Our physiology isn't really any different, except for the artificial alterations thereof. Human is human. Dietary needs may vary based on region, but not time period.
Also, we don't necessarily "live longer" now either - it depends entirely on a host of conditions, including sanitation, access to food, disease control, lifestyle, sociopolitical factors, and the like; all that varies from one culture to the next. There are some pre-modern cultures that may have lived longer than some modern cultures, and there are certainly some that lived proportionally as long or longer, once you factor in death causes that have nothing to do with health. (Consider also how many health problems are modern, first-world phenomenons.)
Thus, "we're healthier now than before" is more of a blanket statement that fails to account for thousands of factors. It's difficult to do a complete comparison straight across the board (e.g. we don't have to fear attacks by warring Mongols that cut our life expectancy short), so we look at individual topics without drawing artificial correlations between them. We don't have a life expectancy of 70-80 years because of our diet and lifestyle - we have one of the most nutritionally devoid diets in history, and we tend to be dangerously sedentary - we live long because of healthcare. When we make comparisons in history, we can't just look at "when did they die;" we MUST look at "WHY did they die?" A population dying of smallpox means we can't make the claim "they died young because of poor diet," because their diet may in fact have been quite good (or bad). This is why forensic anthropology is a thing.
As always, those who fail to learn from history are doomed to repeat it.
(Note, I've said nothing herein about the breakfast vs. no-breakfast topic; there are some interesting and valid points on both sides that warrant further inquiry. History has examples of both.)
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