Being underweight is associated with an increased risk of death, just as being obese is, but the causes are fundamentally different. The risks of obesity are owed largely to having too much fat mass, while those of being underweight are owed primarily to having too little muscle mass.
Regardless, being underweight is nowhere near as large of a health concern considering how much more prevalent obesity is.
There is no such thing as “healthy” obesity. Sure, you can be obese and free of metabolic abnormalities, but you still have a markedly increased risk of developing obesity-related diseases in comparison with normal weight individuals.
Not only are your ever-expanding fat cells giving out more and more stress signals as they fill up, providing a beautiful setting of low-grade, chronic inflammation, but if things don’t change, you’re going to pass your personal fat threshold at some point. Hello diabetes, fatty liver, and heart disease!
Eat fibrous vegetables and proteins first during meals, saving your starches for last, and you can drastically reduce the blood glucose and insulin response to that meal.
Calories and macronutrients matter, but so do the foods we eat. We need to appreciate the role that the food matrix plays in health and disease. To ignore it is ignorance.