The moment you eat less, metabolic rate decreases: https://www.ncbi.nlm.nih.gov/m/pubmed/2204100/
This is why static programs produce zero continued results. If you just eat less, the body simply slows down to improve its chances of storing energy. There must be a progressive strategy, a week-over-week, day-over-day, season-over-season progression in stimulus. One stimulus (ie - eat less) provides one effect. One additional stimulus (ie - move more) provides one more effect. Done. No more effect.
This creates a genuine bottleneck for people who want to make consistent progress in health and fitness. There must be a model in place wherein that person can objectively point to week 3 being greater stimulus than week 1, week 6 greater than 3, and so on.
However, this often requires more time. Ultimately, limits present themselves, namely rest, recovery and sleep. And then, on that end of variables, we have another issue. Less sleep means increased fat storage, especially for certain demographics: https://www.ncbi.nlm.nih.gov/m/pubmed/26538305/…
Yet again, this is why I caution people about worshipping the calorie myth. I’ve known many compliant enthusiasts who are eating fewer calories than they burn only to discover they’re gaining fat. Eating less and exercising more doesn’t obligate the body to maintain muscle and bone tissue. And it doesn’t obligate the body to burn fat. You can burn lean tissue (the body prefers this, since it additionally lowers metabolic rate) and often will without major progressive strategy at play.
Be careful. Because eat less usually means gain fat.