How do I tell hair thinning from an energy deficit on 900 kcal a day?

What I am working with: hair thinning · 900 kcal.

This is not behaving the way I expected and I want to understand the discrepancy before I act on it.

I have photographed the current state and recorded the conditions, so I can answer follow-up questions precisely.

Should I be treating this as a failure or as noise?

The 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.

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.

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

Answering this needs to know the size of the deficit, since fatigue tracks it closely and predictably.

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.

Concretely, 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.

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

Research-use compounds are not approved for human use.

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.

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

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

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.

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.

Ferritin as an acute-phase reactant is a recognised limitation and is why it is interpreted alongside an inflammatory marker.

Nothing here is medical advice.

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