Obesity is a multifactorial condition that causes chronic deleterious implications on health and affects one’s quality of life. It is recognized as an important cardiovascular risk factor which contributes to the reduction of cardiopulmonary functional capacity and exercise tolerance not only because of decreased skeletal muscle strength and elevated metabolic cost, but also because of progressive gait inefficiency.^1,2

In Brazil, an increase in the prevalence of overweight and obese individuals has been observed in the last decade.^1,3 In a recent report published by the Ministry of Health, VIGITEL Brasil 2016,⁴ it was stated that more than half of the population is overweight and that 18.9% of Brazilians are obese. These statistics show an increase of 60% in obesity taxes over the last 10 years, which contributes to a growth in hypertension (14.2%) and diabetes (61.8%) in the same period.

The six-minute walk test (6MWT) is a useful tool in the assessment of functional capacity and of the response to the demands required in daily life activities. In the last two decades, especially after the American Thoracic Society (ATS) published a guideline for the 6MWT, reference equations for the distance predicted in the test have been indicated for different populations, both for healthy individuals and for patients with several chronic diseases.^5–8

In 2009, Iwama et al.,⁹ proposed a reference equation for healthy Brazilian subjects. In 2011, Soares¹⁰ also presented a reference equation for healthy adults in Brazil, taking into account the body mass index (BMI) in their composition. In 2013, based on a multicentric study, Britto et al.¹¹ published reference equations for the Brazilian population, which illustrates the growing interest in the 6MWT in our context.

The use of the 6MWT for the evaluation of obese patients has recently been observed, as shown in different already published studies.^12–15 Thus, this study aims to establish a reference equation for the distance predicted in 6MWT in obese Brazilian patients.

The sample size was defined by convenience, patients were recruited according to their admission to the Multidisciplinary Program for the Treatment of Obesity and Bariatric Surgery (PROCIBA) at the Clementino Fraga Filho University Hospital of the Federal University of Rio de Janeiro (HUCFF-UFRJ - Brazil) between August 2007 and August 2017. All patients had 6 or more years of schooling and signed a Free and Informed Consent Form. This study was approved by the UFRJ Research Ethics Committee (CEP-HUCFF-CAAE, logged under project number 1041.0.197.000-05).

Patients with unstable angina, myocardial infarction, cardiac arrhythmias, heart rate (HR) greater than 120 beats per minute at rest, blood presure (BP) ≥ 160/100 mmhg at rest, peripheral oxygen saturation lower than 95% at rest, neurological and /or orthopedic conditions capable of restricting gait were excluded.

Using a standardized technique,⁽¹⁶⁾ the patients’ weight (w) was obtained with a Filizola calibrated balance (0.1 kg of precision) when they were barefoot and wearing light clothes. The patients’ height (h) was measured with a stadiometer (0.5 cm accuracy). Afterwards, their body mass index (BMI) was calculated using the following formula: BMI = w (kg) / h (m²).

The 6MWT was applied to all participants in accordance with the “ATS Statement: Guidelines for the Six-minute Walk Test”.¹⁶ Patients were advised to walk as fast as possible in a quiet and unobstructed hallway, 30 meters long, with each meter marked. Encouragement instructions were standardized and given every minute. The walk could be interrupted at any moment by the patient or the examiner in the presence of chest pain, intolerable dyspnea, dizziness, pallor, sweating, cramps, imbalance, and physical exhaustion. Before and after the test, the patients’ HR and peripheral oxygen saturation were recorded with a pulse oximeter (Onix II, Nonin Medical Inc., Plymouth, Minnesota). Their BP was measured with a cuff diameter corresponding to the group of patients under study,¹⁷ and their subjective perception of effort was graded according to the Borg scale.¹⁸ The distance reached at 6 minutes of walking (D6MW) was recorded and compared to predicted values based on three reference equations employed in other populations.

