I wonder if there any difference in how many times I eat per day?
Let's say I eat one big meal in the morning at 2000 kcal (at 9am). Is that any different than having 4 meals at 500 kcal throughout the day (at 9am, 1pm, 5pm, 9pm)?
I think I will be hungry by the end of the day if I eat just once in the morning. But I won't be hungry if I eat 4 times a day at smaller portions? How is that possible?
Which way is it better to eat? Maybe twice a day at 1000 kcal? (9am and like 4pm?)
There have been quite a few studies on the influence of meal frequency, but usually they focus on weight loss.
I could find one pilot study comparing people receiving either one or three meals a day, for a total of 8 weeks, while receiving the same amount of calories in total. All study subjects maintained their body weight. The study concluded:
There were no significant effects of meal frequency on heart rate, body temperature, or most of the blood variables measured. However, when consuming 1 meal/d, subjects had a significant increase in hunger; a significant modification of body composition, including reductions in fat mass; significant increases in blood pressure and in total, LDL-, and HDL-cholesterol concentrations; and a significant decrease in concentrations of cortisol
Note, however, that over a quarter of study subjects withdrew during the study. The authors report that it was for unrelated reasons, but such a high drop out rate is still a cause for concern. A concern you voice in your question is hunger, and when eating only one meal a day, that was something the study subjects reported significantly more than when eating three meals a day.
As I said, most studies focus on weight loss,and most have few study subjects. A recent meta-analysis (analyzing the existent literature on the topic) found
Three randomized controlled clinical trials of fasting in humans were identified, and the results were published in 5 articles, all of which evaluated the effects of fasting on surrogate outcomes. Improvements in weight and other risk-related outcomes were found in the 3 trials. Two observational clinical outcomes studies in humans were found in which fasting was associated with a lower prevalence of CAD or diabetes diagnosis. No randomized controlled trials of fasting for clinical outcomes were identified
But ultimatively concluded:
Whereas the few randomized controlled trials and observational clinical outcomes studies support the existence of a health benefit from fasting, substantial further research in humans is needed before the use of fasting as a health intervention can be recommended.
Health effects of intermittent fasting: hormesis or harm? A systematic review
Intermittent fasting isn't exactly like what you are describing, but it comes close, and there's more studies on it than eating just one meal a day, which I suppose many people find unsustainable.
As an example for what I mean when I say studies are usually small, one that I saw cited a lot is: Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism. That study bases its conclusion, that Intermittent Fasting doesn't affect insulin sensitivity and various other factors on a study in just 8 healthy, lean males.
Overall, I don't think there's a recommendation based on evidence that can be given - no frequency of meals that we can point to and say "this is the optimal frequency for being healthy". Or, at the very least, there isn't one yet (personally, I doubt there ever will be). No meal frequency seems to be associated with dramatic negative effects on health, and some may be associated with small beneficial changes. As it is, any meal frequency that person finds sustainable and satisfying is probably good.
