We were interested to read the recent review article in Clinical Science concerning OSA (obstructive sleep apnoea) and its connections to obesity, diabetes and hypertension . That article looked into the evidence supporting the association of OSA with hypertension, and described the different associated health problems of obesity, hypoxia and cardiovascular disease and the difficulty in finding successful treatments, even using animal models. Similar points have been laid out recently in an editorial published in the New England Journal of Medicine . However, one important aspect that appears to have been overlooked in the review  and in the editorial , is, with sleep apnoea, the mouth is open sometimes for considerable periods of time. With chronic open mouth breathing there is considerable loss of water leading to chronic dehydration. Appropriate physiological responses to intracellular dehydration (increased osmolality) are thirst (drinking) and vasopressin release, and, in response to extracellular dehydration (hypovolaemia) thirst, vasopressin release and activation of the renin–angiotensin–aldosterone system . In cardiovascular disease, antagonists of the renin–angiotensin–aldosterone system make up the majority of treatments [4,5]. As mentioned in many reviews, these same medications are used in patients with apnoea. This would suggest that hypovolaemia, or extracellular dehydration, is a prominent part of OSA.
Another problem associated with dehydration, and especially hypovolaemia, is anaemia leading to hypoxia and thus to an increase in viscosity . This is an adaptive mechanism to low blood volume or hypovolaemia. The correct physiological response would be to reduce red blood cell number, which is also observed in obesity and diabetes [7,8] and, of course, to increase arterial stiffness, leading to cardiovascular disease . It would not be surprising then to understand that all of these effects also lead to resistant hypertension.
Recommendations to increase water drinking to reduce dehydration would be necessary, and perhaps especially at night, in order to help reduce any increase in viscosity . An increase in fluid, especially water, intake would over time restore blood volume, which would reduce blood levels of angiotensin, vasopressin and aldosterone, the three major hormones involved in hypertension. Furthermore, increased fluid intake would help improve blood volume and thus red blood cell number, thus assisting in decreasing hypoxia and leading to a possible reduction in obesity and diabetes associated with OSA.