Given the goal is lifespan extension, the implication seems to be the person is trying to do this for an entire lifetime. You can't eat at a calorie deficit in

Given the goal is lifespan extension, the implication seems to be the person is trying to do this for an entire lifetime. You can't eat at a calorie deficit in the long term without starving at some point, so that means 1500 KCal at some point has to be equal to your TDEE. There is no way to get that low permanently while having any meaningful amount of muscle. The comment below claiming TDEE of 1900 for a 190 lb "active" male is interesting. I currently use the Macro Factor App to track meals and TDEE, make everything from scratch from raw ingredients, weigh and measure absolutely everything, including any cooking oil, so the estimates are good as you can get without checking into a metabolic ward, and I'm a 41 year-old male who currently weights 187 lbs and my TDEE is roughly 2900. That's at an activity level of lifting for 60-90 minutes 6 days a week, walking usually one to three hours a day at a pretty leisurely pace, usually while listening to a podcast or playing Pokemon Go, and I live in a four-story house so walk up and down stairs quite a bit. The famous study on Amish body composition and activity levels had them at an average of 18000 steps and 9% body fat, and they were eating ungodly amounts of food, pounds of potato and grain, upwards of 4000 KCal a day in most cases. I suspect what a western white collar worker considers "active" is skewed a bit. I doubt there were many pre-industrial adults outside of nobility who got by on 1500 a day. Maybe some had no choice during a very harsh winter, but not as an everyday thing for years.