Is resistance training twelve times a week enough at 900 kcal a day?

Stated plainly: twelve · 900 kcal.

I am trying to build something sustainable rather than something thorough that I will abandon.

I have already decided the broad direction; this is about the specifics.

How would you structure this, and what thresholds would you set in advance?

Twelve sessions a week against 900 kcal a day — 6300 kcal for the week — and at that intake the binding constraint is almost never the session count. Protein at 4 kcal per gram is the reason: 100 g a day is 400 kcal, 44 per cent of 900, and 2800 kcal of the 6300 for the week spoken for before anything else is eaten. 12 sessions also has to be recovered from on 900 kcal, and recovery is the first thing an energy deficit takes. Load matters more than frequency here: the stimulus that preserves mass is mechanical tension, so keep the weight and cut the volume rather than the reverse. None of this is medical or dietary advice.

The relevant constraint is recovery: a deficit reduces recovery capacity, so the sensible adjustment is volume down slightly and intensity held.

Two sessions per muscle group per week outperforms one at matched volume in most of the published comparisons, mainly through better session quality.

Session performance is the practical readiness signal. Two consecutive sessions where loads drop at matched effort usually means recovery, sleep or intake rather than programming.

Nothing here is medical advice.

Consistency beats optimisation by a margin nobody wants to hear.

The honest answer is that almost any structured programme performed consistently beats an optimised programme performed sporadically.

Load is the retention signal. Reducing weight and adding repetitions preserves the workout and weakens the stimulus, which is exactly backwards for this objective.

More usefully, protein timing matters far less than protein total, but a serving within a few hours either side of training is a free optimisation.

Load-matched comparisons show that maintaining intensity preserves strength and lean mass in a deficit better than reducing load and increasing repetitions.

Research-use compounds are not approved for human use and are not a substitute for a training stimulus.

One to three reps in reserve. Failure costs more than it returns here.

To be exact about it, compound movements give the most stimulus per unit of recovery, which matters more when recovery is the limiting resource.

Beginners retain the ability to gain muscle in a moderate deficit for several months; trained individuals should expect maintenance and be satisfied with it.

Around ten hard sets per muscle group per week is the region where most of the retention benefit is realised; more produces diminishing returns and, in a deficit, a recovery cost.

The caveat is that programme design is individual, and anyone with joint, cardiac or metabolic constraints needs a professional rather than a forum.

Compound movements first, because recovery is the scarce resource.