Recently, I noticed that one of our team members, Paula, had a new, very large, metered, refillable bottle that contained water. Sometimes I would see a fruit wedge bobbing around at the top of the container, but most often, there was only water and ice in the bottle.

Curiously, I queried as to the water-drinker’s intentions for this new “hydration project.” Paula told me that she knew she didn’t drink enough water and had heard that increased water consumption was helpful in losing weight. Thus, she was increasing her daily water consumption to a measured 64 ounces per day. Naturally, after two weeks, I questioned Paula about her goal progress and any other changes that she had noticed on her water-drinking journey. Note: Paula has a documented history of allergy, dry eye, and keratoconus. She wears contact lenses and sometimes reading glasses over her lenses for working at the computer.

To my surprise, Paula told me that what she was most pleased about, besides a small drop in weight, was the improvement in the comfort of her eyes. She was using artificial tears much less often. Paula felt that her vision was better, she was using her reading glasses less often, and, surprisingly, her eyes were less itchy, particularly at the time of year when historically she was most troubled by ocular itch.

Whole-Body Hydration

Paula is my “n of one.” Her story prompted an updated review of the literature on whole-body hydration and how it relates, or does not, to tear osmolarity and dry eye.

Blood plasma (serum) osmolality measures the body’s electrolyte-water balance. In the steady state, our total body water content and salt content remain constant. An increase or decrease in water and salt intake is paralleled by an equivalent change in renal water and salt excretion. But does increased plasma osmolarity (Posm) translate into increased tear osmolarity (Tosm) to precipitate or aggravate dry eye? A 2012 study asked: Is whole-body hydration an important consideration in dry eye? (Walsh et al, 2012).

Results of this study showed that individuals classified as dry eye patients have higher Posm, indicating suboptimal hydration, compared with non-dry eye patients. The authors concluded that whole-body hydration is an important consideration for dry eye patients.

In 2017, a separate study examined the same question (Kobayashi et al, 2017) and attempted to correlate Posm and Tosm in diabetic and non-diabetic patients. This study found that there was no correlation between Posm and Tosm. Correlations between Posm/Tosm and each dry eye sign tested were also not found. Of note, Posm was significantly higher in diabetic than in non-diabetic patients. There was, however, no significant difference in Tosm between diabetic and non-diabetic patients. The study concluded that Tosm is independent of Posm. It, therefore, was suggested that Tosm elevation in dry eye occurs by local mechanisms.

It is generally accepted that total body water turnover is complex and that there is no accepted technique to measure hydration in all situations (Armstrong, 2007). It would be nice, for example, to use Tosm as a non-invasive metric of hydration status during exercise and post-exercise rehydration. It seems Tosm is a valid indicator of changes in hydration status when looking at a group mean in exercise-induced dehydration; however, large differences among individuals in the Tosm response to hydration changes limits its validity for individual recommendations (Ungaro et al, 2015).


It seems that we just don’t know the impact of whole body hydration on Tosm. Considering my “n of one,” however, I will continue to suggest general recommendations for total daily water intake of approximately 2.7 liters (91 ounces) for women and approximately 3.7 liters (125 ounces) for men (Institute of Medicine of the National Academies, 2004). CLS

For references, please visit and click on document #262.