Imagine following nearly 13 million real people for more than 10 years just to see what cholesterol level helps them live the longest. That is exactly what Korean scientists did in a giant 2019 study. They tracked 12.8 million adults who had check-ups between 2001 and 2004, and they kept watching until 2013. More than 694,000 people died during that long stretch. The results were clear and surprising. Cholesterol and death formed a U-shape. Very low cholesterol was linked to higher death rates. So was very high cholesterol. The sweet spot for the longest life was 210 to 249 mg/dL for most adults. That is higher than many doctors call “ideal.” Younger people had a slightly lower best range, but still not the super-low numbers often pushed today. Because the study lasted so many years, reverse causality is less likely to explain the low-cholesterol danger. Reverse causality means a hidden illness might lower your cholesterol right before it kills you. With more than a decade of follow-up, early sickness cannot easily account for the extra deaths seen in people who started with low cholesterol. The pattern held across ages and for both men and women. The huge number of people—almost the entire adult population of a country—makes the findings hard to ignore. Smaller studies can be wrong by chance. This one is massive. It shows that moderately higher total cholesterol, not the lowest possible number, went with the best chance of living longer in this large group. In short, the biggest study of its kind found that cholesterol levels in the low-to-mid 200s were tied to the longest lives, and the long tracking time makes it tougher to blame the higher death rates at low levels on reverse causality alone.