Discussion
Movement, physical activity and position are known to influence a broad spectrum of mechanisms in the human body. Therefore, we set out to explore the impact movement may have on urine production. There is ample existing body of literature showcasing the effects of movement, physical activity and position, however, there is no existing data on the impact on urine production 9-11.
Our study was performed in a population of healthy patients with age varying from 26 years to 54 years with a mean of 35 years old. Due to multiple variables and the complex interaction between these, we opted for mixed model statistical analysis, A positive association was found between diuresis and physical activity, movement and position for all three timeframes i.e. 24 hours, daytime and night-time. Negative associations were found between osmolality and creatinine and physical activity, movement and position. The association between creatinine and physical activity, movement and position were significant for all three timeframes, however, this was not the case for osmolality where the association was only significant for 2 timeframes being 24-hours and night-time. Furthermore, there were no significant associations between sodium and physical activity, movement and position except a very slight statistically significant value for the model of daytime.
As this is a pilot study, there is a paucity of data exploring the topic of movement and urine production. There are, however, other studies exploring the association of physical activity and glomerular filtration rate (GFR) as well as kidney function. The general consensus states a positive association between physical activity and GFR as well as kidney function 9,12-14. Moreover, Wolin et al. state that physical activity may provide a strategy for the management of benign prostatic hyperplasia related outcomes, particularly nocturia11.
Physical activity is known to increase blood pressure in the acute setting. An increase in blood pressure would theoretically increase the blood flow to the kidneys, without counting for any diversion of blood flow to skeletal muscle or cerebrum in cases of flight-and-fright responses. Nonetheless, an increase in renal blood flow, would increase the flow within the glomerulus. This would result in an increased glomerular filtration rate, which could be a potential hypothesis for increased urine production. Nonetheless, one must account for processes of reabsorption and filtration that occur within the renal tubules, which brings us to our second hypothesis 15,16.
The renin-angiotensin-aldosterone system (RAAS) is a hormone system that regulates fluid balance and blood pressure and works by increasing blood volume and hence blood pressure. With increased physical activity, blood pressure rises likely due to a combination of the baroreceptor reflex, increased cardiac output and increased systemic vascular resistance secondary to the effect of the sympathetic nervous system. As such, there is a possibility that RAAS activity may not be as required by the body’s physiology since the blood pressure is already autoregulated and no increase in blood volume is required to increase blood pressure. Nonetheless, a simple increase in blood pressure causing an increased glomerular filtration rate with unchanged reabsorption dynamics, could very well be the principle cause of an increased diuresis associated with physical activity, movement and position. This would clarify the phenomenon of increased urinary excretion without impacting on urinary sodium, however, it is likely it is a combination of multiple systems that play a role 10,15-17.
The strengths of our study are that it has been performed in a well-designed sample that is representative of the general population. Furthermore, the study design is prospective, and the research topic is completely novel; hence the pilot study. However, some limitations must be noted such as the lack of data on serum. Due to this, we could not calculate the GFR and an estimate was made using BMI and age, however, we felt this would be too vague and not accurate enough. Furthermore, as the initial objective of the questionnaires was to compare the basic and extended urine sampling, targeted data for the movement was not specifically outlined. Urine samples were taken every 3 hours and were objectified; whereas, the data on movement was given a subjective score every hour by the patient. This data was then transformed to make the analysis more transparent. In addition, the sample of our study might be considered too small to note all the differences between certain confounders. As such it is difficult to amplify the results to larger patient groups. It is important to note that calculating the low level-1 variations of the different mixed models may have led to the loss of matching previous results within the same patient. As such all the data and its results could only be made with relation to intraindividual differences.
It remains clear that more research is necessary within this domain. As many studies have suggested physical activity is a very easy, non-invasive way to improve quality of life 9-11,13. Here it is vital to focus on a larger and more varied sample, as this could lead to more exact data. As there are many unanswered questions within this field of research, we must try and find a better understanding of the pathology and increase the methods and tools available to analyse the same. In an ideal situation, the researcher must gain information on the exact timing of movement, the exact intensity of movement, and finally how this impacts the renal system.
In the future, the use of automated monitoring devices for factors like voiding, urine production and residual volume, but also for diet, blood pressure, pulse and sleep will be necessary. In addition to this, fitting applications, such as bladder diaries and intake diaries, will be needed as well in support of these new monitoring devices. Technology is already available in the form of a smart flow, a smart diaper, but also a smart mattress that can supply the required information. Furthermore, the necessary applications are available or easy to make. The main problems to solve are validation of the applications and devices for the purpose of exploring nocturnal LUTS and the financial incentive for companies to this low-volume work. However, community-driven sponsoring might be a solution for this.