You could have good blood sugar, blood pressure, and eat right and still be over weight, but that is still considered healthy.
Now look at another example a 35 year old male who eats junk food and doesn't exercise could be 6 foot and 150 pounds. Would you consider him healthy or well?
It cause weight loss because the diets make you avoid certain food that have higher amount of some thing’s such as carbs or fat in them. If you avoid less of something then you will overall be getting less it therefore you will lose weight
