If he’s not exercising and is largely sedentary then 1400 calories for an adult male is a serious deficit, but not extreme. If he added in an hour of high intensity interval training or something that would be extreme and potentially hazardous.
Severe caloric restriction diets are actually not particularly effective if your goal is to permanently lose significant quantities of weight and keep it off - once you plateau it really sucks. Of course I mean if you power through and keep at it you will, but IME, periodic fasting is a much easier way to lose weight and keep it off.
Not hazardous, really, as you point out. The whole point of stored fat is to be used.
Citations available on request, I have NCBI links for each of my points but it takes a while to dig up.
Usually one big meal after noon and that's about it, plus occasional coffee or cup of cocoa. If I have event in the evening I just don't eat during the day or get a milky beverage to get me by, then depending on how much I ate I either delay dinner the next day or skip the next day altogether.
Here's another interesting one. Check out the Energy Expenditure section of [2].
> The gross BMR decreased in all the subjects during semistarvation, the average decline being 17.1% after 19 days (54) and 21.4% after 13 days (55). [3]
Based on this data, I would expect your BMR to go down by 20% over a period of three weeks, and remain there for the rest of your life were you to continue this for a while. So you'd have to, as an 'average' person, reduce intake by 500kcal to account for metabolic adaptation, then another 500kcal to lose 1lb per week.
That aligns with your suggestion that 750kcal-1000kcal reduction should show results for most people.
[1] https://pubmed.ncbi.nlm.nih.gov/27136388/
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673773/
[3] https://journals.physiology.org/doi/abs/10.1152/jappl.1958.1...
The important thing here is that a 1200 calorie diet for someone that big is an extreme outlier and I’d like to know what the actual protocol was.
If there wasn’t a lot of consideration about actual composition of diet or if this was a longer term protocol I’d be extra leery of it.
You can be healthy with a zero calorie diet if done correctly and for the proper time period, but you should be consulting an expert if you do.
And for anecdata I’m 6’0, 215 and 45. 1700 calories is a crash diet for me.