities (hypertension and diabetes), a significant decrease in the prevalence of these diseases can be observed in rated obese individuals after bariatric surgery, similar to most studies. ³⁵

Some studies have pointed out the improvement of renal function after bariatric surgery, but not in other studies. ^{36 – 38} In the present result, an improvement in renal function was observed after bariatric surgery.

In this study there was trend towards an improvement in the GFR in the bypass technique in relation to the sleeve as opposed to the one observed in the literature. ³⁹ The explanation for this would be the evidence of the relationship of the bypass technique with urinary calculus formation, as well as malabsorption syndrome. ³⁹

Kidney disease can be caused by hypertension, diabetes as well as obesity. ^{10 , 40} However, obesity is also associated with the emergence of diabetes, hypertension and kidney disease. In this sense, bariatric surgery promotes a reduction in BMI, glycemic control and blood pressure, and is even associated with improved renal function. ^{6 – 12} Therefore, it is hard to know what came first in this cascade of improvements.

After performing a bivariate analysis between outcome of glomerular filtration rate and factors such as outcome of hypertension, outcome of diabetes and surgical techniques, no association of these factors with improved renal function was found. Furthermore, the type of GFR outcome under the effect of bariatric surgery does not affect the result of BMI which reinforces evidence suggesting that there is a strong and independent association between obesity and development of kidney disease. ²⁴

After bariatric surgery, there was a significant improvement in the GFR, abdominal circumference and BMI, in addition to the control of associated comorbidities in obese patients with diabetes and hypertension. After bivariate analysis, no association was found between the improvement in GFR in the post-operative period of bariatric surgery and the analyzed variables. However, other prospective and more robust studies, as well as those with a larger sample size are needed to answer this challenging question: “Does bariatric surgery have an effect on improving renal function as an independent factor?”

Bezerra JAB: collected the data, contributed in interpretation of data and prepared the manuscript.

Brandt CT: contributed in interpretation of data and made the final revision.

Santos EPR: contributed in interpretation of data.