S. Weissenberg

The term “insensible water loss” is discussed in connection with the respiratory system, water balance, and thermoregulation in both human physiology and animal physiology courses. Students are not always aware of the quantity of water lost through this channel and its significance to water balance. To help students appreciate this issue quantitatively, they are offered the following theoretical exercise. The exercise provides a basis for a discussion on water balance at different ambient conditions (dry/humid or hot/cold climates) or special physiological situations (artificial ventilation or premature babies in incubators). The exercise consists of a question, instructions for answering the question, and relevant data. The calculations require integration and manipulation of data from empirical equations and physiological measurements obtained from different sources. Most data are taken from text books that are accessible to our students (but it may be found in other sources as well).

The Question

A person weighs him/herself before and after sleep. During 6 h of sleep, s/he loses about 230 g in weight. Why did s/he lose weight?

Instructions for the Answer

  1. Explain the possible reasons for weight loss (body mass and water).

  2. Calculate the expected weight loss during the sleep in the example above, based on the data in Tables 1 and 2.

    1.Calculate the amount of water lost through the different channels of water loss.

    2.Estimate the basal metabolic rate (BMR) and calculate the decrease in body mass due to metabolism. Take into consideration metabolic water gain (3, 9).

  3. Suggest an experiment to validate your findings.

    1.Explain how to measure (experimentally) weight loss through every channel.

  4. Explain the effect of the following factors on evaporative/insensible water loss:

    1.A change in temperature and/or humidity of the inhaled or exhaled air.

    2.A change in ventilation rate, BMR, or body surface area. (For example, compared with adults, newborns have a higher metabolism and surface area.)

  5. Explain the effect of insensible water loss on thermoregulation and energy balance.

View this table:
Table 1.

Data for calculations

View this table:
Table 2.

Metabolic data


  1. Reasons for weight loss.

    Weight loss can be caused by loss of body mass due to metabolism (exploitation of body fat, carbohydrates, or tissue proteins for energy) or by water loss as insensible water loss, sweating, or excretion (in feces and urine). Feces and urine are ignored in this example.

  2. Calculations.

    1.Insensible water loss through airways.

    The volume of water vapor (VH2O) lost during breathing is the difference between the water in exhaled air (VH2Oex) and inhaled air (VH2Oin).

    VH2O in exhaled or inhaled air is the product of the water vapor fraction in the air (Fr; in %) and the ventilation rate (V), respectively: VH2O = V × Fr Math where Ph2o is water vapor partial pressure and Pb is barometric pressure. Math where the subscript ex denotes expired air and the subscript in denotes inspired air. Math With the use of the data shown in Table 1, Math For 6 h, Math Math

    2.Insensible (evaporative) water loss through skin and sweating.

    Assuming that the rates of evaporating from the skin and sweating are even during day and night,1 the estimates for 6 h of sleep would be as follows. Math Math [1This assumption is inaccurate. Rates change during day and night with changes in activity and ambient temperatures and humidity (7).]

    3.Loss of body mass.

    A theoretical O2 consumption of 250 ml O2(STP)/min [where STP is standard temperature (0°C, 273 K) and pressure (760 mmHg)] was chosen as the metabolic rate during sleep. For more accurate values of BMR, calculate the predicted BMR according to your personal data. Choose any suitable empirical equation used in physiological tests such as the Schofield equation (10) or Harris-Benedict's equation (6). For the sleeping metabolic rate, you can also use the value of 4.6 or 4.2 kJ/min for males and females, respectively (1). Assuming fat is metabolized during night, use the caloric value of fat: 19.8 kJ/l O2. In such a case, the weight of O2 consumed is about the same as CO2 produced (12). (You can calculate also for carbohydrates or mean diet.) Math where Vo2 is O2 consumption. Math Assuming 7,000 kcal (29.3 MJ) are required to lose 1 kg of body weight (11), this will bring a reduction in body weight of about Math

    4.Gain of metabolic water.

    The gain of metabolic water (H2Om) can be calculated as follows: Math Assuming the fat metabolism and metabolic rate are as calculated previously (see above), Math

Results and Conclusion

Weight loss and water loss calculated in this example are shown in Table 3. It seems that the main reason for weight loss during sleep is insensible water loss from airways and skin (about 83 % of the total weight loss, not including feces and urine).

View this table:
Table 3.

Weight loss during 6 h of sleep


The author thanks Prof. I. Choshniak (Department of Zoology, Tel-Aviv University, Tel-Aviv, Israel) for helpful comments on the exercise.