How do I tell vomiting from an energy deficit on 800 kcal a day?

Details up front: vomiting · 800 kcal.

I have a result I cannot explain, and I would rather diagnose it than guess.

I have checked the obvious explanations and eliminated the two easiest ones.

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 vomiting 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 vomiting 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.

On the detail: 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 is a variable here too, and no amount of eating fixes sleep debt.

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

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.

Specifically, 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.

If it persists at an adequate intake, get bloods rather than more theories.

Start with the actual intake, because a substantially suppressed appetite produces deficits far larger than intended and fatigue is the first symptom.

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.

In practice, fatigue that appeared abruptly, rather than gradually with the deficit, is a different pattern and points away from intake.

Check fluid and sodium before anything more exotic.

edited 16 Mar 2025 by sian_llewellyn — corrected a unit error in the worked example

The part that matters: dehydration and low sodium intake produce fatigue that is indistinguishable subjectively and much easier to fix.

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.

The caveat is that fatigue with breathlessness, chest symptoms or sudden onset needs assessment rather than dietary adjustment.

Abrupt onset points away from the deficit and towards something else.

The relevant physiology is that a large deficit reduces both available substrate and spontaneous activity, and the second is often mistaken for the first.

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

Research-use compounds are not approved for human use.

Weigh three days of intake honestly. That answers this most of the time.