Why Your Weight Can Fluctuate by a Few Pounds a Day

by | Sep 8, 2017


Writing for Women’s Health Magazine, nutritionist Alexandria Caspero weighed herself every hour over a 12 hour period.  What happened? Her weight often fluctuated by a pound or two on back-to-back weigh-ins and varied by nearly 7 pounds over the course of the day! How is this possible? This schematic helps explain:


Throughout the day we consume and then expel about 13.5 pounds. We intake food, water and oxygen and expel solids, water and CO2. Whether we’ve recently consumed a few cans of LaCroix or gone to the bathroom can have a material effect on the numbers on the scale. Where your body is with respect to the intake vs. output side of the process displayed above is the reason for short-term weight fluctuation. Realize that gaining a pound is around 3500 excess calories and weight fluctuations over a few days can’t possibly be reflective of true weight gain or loss.*

If you weigh yourself regularly, advice from experts is to weigh on the same scale first thing in the morning while naked. Even then, do not place too much credence on any one measurement – average a few days and take note of trends over the course of a week or two.

An interesting point highlighted in the schematic above – note that the “solids” we excrete only weigh on average 0.3 pounds – 7x less than the weight of CO2 we breath out. Is that correct? While weight of our stool varies based on our diet, age and gender, as a species we average 128 grams a day of feces. A pound is 450 grams, so the 0.3 pounds number above is a good approximation. Higher fiber intake leads to more weighty fecal output and weight of fecal output is inversely correlated with colon cancer risk (i.e. higher stool weight is associated with lower colon cancer risk). Link to feces cancer correlation study: https://www.ncbi.nlm.nih.gov/pubmed/1333426

*With respect to the 3500 calories per pound guideline – whether all calories are equal is a controversial topic and may be the topic of a future IFOD.


  1. My scale is seismic…

  2. My scale doesn’t do the metric system but I’m pretty sure it is way more than 128 grams.


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