Researchers have discovered a significant relationship between inadequate hydration and an increase in body mass index (BMI) and obesity, even after adjusting for confounding factors.
Study: The Relationship Between Hydration and BMI
"Our findings suggest that adequate hydration may play a role in body weight and support further conversation regarding adequate hydration during weight management counseling", says Dr. Tammy Chang, a public health professor at the University of Michigan Department of Family Medicine in Ann Arbor, and her colleagues.
If people do not hydrate adequately or eat when they think they are hungry but are actually thirsty, coaching can help differentiate the signals, the authors continue.
The researchers analyzed a nationally representative sample of adults aged 18 to 64 years (n=9528; weighted, n=193.7 million) from the US National Health and Nutrition Examination Survey. In the sample, 50.8% were female, 64.5% were non-Hispanic Caucasians, and the mean age was 41 years.
They analyzed BMI to determine obesity status and its correlation with urinary osmolality values of 800 mOsm/kg or more, which indicate inadequate hydration. They adjusted for documented confounding factors, such as age, race/ethnicity, sex, and poverty income ratio.
They found that adults with inadequate hydration had higher BMIs than those who hydrated adequately (1.32 kg/m2 higher on average; 95% confidence interval [CI]: 0.85 – 1.792; p <0.001), as well as greater odds of being obese (odds ratio [OR]: 1.59; 95% CI: 1.35 – 1.88; p <0.001).
Urinary Osmolality Is a Better Measure Than Water Intake
Urinary osmolality is considered an excellent measure of hydration and is a better indicator than water intake alone because it takes into account water and solutes ingested in food and other beverages, the authors explain.
To exclude confounding factors that alter urinary osmolality, the researchers conducted a sensitivity analysis and excluded people who had a diagnosis of diabetes, had an elevated HbA1c concentration (6.5% or higher), or were prescribed a diuretic during the study.
Although drinking water often comes up in discussions about weight management by clinical professionals and reports in the media, adequate hydration is not included in treatment guidelines.
Water Needs Increase As BMI Increases
The authors note that clinical professionals may not be aware that higher-BMI individuals need more water, and they may not be adequately advising on hydration levels.
"Obese individuals have higher water needs than nonobese individuals, since water needs depend on metabolic rate, body surface area, and body weight," they explain.
The authors also note that inadequate hydration has been linked to worsening mental, physical, and emotional health and may affect performance in tasks that require attention, psychomotor skills, and memory. Decreased mood, headaches, and poor kidney function have also been linked to inadequate hydration.
Some limitations of the study were that the data show a correlation but not a causal relationship. Similarly, a single urinary osmolality reading for individuals available in the US National Health and Nutrition Examination Survey data may not represent habitual hydration levels. Additionally, since only people aged 18 to 64 years were included, the results may not be generalizable to children or older adults.