When the
number of calories you consume matches up with the
calories you burn through your metabolism and physical activity such as exercise, you'll be able to maintain the
proper caloric balance.
Cross multiply, then divide, to get the
proper number of grams or ounces
of food you need to eat to fullfill the
number of calories you picked for you meal (let's say 450 cals).
They didn't have any knowledge
of nutrition, they weren't able to eat nutritious,
calorie dense food whenever they wanted due to the absence
of agriculture, and their immune systems were likely weaker than ours (living together in large
numbers placed enormous selective pressure on our early agricultural ancestors to develop strong immune systems, keep in mind that early human civilizations did not have indoor plumbing... so they were sometimes exposed to fecal matter both from fellow humans and from livestock and they didn't have the kinds
of disinfectants and anti-biotics we have today,) so for them to have serious health complications makes perfect sense, nature can be very harsh and doesn't care how long its been since your last meal or what your
calorie and micro nutrient needs are... a lot
of people died at very young ages back then simply because they got sick and didn't have
proper medical treatment or due to malnutrition or starvation.