Caloric Restriction (CR) is a diet in which calorie intake is reduced, compared to diets that do not limit consumption, i.e., ad libitum (AL) diets. CR has been effective in extending the lifespan of many species through biochemical mechanisms that are still not well understood. Mice fed calorically restricted diets generally outlive mice fed AL diets by significant margins. It is not known yet whether CR will extend life on humans, but it is known that being slim, with a Body Mass Index around 21, reduces the incidence of diseases such as cancer, heart disease, and diabetes. Many people have already adopted CR diets to stay healthy and hoping to prolong their lives. To practice CR safely, it is necessary to select foods that provide all the nutritional requirements, but which are low in calories.[1]

How do you measure Caloric Restriction?
In animal experiments, scientists divide a set of genetically homogeneous animals into an experimental group and a control group. They measure how much a control (AL) group eats, and then use that amount as the basis for determining how much to feed the experimental CR group, e.g., 30% less, etc. There is no hope of finding genetically homogeneous controls for humans. However, predictive energy expenditure equations like Harris-Benedict[2] and Mifflin-St Jeor[3], in addition to the Body Mass Index (BMI) formula, make it possible to determine the caloric requirements of a person with your physical characteristics, but having a BMI of 22.0 which corresponds to the middle of the normal range. This theoretical person will be your "Control Twin" and will have your same height, age, sex, and exercise habits, but a weight corresponding to a BMI of 22.0. The BMI is the ratio of the weight to the height squared. It is calculated by dividing the weight in kilograms by the height in meters squared (BMI = weight / height²).

The Harris-Benedict and the Mifflin-St Jeor equations provide an estimate of the Basal Energy Expenditure (BEE), also called the Resting Metabolic Rate (RMR), or Basal Metabolic Rate (BMR). Predictive energy equations are routinely used in hospitals and nutrition clinics to determine the caloric requirements of various patients. Of the four most commonly used predictive energy equations, the Mifflin-St Jeor equations give the most reliable results.[4]

The Mifflin-St Jeor equations are:

These equations require the weight in kilograms, the height in centimeters, and the age in years. To determine your total daily calorie needs, the BMR has to be multiplied by the appropriate activity factor, as follows:

• 1.200 = sedentary (little or no exercise)
  
• 1.375 = lightly active (light exercise/sports 1-3 days/week)
  
• 1.550 = moderately active (moderate exercise/sports 3-5 days/week)
  
• 1.725 = very active (hard exercise/sports 6-7 days a week)
  
• 1.900 = extra active (very hard exercise/sports and physical job)
  

If your BMI is higher than 22.0, your weight will be more than the weight of your control twin, and the predictive energy expenditure equations will calculate a higher caloric requirement for you than for the control. The opposite is true if your BMI is below 22.0. Your percent of caloric restriction is determined only by the number of calories that you actually consume relative to those required by the control.

Warning!
Inadequate low calorie diets can result in malnutrition or death. You should not attempt a low calorie diet without the aid of nutritional tracking software, a good understanding of nutrition, and competent medical supervision.

Caloric Restriction Decreases BMR
The Calorie Restriction Society and the CRsupportgroup provide forums where members can exchange information about their practice of CR. Below is the case of a practitioner whose weight and eating habits have remained constant for over 20 years. Analysis of the data provides some interesting insights.

A Case Study:
Lightly active female, age 51, 5 feet 7 inches tall, weighs 120 lb, and eats about 1000 kcal per day.

Using the Caloric Restriction Calculator we find that the subject's BMI is 18.8 and her Percent CR is 43.4%. The estimated calories for her level of activity are 1640, but she actually eats only 1000 Calories. This discrepancy of 39% raises concerns about whether the calories consumed were counted correctly, or whether the subject is still losing weight. Verification by electronic mail was used to establish that the subject's weight had been steady for many years and that the calories consumed were correct.

Data from other long-term CR practitioners reflect similar discrepancies between actual caloric requirements and those estimated by energy equations. This makes it necessary to conclude that a calculated caloric deficit does not imply that a person will lose weight. When a person achieves material and energy equilibrium with fewer calories, the calculated caloric deficit reflects an increased efficiency of metabolism compared to the subjects from which the equations were derived. This lower basal metabolic rate is one of the objectives of long-term Caloric Restriction with Optimum Nutrition (CRON). The Biosphere-2 experiment[8] showed that the metabolic rates of the biospherians were approximately 6% lower than those of the control subjects after adjustment for age, sex, fat-free mass, and fat mass.  Other experiments have shown that the basal metabolic rate can decrease by approximately 12% in three weeks of a 40% calorie restricted diet, but without reaching material equilibrium.[9]  A pioneering six-month semistarvation study by Keys[10] showed that severe energy restriction decreased BMR in absolute terms by 39% and also relative to the weight of metabolically active tissue by 16%. 

