Never Get Fat. If You're Fat, Lose Weight.
http://www.eurekalert.org/pub_releases/2011-03/mu-tlw032011.php Of course this is correlation, but I’ll weakly bet that it’s causal. Even briefly being extremely fat means you’re more likely to die (for the rest of your life). You have something like a permanent 100% increase in relative mortality risk (how likely you are to drop dead each day) if you’re fat for 7 years (but then lose the weight). After that, it’s more like 4% per year you stay fat. I don’t know how quickly the risk concentrates in the first year of getting fat, but it’s probably front-loaded. I also don’t know if yo-yo fattening/defattening is worse. You should never get fat. Another reason not to, is that fat cells are hormonally active, and don’t retire when you lose weight; they just become less full of lipds (like an empty balloon). Supposedly an empty fat cell creates roughly as much hunger as a full one. (I don’t remember the evidence for this, but I believe it). Disclaimer: I don’t mean stay model-skinny. Just don’t get fat.