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Michael R posted a topic in CR PracticePeople starting CR would very much like to have definitive, hard-and-fast answers to these questions, and unfortunately, there just isn't any way to give one in free-living humans! No one can tell you the exact point at which you're "on CR" or what "%CR" you are. These are guidelines and principles for entering the "CR continuum." Under laboratory conditions, it's easy to put a mouse or rat "on CR" and prescribe a "%CR" to it, because scientists have full, lifelong control of its genetics, diet, and environment. To establish a healthy ad libitum (AL) intake for the control group, researchers begin with a few hundred mice from a genetically-homogeneous strain, and watch what they eat when given unlimited access to lab chow under their laboratory conditions. In the best studies, researchers then cut back about 10-15% from that to keep them from getting overweight: it's this slightly "restricted" diet that actually constitutes the "AL" baseline Calorie intake for the controls. The CR group then has Calories cut from this baseline, and that's the "%CR" of the CR animals. For humans, however, we don't have anything like this level of precision, unless you've got a colony of a hundred or so identical twins that you can lock up and force to eat the same AL diet and get the same amount of exercise for the first two decades of life. It's therefore quite impossible to judge the exact point at which a person transitions to CR — let alone determining exactly what a given person's "% CR" is as a translation of the rodent phenomenon — in free-living, genetically unique humans based on some arbitrary BMI, %body fat, or Caloric intake standard. If you are lucky enough to have had a clear, healthy 'setpoint' in your youth — a weight to which you tended to gravitate when you were in your early twenties, and that was within the healthy BMI range — take that as your baseline, and restrict Calories down to a level that keeps you at least 15% below that. Unfortunately, of course, since the 1980s, fewer and fewer people at that point in their lives have been in the healthy range, or had a stable weight rather than a slow upward creep during that period — and once a higher weight is established, the body resets its setpoint to the new, abnormal fat mass. If that's you, you're stuck with a somewhat more arbitrary starting point, somewhere around the middle of the 'healthy' 18-25 BMI range, but taking some account of your build, % body fat, waist-to-hip ratio, and (ideally) visceral adiposity. if you're extremely lean and muscular at BMI 25 and only moderately active, that should work fine; if you can still "pinch an inch" and can't count your ribs at BMI 22, you probably need to start from a lower baseline. But CR is not about your weight or body fat: the effect comes from Calories, Calories, Calories. That's probably a big part of the reason why the epidemiology fails to find a longevity benefit to low BMI: the vast, vast majority of people with low BMI aren't on CR. Just having a small body frame can give a low BMI, and people of South Asian descent carry substantially more visceral adipose tissue than Caucasians, resulting in metabolic syndrome linked to excess metabolically-active fat in people who look slim to all the world. Some skinnier people are eating tons but getting lots of exercise, or have pre-existing illnesses or malabsorption issues (Crohn's disease), hyperthyroidism (associated with high cardiovascular and other risks), high Non-Exercise Activity Thermogenesis (NEAT), use amphetamines, smoke, have tapeworms, or what have you. These people are merely slim — they're not on CR. Wherever you start from, you need to cut Calories. Ultimately, the goal is to keep Calories lower than your physiology 'thinks' it needs, and a level of Calorie intake that only normalizes an overweight body will simply return you to the historical norm for our species, not induce the anti-aging metabolic shift that characterizes CR. You should also look for the endocrinological (TSH, T4, and T3; IGF-1 and IGF-BP3 if possible) and risk factor (blood glucose, fasting insulin, cholesterol profile, etc) signs discussed in The Longevity Diet by CR Society President Brian M. Delaney and Emeritus Board member Lisa Walford, p. 36.