The Ob mouse is a strain of mouse that has a genetic mutation that makes it obese and unhealthy. It has been used for many years as a model of obesity to do research on, though the reason that it was obese had eluded scientists. This changed when, in 1994, Jeffrey Friedman discovered that this mouse lacked a previously unknown hormone called leptin, and when it was injected with leptin it became thin, vibrant, and very healthy within weeks. This made headlines around the world, "the cure for obesity found" and pharmaceutical companies started tripping over themselves with trillion dollar signs in their eyes to be the first to genetically manufacture leptin on a large-scale. This did not last long. When people were tested for leptin, it was found that, unlike the Ob mouse, they did not lack leptin; on the contrary almost all overweight and obese people have excess leptin. These people were "leptin resistant" and giving extra leptin did little good. The financial disappointment was extreme and scientists working for pharmaceutical companies said that leptin wasn't important anymore since they could not find a drug to control it, and therefore the industry couldn't make money on it. To make big money in medicine one needs a patent and this generally means remedies which are not commonly or easily available -- that are not natural. This illustrates two extremely unfortunate principles in modern medicine; only those therapies that will make lots of money (generally for the pharmaceutical industry or hospitals), ever get pursued and then taught to physicians (since most of medical education after medical school takes place by drug reps), and these therapies, almost by definition, will be unnatural. This inhibition of extremely important knowledge is not only unfortunate, it is deadly, and is exemplified by how few people, including doctors, know anything about leptin, though I would consider it to be the most important chemical in your body that will determine your health and lifespan.
Each and every one of us is a combination of lives within lives. We are made up of trillions of individual living cells that each must maintain itself. Even more significantly, the cells must communicate and interact with each other to form a republic of cells that we call our individual self. Our health and life depends on how accurately instructions are conveyed to our cells so that they can act in harmony. It is the communication among the individual cells that will determine our health and our life. The communication takes place by hormones. Arguably therefore, the most important molecules in your body that ultimately will decide your health and life are hormones. Many would say that genes and chromosomes are the most important molecules, however once born your genes pretty much just sit there; hormones tell them what to do. Certainly, the most important message that our cells receive is how and what to do with energy, for metabolism and therefore life cannot take place without that. The two most important hormones that deliver messages about energy and metabolism are insulin and leptin.