Stated plainly: headache · 1,100 kcal.
I noticed this today and I have not touched anything since, in case the state is diagnostic.
I have not discarded anything yet, so a test is still possible if that is the recommendation.
How do I distinguish the benign explanation from the one that matters?
Start with what 1,100 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 — 36 per cent of the entire day's energy — and everything else has to come out of the remaining 700. A day that misses protein at 1,100 kcal has missed it by a wide margin, and headache 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 1,100. Log intake, protein and headache on the same daily row for a fortnight and the two patterns separate without any further cleverness. Nothing here is medical advice.
The honest answer is that this is usually fixable by eating more, which is unwelcome advice in this context.
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.
| Event | Active arm | Placebo arm | Timing |
|---|---|---|---|
| Nausea | 40–45 % | 15–20 % | Peaks 1–2 wk after each step |
| Vomiting | 15–25 % | 5–8 % | Follows nausea |
| Diarrhoea | 20–30 % | 10–15 % | Early, variable |
| Constipation | 20–25 % | 8–12 % | Later onset, persistent |
| Discontinuation for GI events | 4–7 % | 1–2 % | Mostly during escalation |
Ranges span agents and doses; read the specific prescribing information for a specific figure.
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.
Nothing here is medical advice.
Check fluid and sodium before anything more exotic.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe short version: check intake, check hydration, check iron and thyroid if it persists, and only then attribute it to the compound.
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 quality often changes during rapid weight loss in both directions, and sleep debt produces fatigue that no amount of dietary adjustment will fix.
Ferritin as an acute-phase reactant is a recognised limitation and is why it is interpreted alongside an inflammatory marker.
The caveat is that fatigue with breathlessness, chest symptoms or sudden onset needs assessment rather than dietary adjustment.
If it persists at an adequate intake, get bloods rather than more theories.
Answering this needs to know the size of the deficit, since fatigue tracks it closely and predictably.
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.
The underlying point is that fatigue that appeared abruptly, rather than gradually with the deficit, is a different pattern and points away from intake.
Sleep restriction produces measurable decrements in subjective energy and in training performance independently of energy intake.
Sleep is a variable here too, and no amount of eating fixes sleep debt.
The relevant physiology is that a large deficit reduces both available substrate and spontaneous activity, and the second is often mistaken for the first.
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.
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.
Weigh three days of intake honestly. That answers this most of the time.
Stated carefully, this is the complaint with the widest differential and the one most often attributed too quickly.
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.
The pooled gastrointestinal adverse-event rates across the STEP programme and the SURMOUNT programme are reported in the primary publications and in the FDA and EMA assessment reports, and the assessment reports are more useful because they give the placebo-arm rates alongside the active-arm rates in the same table.
Research-use compounds are not approved for human use.
Abrupt onset points away from the deficit and towards something else.
edited 26 Jun 2025 by sian_llewellyn — updated for the 2026 guidance change
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