I&#x27;ll reply to all the dissenting comments just here to avoid duplication and since you were actually civil about it.<p>Yes that is the most likely answer I agree, however it doesn&#x27;t make it the ONLY one. I have dieted using a &#x27;food diary&#x27; and been amazed at how many calories you can eat&#x2F;drink without realising it, it&#x27;s really quite something.<p>HOWEVER, if she&#x27;s so serious as to exercise that much, and knows the calories and it&#x27;s at that low a level, it seems the error bars would likely preclude this.<p>I&#x27;ve noticed weird results when I&#x27;ve dieted where the &#x27;energy equation&#x27; just does not match reality when I have dieted STRICTLY using a food diary and known-calorie food&#x2F;drink. The &#x27;dieting plateau&#x27; is famous and encountered by many so maybe there are more factors here.<p>Factors that might be at play (other than self-delusion):<p>1. Water&#x2F;body fluid retention - This can vary quite a bit and is often (apparently) the explanation for dramatic results at the beginning of a diet. Water is dense enough for this to cause there to appear to be no weight change. In this case, the body fat _would_ be being used by the body as an energy source it just wouldn&#x27;t show up in weight changes.<p>2. Inaccurate scales - It&#x27;s surprising how often bathroom scales can be significantly off. Usually they are consistent + their delta, however maybe she measured on 2 different scales before and after (seems unlikely given how often we dieters like to check)<p>3. Use of muscle mass as an energy source - This can and does happen, though as I understand it usually as a &#x27;last resort&#x27; by the body.<p>4. Reduced body temperature&#x2F;energy usage - The body is capable of reducing energy used in starvation situations (though it renders you not feeling great often), this could happen here.<p>5. Change in efficiency of energy extracted from food - The body is complicated, it&#x27;s not obvious that energy extracted from food will always remain the same.<p>6. Difference in diet composition - calories aren&#x27;t the entire answer as some food is more easily digested than other food, and some types of food are more likely to result in weight gain than others.<p>7. Hormonal changes - Hypo and hyper-thyroidism can change body mass accumulation without ANY change in food intake, so this could easily apply to other hormonal changes.<p>8. Constipation - It sounds silly, but perhaps more retention of feces in the system could result in apparent lack of weight change.<p>9. Change in weather - Perhaps a hotter climate requires less heating and hence less energy usage by the body?<p>My principle point here is that things are just not as simple as energy in -&gt; energy out - yes &#x27;basic physics&#x27; holds that has to be an equilibrium, however you might not be measuring what these are correctly. A car must maintain an equilibrium too, but the type of car, its condition and what you do with it significantly varies the energy it requires despite the same distances being traversed.