Rodent Experiments
One feature of mice experiments is that the mice are started on calorically restricted diets after 9 weeks, shortly after weaning. The mice on 40% calorie restricted (CR) diets grow to be adults which are approximately 51% of the size of mice fed ad libitum.[5]  Considering that the CR mice receive 60% of the food and that they have an adult weight of 51% of the weight of AL mice, CR mice eat 18% more than AL mice on a body weight basis.

Masoro[6] also reported that rats started on a 40% CR diet at 6 weeks of age had lower weights. Masoro's data for rats from 10 to 20 months old can be used to calculate that the 40% CR rats had only 53% of the weight of the AL rats. The restricted rats got 57.9% of the food, but received 9% more calories per gram of body weight than the AL rats. Masoro only remarks about the number of calories that the CR rats consume over their lifetime, but his data indicates that the CR rats routinely ate more calories per gram of body weight from the age of 3 months, and also had a substantially longer lifespan.

What does this mean for humans?
The paradox of increased food consumption per unit of body weight for calorically restricted rodents has been interpreted to mean that it is only the reduction in total calories that really matters for longevity. Humans starting caloric restriction after maturity cannot achieve higher consumption-to-weight ratios than the controls. In Case 1, above, the actual calories divided by the subject's weight gives 1000/120 = 8.3 Calories/pound, whereas the corresponding control requires 1766/140.2 = 12.6 Calories/pound.

Animal experiments on caloric restriction after maturity indicate that adult-onset CR also increases longevity.[7] This has provided encouragement to the growing number of human CRON practitioners. Human CR diets must be phased in gradually over several years, and moderation is recommended since the optimum percentage of caloric restriction for human adults is still not known.

References

1. Roy Walford, M.D., "Beyond the 120 Year Diet: How to Double Your Vital Years", 2000. Describes Caloric Restriction with Optimal Nutrition (CRON) as a way of losing weight, retarding aging, and increasing life span. Numerous scientific studies are referenced to support the claims.
2. J. Arthur Harris and Francis G. Benedict, "A biometric study of basal metabolism in man" Published by the Carnegie Institution of Washington, Washington 1919. NOTE: The equations are on page 190.
   The Harris-Benedict equations are:
   Male: BMR = 66.47 + (13.75 × weight) + (5 × height) - (6.76 × age)
   Female: BMR = 655.1 + (9.56 × weight) + (1.85 × height) - (4.68 × age)
   
3. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals.", Am J Clin Nutr., 1990 Feb;51(2):241-7. PMID: 2305711
4. Frankenfield D, Roth-Yousey L, Compher C., "Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review", J Am Diet Assoc., 2005 May;105(5):775-89. RESULTS: Four prediction equations were identified as the most commonly used in clinical practice (Harris-Benedict, Mifflin-St Jeor, Owen, and World Health Organization/Food and Agriculture Organization/United Nations University [WHO/FAO/UNU]). Of these equations, the Mifflin-St Jeor equation was the most reliable, predicting RMR within 10% of measured in more nonobese and obese individuals than any other equation, and it also had the narrowest error range. PMID: 15883556
5. Mattson, et al. "Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake", Proc Natl Acad Sci USA, 2003 May 13; 100(10):6216-6220.
6. E J Masoro, et al, "Action of food restriction in delaying the aging process", Proc Natl Acad Sci U S A., 1982 July; 79(13): 4239-4241.
7. Weindruch R, Walford RL., "Dietary restriction in mice beginning at 1 year of age: effect on life-span and spontaneous cancer incidence." Science, March 12, 1982; 215(4538), pages 1415-8. PMID: 7063854
8. Christian Weyer, Roy L Walford, Inge T Harper, Mike Milner, Taber MacCallum, P Antonio Tataranni and Eric Ravussin, "Energy metabolism after 2 y of energy restriction: the Biosphere 2 experiment", American Journal of Clinical Nutrition, Vol. 72, No. 4, 946-953, October 2000.  Free Full Text
9. Friedlander AL, et al. "Three weeks of caloric restriction alters protein metabolism in normal-weight, young men" Am J Physiol Endocrinol Metab., 2005 Sep;289(3):E446-55. Epub 2005 May 3. PMID: 15870104
10. Keys A, Brozek J, Henschel A, Mickelsen O, Taylor HL. "The biology of human starvation", Minneapolis: University of Minneapolis Press, 1950.