I guess it depends on how underweight you mean, but aren't out hunter-gatherer ancestors supposed to have lived in a state of semi-starvation for some parts of the year?
If you have a choice between "underweight" and "overweight" why don't just go for "neither"?
I personally think it depends on what you are eating. I'm happy to eat veggies until my heart is content, I have a huge piece of steak for dinner and I'm full. It's double what I used to eat but I'm still losing weight and feeling fantastic. I personaly would be very wary of people telling you to be undernourished as that in itself can be very dangerous and can SOMETIMES lead to an eating disorder (I only say that from self experience)
I struggle with that myself. I have never been overweight, but I have been too thin several times in my life. Not because I have an eating disorder, but because I just don't eat a lot (although I have eaten healthy most of my adult life). I had gained about 10 lbs over the last 3 or 4 years and it disturbed me, (I had gotten a bit lazy and was eating too much bread, crackers etc), so I started Crossfit (I was a runner for many many years and had slacked on that a bit) and exploring the Paleo diet. After cutting out grain products, except for rice and some granola (I make my own), I have lost almost all the 10 lbs in short order. HOwever, I can tell if I don't up my food intake I will lose down to being too skinny again. It's a narrow line I walk every day. I am 5' 10", 138 lbs and 49 years old.
It is worse to be undernourished.
There is some growing evidence that "obesity" may be caused in part by being under nourished, in terms of vitamins, minerals, and good food. Thus, it is not just calories, but the whole diet that comes into play.
Your body, when you listen to it, usually tells you when you are overnourished. If you cannot stand the sight of liver after eating it the past week, or your body says "ya know, I'm tired of beef, lets have some fish," you will get those signs.
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