We read with interest the article by O’Connor, Walton and Flyn reporting water intakes in Irish adults (published on line 6th Dec 2013, O’Connor L., Walton J. & Flynn A. Water intakes and dietary sources of a nationally representative sample of Irish adults. J Hum Nutr. Diet. doi:10.1111/jhn.12189). They drew attention to the important fact that water contained in food (‘food moisture’) is understudied. However we were surprised that the authors’ careful analyses did not take into account the contribution to intake due to metabolic water (i.e. water produced as a result of the oxidation of dietary macronutrients). Oxidation of protein produces 24.6mg of water per kJ of energy released, fat 28.4mg/kJ, carbohydrate 35.9mg/kJ, and ethanol 40.4mg/kJ. Assuming that the macronutrient composition of the diet was represented by that reported by Harrington, about 15% protein, 35% fat, 45% carbohydrate and 5% alcohol, it is possible to estimate the amount of metabolic water produced as 31.8mg per kJ total energy expended. Comparison with Table 1 of O’Conner et al. indicates that this corresponds to 8.4-14.5% of water obtained from that preformed in food in beverages. Presumably precise data could be calculated on an individual basis from the dietary records made in NANS, if so desired. We feel that attention to the metabolic contribution is sufficiently large for it to be discussed in the context of water sources in general.
Harrington, K.E, et al. (2001) Public Health Nutrition, 5, pp 1051-1060
Dr Les Bluck and Dr Gail Goldberg,
MRC Human Nutrition Research,
Elsie Widdowson Laboratory,
Response from the study authors
We would like to thank Dr Les Bluck and Dr Gail Goldberg for their recent comment on our article.
Metabolic water does indeed contribute to water balance, however the focus of the current article was on intakes and sources of preformed water rather than on water balance, as such we felt that it was outside the scope of our aims.
As you have noted, our data does allow for calculation of metabolic water at the level of the individual. We have calculated it using the conversion factors suggested as 305 ± 88 mL/d in men aged 18-90 years and 220 ± 63 mL/d in women; 11% in men and 9% in women additional to that preformed in food.
Laura O’Connor on behalf of the authors.