For reference: fatigue · 1,000 kcal.
An unexpected observation, and I would like a differential rather than reassurance.
The conditions were within what I understood to be the acceptable range, which is why I am asking.
What is the differential here, and which test discriminates between the options?
Start with what 1,000 kcal a day leaves you room for, because at that intake the deficit is a live competing explanation rather than a footnote. Protein is 4 kcal per gram, so 100 g costs 400 kcal — 40 per cent of the entire day's energy — and everything else has to come out of the remaining 600. A day that misses protein at 1,000 kcal has missed it by a wide margin, and fatigue that tracks those days is a nutrition signal. Then separate the two by timing: a dose effect is periodic and phase-locked to the injection or to an increase, and a deficit effect is monotonic and gets worse the longer the intake stays at 1,000. Log intake, protein and fatigue on the same daily row for a fortnight and the two patterns separate without any further cleverness. Nothing here is medical advice.
The relevant physiology is that a large deficit reduces both available substrate and spontaneous activity, and the second is often mistaken for the first.
A deficit beyond about a thousand kilocalories a day reliably produces fatigue, reduced training performance and reduced spontaneous movement. With appetite suppressed, deficits of that size arrive by accident rather than by plan.
| Feature | Local reaction | Sterile abscess | Cellulitis |
|---|---|---|---|
| Onset | Hours to 2 days | Days | 1–4 days, progressive |
| Warmth | Absent or minimal | Mild | Marked |
| Expansion | Static or shrinking | Slow | Expanding |
| Texture | Firm, flat or raised | Fluctuant | Diffuse, indurated |
| Systemic features | None | None | Fever, malaise possible |
| Action | Observe, rotate site | Clinical review | Same-day clinical review |
In practice, dehydration from reduced fluid intake alongside reduced food intake produces headache, lethargy and postural dizziness, and is the cheapest thing on the list to correct.
Energy deficits above roughly a thousand kilocalories a day are associated with measurable reductions in resting metabolic rate, spontaneous activity and subjective energy in controlled studies.
Attributing a symptom with a wide differential to the most recent change is a common and expensive error.
Weigh three days of intake honestly. That answers this most of the time.
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Browse resultsAnswering this needs to know the size of the deficit, since fatigue tracks it closely and predictably.
Three days of honest weighed intake usually settles this. Estimated intake in this situation is systematically wrong and usually wrong in the direction that hides the problem.
Hypothyroidism, sleep apnoea, depression and anaemia all present as fatigue and all become more likely rather than less in this population, so attribution to the compound should be a diagnosis of exclusion.
Sleep restriction produces measurable decrements in subjective energy and in training performance independently of energy intake.
If it persists at an adequate intake, get bloods rather than more theories.
The honest answer is that this is usually fixable by eating more, which is unwelcome advice in this context.
Sleep quality often changes during rapid weight loss in both directions, and sleep debt produces fatigue that no amount of dietary adjustment will fix.
In practice, iron deficiency is common, particularly in menstruating women, and low intake plus a reduced red-meat share makes it more likely. Ferritin is the test, and it is an acute-phase reactant so it needs interpreting alongside CRP.
Ferritin as an acute-phase reactant is a recognised limitation and is why it is interpreted alongside an inflammatory marker.
Abrupt onset points away from the deficit and towards something else.
To be exact about it, if it persists at an adequate intake, it needs blood work rather than more speculation.
Carbohydrate intake specifically affects training performance and perceived energy at a given total intake, which is why very low carbohydrate approaches feel worse in the gym at matched energy.
Self-reported intake underestimates measured intake by twenty per cent or more in doubly labelled water comparisons.
Sleep is a variable here too, and no amount of eating fixes sleep debt.
edited 16 Jul 2026 by ilaria_bertone — expanded the table to cover the lower concentration
The short version: check intake, check hydration, check iron and thyroid if it persists, and only then attribute it to the compound.
Fatigue that appeared abruptly, rather than gradually with the deficit, is a different pattern and points away from intake.
Nothing here is medical advice.
Check fluid and sodium before anything more exotic.
edited 20 Jul 2026 by Dr_Elias_Weiss — added the placebo-arm figures
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