Research

Energy balance

During the initial phase of severe caloric restriction, the majority of rapid body weight loss is due to water loss rather than fat or protein oxidation, resulting in a significantly lower caloric equivalent per kg of weight lost.

When you start a strict low-calorie diet, expect to lose weight faster than your calorie math predicts in the first week. This is largely water, not just fat. Do not be discouraged if the rate slows down after the first few days; the fat loss rate is likely more constant than the total weight loss suggests.

StrongQualifiesVERY_HIGH confidence
a large part of the loss derived from energy balance and nitrogen excretion, indicated that a large part of the loss was due to water. During the first few days of restriction the caloric equivalent of the loss was about 3000 Cal/kg... to 8700 Cal/kg during the last days of restriction.
John Brozek et al. · Journal of Applied Physiology · 1957

Why this rating

Controlled clinical experiment with direct metabolic measurements (nitrogen, oxygen consumption) and precise caloric control.

Source

<i>Changes in Body Weight and Body Dimensions in Men Performing Work on a Low Calorie Carbohydrate Diet</i>

John Brozek et al. · Journal of Applied Physiology · 1957

DOI 10.1152/jappl.1957.10.3.412

crossover · n=19Cited 41×
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DOI resolved against Crossref · corpus check 2026-06-10

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