Estimating Fluid Requirements in the Obese

Celebrating National Nutrition Month

Estimating Fluid Needs for Obese Patients
Water is the single largest constituent of the human body and is essential for homeostasis and life. Fluid intake is important to health and is considered a critical component of nutrition assessment. As the population in the United States becomes more obese, many dietitians are questioning how to estimate fluid needs in an obese patient.  When assessing fluid needs, is it necessary to adjust the IBW for a patient who is classified as obese (BMI>30)?
An on-line attempt at a literature review indicates there are virtually no published studies on this topic. However, published nutrition assessment guides currently available indicate that the actual body weight should be used for fluid estimates, not adjusted body weight. As with other nutrition assessment tools, calculation of fluid needs should be used in conjunction with assessment of fluid intake and signs and symptoms for dehydration. The clinical practitioner should use his/her clinical skills to judge if fluid provided is adequate.

​There is, however, increasing evidence for a relationship between obesity and underhydration. Individuals with an osmolality of 800 or more were considered underhydrated. Inadequately hydrated patients weighted more than adequately hydrated patients.  This 2016 research suggests calculating 40-50 ml/kg/day in a healthy population.

Formulas that are recommended for fluid estimates (assuming normal renal and cardiac function) include:

1. 30 cc/kg body weight (Brummit, Kobriger) *
2. 1 cc/kcal intake (Kobriger)
3. 100 ml/kg for the first 10 kg, plus 50 ml/kg for the next 10 kg, plus 15 ml/kg for the remaining weight (Brummit, Kobriger)
4. (kg body weight – 20) x 15 + 1500 (Brummit)
5. 3.7 L fluid/day (at least 3.0 liters from beverages and the remainder from food) for men and 2.7 L/day (at least 2.0 L from beverages and the remainder from food) (Dietary Reference Intakes)

* The AND Nutrition Care Manual recommends the following:

• Average healthy adult   30-35 ml/kg body weight
• Adult 55-65                   30 ml/kg body weight
• Adult > 65 years             25 ml/kg body weight

Comparison of these formulas produces a wide variety of fluid recommendations for an obese patient. Using a fictional woman who is 65 inches tall and weighs 248 pounds (112.6 kg) with a BMI of 41.3, estimated fluid needs using actual body weight are as follows:

1. 112.6 x 30 =3378 cc
2. 2000 cc (assuming she consumes approximately 2000 kcal/day)
3. 100cc + 50cc +(15cc x 92.6) =1539 cc
4. (112.6-20) x 15 + 1500 =2889 cc
5. 2700 cc (at least 2000 cc from beverages)

When fluid needs are calculated using recommended formulas, the fictional patient’s fluid needs vary from a low of 1,595 cc to a high of 3,378 cc. This is a reminder that fluid needs, no matter what formula is used, are at best an estimation.
Implications for Dietetics PractitionersDietetics practitioners should use actual body weight when calculating fluid needs for obese patients. There are several formulas available for estimating fluid needs; the practitioner should select the method that they are comfortable using. This method should be indicated on the nutrition assessment form so the results can be justified if necessary.
Nutrition AssessmentIn nutrition assessment and progress notes, document that fluid needs are estimated and/or provide a range of estimated fluid needs (such as 25-30 cc/kg body weight). Adequate fluids should be made available, preferably in a calorie-free form that will not contribute to excess weight gain in an obese patient. Practitioners can allow for the fact that up to 19% of fluids are provided by foods and the body’s metabolic processes (Dietary Reference Intakes), or up to 700 cc/day (Kobriger) by documenting that the fluids available in foods contribute to the overall fluids available to the patient.
Monitoring Hydration StatusHydration status can be monitored by assessing food and beverage intake and urinary output (when available). Abnormal laboratory values such as serum osmolality and electrolytes should be addressed as needed. The patient should be monitored for signs and symptoms of dehydration, including weight loss, poor skin turgor, decreased or concentrated urine, dry skin and mouth, sunken eyes, and decreased functional ability. If dehydration is noted in obese patients, fluids provided should be increased and this should be documented in the patient’s progress notes.
References:
AND Nutrition Care Manual. Available to subscribers at www.nutritioncaremanual.org.
Brummit, P. Dietary Documentation Pocket Guide. Developed as a Joint Project by the Consultant Dietitians in Health Care Facilities DPG and the Dietary Managers Association, 2002.
Kobriger, AM. Hydration: Maintenance: Dehydration, Laboratory Values, and Clinical Alterations. Chilton, WI: Kobriger Presents, Inc., 2005.
National Academy of Science. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Water, Potassium, Sodium Chloride, and Sulfate. 2004. Available at http://fnic.nal.usda.gov/nal_display/index.php?info_center=4&tax_level=4&tax_subject=256&topic_id=1342&level3_id=5141&level4_id=10592.
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https://pmc.ncbi.nlm.nih.gov/articles/PMC4940461/

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