How do I tell injection-site erythema from an energy deficit on 800 kcal a day?

The specifics, since they change the answer: injection-site erythema · 800 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 most likely explanation, and how would I confirm it?

Start with what 800 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 — 50 per cent of the entire day's energy — and everything else has to come out of the remaining 400. A day that misses protein at 800 kcal has missed it by a wide margin, and injection-site erythema 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 800. Log intake, protein and injection-site erythema on the same daily row for a fortnight and the two patterns separate without any further cleverness. Nothing here is medical advice.

Answer first: fatigue in this context is usually an energy-intake problem before it is a drug effect, and the arithmetic is the first place to look.

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.

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.

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.

To be exact about it, if it persists at an adequate intake, it needs blood work rather than more speculation.

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.

Concretely, 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.

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.

The short version: check intake, check hydration, check iron and thyroid if it persists, and only then attribute it to the compound.

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.

To be exact about it, 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.

Sleep restriction produces measurable decrements in subjective energy and in training performance independently of energy intake.

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.

This is the complaint with the widest differential and the one most often attributed too quickly.

Sleep quality often changes during rapid weight loss in both directions, and sleep debt produces fatigue that no amount of dietary adjustment will fix.

Sleep is a variable here too, and no amount of eating fixes sleep debt.

edited 22 Aug 2026 by cal_hennessy — added the citation requested in comments