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A perennial discussion among people who follow the CRON diet is the relationship between weight and calories. Many people find it easier to track weight rather than calories. And most people lose weight when practicing CRON. So the two are usually closely related.

But bottom line: it is the calories that count, and weight can sometimes*be a convenient proxy for calorie intake. But this isn't always the case, and it is something that new and veteran CRONies should keep in mind. Here is a helpful discussion on the issue ("CR" and "CRON are used interchangeably):

The fundamental misunderstanding underlying many people's view of CRON dieting ia based on the notion that CR is all about weight loss. It is not.

To be on CRON means to reduce one's intake of calories by a fixed percentage, while maintaining adequate/optimal nutrition. The reduction is relative to the mean intake of an Ad Lib person of the same age/sex. Additional variables, within very tight parameters, are: body build, activity level (tight parameters means: if your activity level requires that you fall significantly outside the mean, like a sportsman that needs 6000 cal/day, you *cannot* be on CRON), environment (cold exposure). Note, there is not a word here about weight loss.

CRON most often *results* in weight loss - but not necessarily. It is possible to be on CRON: take in restricted calories, and still be overweight. For more information, please search PubMed or the CR Society archives for ob/ob mice - mice which were fat, and yet on long term CR, and had comparative life spans to lean CR mice which were on the same amount of calories. Of course, those are rodents, but we don't have human CR experiments. This shows you conclusively, that CR does NOT equal low weight. What makes you be on CR is restricting calories relative to Ad Lib, whatever the percentage may be: 10% to 70%. In simpler animals, such as rodents, you can obtain best results (in lifespan extension) by going to the extreme of 70% or perhaps even a bit more, but in more complex animals, such as monkey (and experimentally, dogs), and therefore by extension, most likely humans, it appears that about 40% is the upper safe level of restriction, though this is an understudied subject. Still - it is not about weight.

So a long-term fat subject may be on CR. At the other end, you may have a very skinny person, who is NOT on CR. At an extreme, they may be a very lean athlete consuming 6000 cal/day - thin, but not on CR. CR means cutting calories relative to Ad Lib. You can consume an average number of calories and be skinny and NOT on CR. What happens when this skinny person goes on CR? THEY DO NOT NECESSARILY LOSE WEIGHT. In other words, e.g. a person of a BMI of, say, 16, can cut their calories by a pretty dramatic amount relative to a mean Ad Lib (i.e. not an athlete, where that would be very easy), maintain the same (modest) activity level, and yet not drop a single pound! How is that possible? Simple: the body adjusts to lower calories through several mechanisms - the body cuts back on energy expenditure. This does not mean "athletic performance". You fidget less, there are fewer *involuntary* movements. This is actually a huge amount of energy. See the literature. The body read justs thermal energy governance. You become colder. A very common CR effect - core temperature drops. This is huge too. See the literature. Finally - this point is controversial and at the moment unsettled, with evidence going both ways - your metabolism may become more "efficient" by about 15% (this is a known "dieter" response, which may simply persist long-term in a CR person).

Bottom line, being skinny or fat has not much to do with CR, though the rule is, that usually, you do lose some weight as a result of going on CR, and so, skinny CR prevails. This does NOT mean that a skinny person who goes on CR will necessarily lose more weight at the same moderate activity level (and same environment, i.e. no extreme cold exposure).

There is no "ideal". You don't look at your weight first. You look at your calorie intake first. That said, CR is a LONG-TERM regimen. Don't assume that the calories you are consuming initially when getting on CR will be the same as when you are in the long-term maintenance mode. A morbidly obese person, who gets on CR could theoretically initially consume fewer calories than they would once they are skinny -- simply because the body is burning the fat in the weight loss stage, and once that is over, you need to up the calories -- though this is a pretty theoretical situation, given that you are supposed to ease into CR pretty slowly, with weight coming off within as long as 2.5 years (and not shorter than about a year). But this is about calories, not about losing weight.

The rule is: watch calories. How do you know if you are taking in too few? *That applies only to the more extreme CR*. Odd are, that on the kind of CR many are here on the list, dropping weight way too low is not going to happen. But what about the extreme CR? Well, once your BMI drops to below 16 for a woman and about 17 for a man, it may be time to up the calories. A side note - BMI is pretty imprecise, as it does not account properly for muscle mass and body fat percentage at the extremes. As always, involve a qualified physician.

-RD