FAQs: Caloric Restriction and Intermittent Fasting

  1. If a human achieves the same caloric restriction as a mouse, e.g. 20%, would be we expect the human to have the same health benefits as a mouse? And what about primates?

    There is no clear answer to this at this time — that why scientists at the Wisconsin Primate Center are currently studying lifelong calorie restriction (30%) in rhesus macaque monkeys — presumably more pertinent to humans than rats. So far, the monkeys in this study have attained the median lifespan of (unrestricted) monkeys, and the death rate from age-related disorders has only been one-third as high in the restricted monkeys as in the restricted ones. It’s not yet clear how this will equate to increased longevity, but there is already clear evidence that calorie restriction can be tremendously protective in primates.

    However, perhaps our best current way to address this problem is to look at Okinawans, a contemporary human experiment in calorie restriction. Studies show that, prior to 1970, Okinawans consumed about 11% fewer calories than would have been considered “weight maintenance” for their lean physiques (averaging BMI 21). By the “BMI 22 reference man” standard, their “true” calorie restriction may have been closer to 12%. But after 1970, the Okinawans were no longer calorie restricted. Yet aging Okinawans now have an average and maximal life expectancy that is about 4-5 years longer than that of current Americans. In rodents, life extension achieved by calorie restriction appears to be more or less proportional to the extent of calorie restriction and the portion of one’s total life spent in calorie restriction. Elderly Okinawans achieved about a 12% calorie restriction for perhaps about half their lives — and now enjoy a 4-5 year advantage in life expectancy. This suggests that humans who managed to achieve a 20% restriction for most of their lives could enjoy a quite substantial increase in life expectancy — provided that this benefit is proportionate in humans as it is in rodents.


  2. How can I estimate my degree of caloric restriction — as understood in Caloric Restriction Experiments — from my measurement of the reduction in caloric consumption that I am experiencing?

    By estimating your average daily calorie intake at baseline, and then once again after your weight has equilibrated after a year or so of CC dieting, you can calculate the percent reduction in daily calorie intake made possible by your new lifestyle.  But many people who embark on CC dieting do so not just to get leaner and improve their risk factors, but also in the hope of achieving some of the aging-retardant benefits observed in rodents and rhesus monkeys who are chronically fed less than their ad-libitum calorie intake.

    How can you estimate how much calorie restriction benefit you are achieving with your CC regimen? This is not as straightforward as it might sound.   Consider two people:  Person A was quite heavy, but has managed to reduce his average daily calorie intake by 30% and lose significant weight with CC dieting.  Yet his daily calorie intake may now be little different than that of a moderately lean person with a comparable height and activity level — because his metabolic rate had been pushed upwards during his years of overeating.  Contrast him with Person B, who may have been quite lean when he embarked on CC dieting — say, a BMI of 20 — and achieves an average daily calorie reduction of 10%.  It is quite conceivable that person B is enjoying a greater calorie restriction metabolic benefit than person A — even though his calorie consumption has fallen by a lesser percentage.

    If you would like to estimate the true degree of calorie restriction you have achieved with CC dieting — so that you can compare your experience with that of other people who are consciously engaged in calorie restriction protocols (or underfed mice or rhesus monkeys!) — we suggest that you visit this website.

    The Calorie Restriction Calculator on this website estimates the daily calories burned by a person who has a moderately lean BMI of 22, but who has a height, activity level, age, and sex similar to your own.  The extent to which your current calorie intake is below this estimated calorie intake can be considered your “true” calorie restriction.  In other words, if your current calorie intake is about 1900 kcals daily, whereas your “twin” with a BMI of 22 is estimated to burn 2200 kcals daily, then the calorie restriction you have achieved would be 300 kcals daily, or 300/2200 = 13.6%.

    To get reasonably accurate results with this Calorie Restriction Calculator, be careful not to overestimate your daily activity level.  The great majority of Americans would fall into the “sedentary” or “lightly active” categories, even those who go to a gym several times weekly.  Only people who are exercise fanatics — substantial exercise most days of the week – or who have physically demanding occupations would fall into the higher activity categories.


  3. I’ve lost a lot of weight with carb concentrated dieting, and now that my weight has equilibrated, I calculate that my daily calorie intake is lower than that of my “reference man” with a BMI of 22 — but my own BMI is still higher than that. In other words, I’m achieving true calorie restriction, but I’m not as lean as I would expect to be. What’s the problem?

    Studies show that, when people lose a lot of weight, their daily energy expenditure tends to decline.   In fact, when their weight equilibrates, their daily energy expenditure tends to be lower than that of people of comparable size and body composition who haven’t lost weight.  Only a small portion of this difference is attributable to a lower resting metabolic rate – most of the reduction is in so-called “non-resting energy expenditure” associated with physical activity.  And it’s not because people who have lost weight are lazier.  Apparently, an adaptation occurs whereby it takes less food energy (ATP) to fuel a given amount of muscular effort; in other words, your body becomes more “energy efficient”.  This may account for the fact that you may not get as lean as other people with comparable activity levels who eat more calories than you do.  More scientific detail →


  4. Mice experiments on caloric restriction compare caloric restricted mice with mice eating as they wish (ad libitum). By contrast, in human dieting, humans eat less calories than they were doing previously at their higher weight. How can the reduction in calories of the human, e.g. a reduction of 20%, be compared and calibrated with the forced reduction of the mice? What role does the reference-man calculation comparing caloric consumption with a same-BMI person of BMI 22 mean here?

    Here is no truly objective way to calculate the degree of calorie restriction you are achieving on a CC diet regimen or other calorie restriction protocols. Obviously, you can calculate your percent reduction of calorie intake from baseline when you go on a CC regimen; this calculation will be most meaningful once your weight has equilibrated at a new lower set point, and you thus are on a sustainable regimen. However, if you had been substantially overweight and were really stoking in the calories, a 20% reduction from baseline in calorie intake — while no doubt beneficial for your body weight and your long-term health prospects — cannot be presumed to provide the same degree of calorie-restriction health benefit that a relatively lean and abstemious person would achieve if he likewise reduced his calorie intake 20% from baseline. That’s why some experts on calorie restriction suggest that we calculate our “true” level of calorie restriction by comparing our calorie intake to that of a person of comparable height who has a relatively lean, healthy BMI of 22 and is taking in sufficient calorie to maintain his weight. Choosing a BMI of 22 was rather arbitrary — 21 or 23 might have served as well. But by employing this standard, we can then seek to correlate one’s “true calorie” restrictions with health outcomes, and perhaps use the results of studies in rats or rhesus monkeys to estimate the life expectancy benefits we might hope to enjoy.


  5. You say that the number of consecutive hours of low insulin is what might matter. But on alternate-day fasting regimens, insulin levels are low for up to 36 hours consecutively. In rodents, we know that this is long enough to get an extension in longevity. Why do you think that 20 hours of low insulin with a CC diet is long enough to get the same effect?

    The main reason we are advocating a CC diet rather than an alternate day diet is that we judge it as much more feasible for large numbers of humans to maintain indefinitely. Moreover, we are emphasizing healthy aging rather than longevity and we believe this will work based on much work related to low insulin diets (see next question). Of course, we don’t know for sure how well it will work and no one will have a definitive answer on this for some time. But one way we might evaluate this experimentally is to look at the impact of blood serum derived from people engaged in specific diets on certain longevity markers in cultured human cells incubated with that serum. This is a strategy pioneered by researchers at the National Institute of Aging (NIH), and there is no reason why we couldn’t compare the effects of serum from people on CC diets with those of serum from people who are doing calorie restriction or intermittent fasting.


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