Lack of resource adds stress to the body, plain and simple. Exercising without a caloric deficit can build muscle. Weight loss without exercise is all loss no gain while maintaining a stressful state on the body.
And "good ol' fashioned gumption" does work, at least for me. (Sample size: 1)
Define “stress”.
Calorie restriction is demonstrably life-extending in multiple species (that it’s feasible to test it in).