The general characteristics of the 460 obese patients, 306 women (67%), are presented in Table 1. A significant difference was observed between men and women for age and D6MC, with p<0.05, as well as for weight, height, and BMI, with p <0.001. Taking the entire group into consideration, the D6MW had a significant correlation with sex (r = 0.102, p < 0.029), age (r = -0.387, p < 0.001), weight (r = -0.312, p < 0.001), height (r = 0.163, p < 0.001), and BMI (r = -0.464, p <0.001). Therefore, the D6MW is explained 1% due to sex, 14.9% to age, 9.7% to weight, 2.6% to height, and 21.5% to BMI.

For multiple linear regression, the best model adjusted for the D6MW predictor was that using the variables of gender, age, and BMI, which together explain 48.7% of the total D6MW variance (Table 2). The equation proposed as reference for Brazilian subjects with obesity is as follows: D6MW = 930.138 + (27.130 x Genderfemales = 0; males = 1) − (5.550 x BMI kg / m²) − (4.442 x Age years).

When we compared the mean of the D6MW for the whole group, obtained with the above equation, to the mean calculated by the Enright ⁵ and Iwama ⁹ equations for healthy individuals, and to the mean calculated by the Capodaglio equation¹³ for obese individuals, it was observed that these equations tend to overestimate the values for D6MW (Figure 3).

Functional capacity to exercise is an important marker of mortality and morbidity. In obese patients, walking requires greater metabolic and cardiopulmonary demand. Moreover, the additional musculoskeletal dysfunction due to joint overload and osteoarthrosis (knees, ankles and hip), associated with eventual loss of muscle mass, can be factors that contribute to a lower performance in walking.^2,6,21 As 6MWT is easy to apply, has a low cost, is safe, and is easily correlated with the activities of daily life, mostly at the submaximal level, it represents a relevant element in the evaluation of obese patients’ functional capacity. Consequently, D6MW becomes a significant prognostic indicator.^6,22

Previous studies have already described age, sex, weight, height, and BMI as variables capable of influencing D6MW.^{5,9,12,13,23,24} For obese individuals, especially those with BMI ≥ 40 kg / m2, as already mentioned, musculoskeletal dysfunctions and loss of muscle mass are factors that have a considerable influence on D6MW, particularly with advancing age^2,25 (Figure 1).

The importance of BMI in the D6MW variance in a population of obese patients has been described in medical literature,^12,15,21,26 despite the significant difference in BMI between the genders (p <0.001) found in our patient group, where the BMI was able to explain 32.6% for men and 20% for women of the D6MW results. It was observed that, for a growing value of BMI, there is a reduction of D6MC in both sexes (Figure 2).

In our study, for the whole group, age was able to explain 14.9% and BMI 21.5% of the results. Data slightly different from those previously obtained by Capodaglio with obese patients,¹³ where age explained 12.9% and BMI 24% of the D6MW results. This difference occurs because our population is older and with a smaller number of male subjects, who are proportionally heavier. However, our results were similar to Capodaglio’s in relation to the total variance of the D6MW, explained by the three chosen variables (gender, age, and BMI), that is, 48% in Capodaglio`s study and 48.7% in our investigation.

A greater number of variables introduced in the equation, as already pointed out by other authors,^(14,20) could provide a better evaluation from a purely mathematical point of view. On the other hand, this study would lose in practicality, since a greater number of variables would lead to a greater complexity in the collection of data in an outpatient environment of basic health care.

Our study, does, however have some limitations. The sample was chosen for convenience, allowing for some selection bias. This study consists of patients with a predominance of grade III obesity, most of whom were female, who sought out bariatric surgery to treat their obesity. In addition, about 50% of the D6MC variance remains unexplained by our regression model. Caution should be taken when using the proposed equation for patients with different characteristics from our cohort. This study should also be repeated with a larger number of participants under the age of 40 and over 60, and with grade I and II obesity.

The reference equation proposed in this study shows a significant adjustment, when compared to the equations already described in the literature, and allows for a better estimate of the submaximal functional capacity of Brazilian patients with obesity, which provides better subsidies for monitoring the therapeutic interventions performed in this population.