Calorie Restriction

Calorie restriction is a reduction in energy intake (food consumption). This may be involuntary, due to reasons such as an inadequate food supply, or voluntary, such as in individuals seeking to lose weight or people with the eating disorder anorexia nervosa. Extreme calorie restriction can result in malnutrition or a deficiency of one or more nutrients.

The human body requires an adequate amount of energy or calories to enable us to grow, perform daily activities, produce hormones, and to minimize the risk of nutrition-related complications. Calorie requirements for individuals vary from one individual to the next and are dependent on variables including gender, age, activity levels, climate and environment, and the presence or absence of medical or nutritional complications. Consequently, it is possible that individuals of similar weight and age may have different energy requirements owing to unique individual needs. In addition to the body requiring energy to perform tasks, energy is required to support activity of the internal organs and to maintain body temperature. This energy is called the basal (or resting) metabolism. The basal metabolic rate (BMR) is determined experimentally when an individual is lying down at complete physical and mental rest under standardized environmental conditions. Research over many years has provided researchers with guidelines on how to assess and determine an individual's BMR using specific calculations.

Estimated daily calorie needs

SOURCE: U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2015-2020. 7th ed. Washington, DC: U.S. Government Printing Office. http://health.gov/dietaryguidelines/2015/guidelines (accessed April 2, 2018).

There have been numerous claims made that reduced-calorie diets extend life expectancy by preventing many of the age-related conditions that typically shorten a life span. Similar claims have been made for intermittent fasting, which alternates between very low–and higher-calorie days. The is little research available to help understand the impact of starvation and severe calorie-restriction on humans. Ethically, it would not be appropriate to impose such situations on individuals, but two major studies did impose such situations in 1919 and 1950. The first of these studies involved the recruitment of 34 volunteers who were underfed for three months to facilitate a weight loss of 10%. This study found that the resting metabolic rate (RMR) of the participants decreased rapidly at first and then more slowly with continuing weight loss. The second study again exposed a group of male volunteers to a reduced calorie intake over a period of 24 weeks to achieve a weight loss of 25%. The study confirmed a decrease in metabolic rate and estimated that 65% of the decrease in metabolic rate could be attributed to the loss of the metabolic activity of the tissue. This study also provided useful information in respect to the symptoms associated with starvation due to restrictive eating patterns. The study highlighted that many of the more behavioral signs and symptoms associated with people with eating disorders are likely to develop in individuals without eating disorders if exposed to an extreme reduction in their dietary intake. These symptoms include an increased preoccupation with food, adoption of unusual eating patterns involving strange concoctions of food, spending a long time consuming food, social withdrawal and a lowering of mood, and a decrease in libido.

More recently, the U.S. National Institute on Aging began a voluntary study on the effects of a calorie-restricted diet in relation to aging and to determine the safety of these types of diets. The study is called CALERIE—Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy. Participants in the CALERIE diet are carefully screened and then followed for two years; the study is ongoing.

Numerous research studies have been performed and are underway to evaluate the effects of a reduced-calorie diet in nonhuman primates, mice, and other common research species. A review of two long-term ongoing studies on rhesus monkeys, published in the February 2011 issue of Institute of Laboratory Animal Resources, stated that scientists have seen reduced rates of heart disease, type 2 diabetes, obesity, and glucose intolerance in monkeys following restricted-calorie diets, among other benefits. However, a study published in Nature in August 2012 did not find evidence that such a diet increased longevity.

Malnutrition occurs when there is a deficiency of one or more nutrients. It may be mild or severe. Severe malnutrition is rare in developed countries but remains prevalent in many developing countries. Weight loss is an obvious sign of a diet too low in calories or energy. Protein-energy malnutrition occurs when the diet provides too little energy and protein. Malnutrition is assessed in terms of degrees of weight loss in adults; in children, height and weight are used and compared with growth charts. Weight loss in adults and failure to thrive in children are the most obvious signs of insufficient energy intake. In adults, this is usually noticed by a drop in the body mass index (BMI), which is a method used to relate weight to height that is used as part of the process in determining an individual's nutritional status. In children, insufficient energy intakes are noticed when children fail to meet expected growth milestones. In the long-term, stunted growth may result as a consequence of insufficient energy requirements and children may not reach an expected height-to-weight ratio.

Severe cases of low energy intake result in starvation and life-threatening conditions called kwashiorkor (characterized by protein deficiency) and marasmus (primarily a deficiency of energy-providing foods). Marasmus is a chronic condition of semi-starvation, characterized in later stages by muscle wasting and an absence of subcutaneous fat and to which children adjust, to some extent, by reduced growth. In kwashiorkor, subcutaneous fat is usually preserved; muscle wasting occurs but is often masked by edema (swelling). The physical effects of malnutrition are dependent on what nutrients are missing from the diet, for example, calories, protein, fat, vitamins, or minerals. Usually, when an individual's dietary intake is inadequate, there is likely to be a deficiency.

Some of the general effects of starvation include:

As the body starts to lose weight as a result of insufficient energy intake, the ability of the brain to function correctly is impaired. The symptoms associated with changes in the way the brain functions are referred to as cognitive changes.

Cognitive and behavioral changes associated with starvation include:

Children who have an insufficient energy intake over prolonged periods of time may not reach expected growth rates. Furthermore, they may lose weight and ultimately experience stunted growth and develop nutrition-related complications. Parents should ensure that a physician monitors growth rates regularly.

See also Anorexia nervosa ; Body image ; Body mass index ; Calories ; Intermittent fasting ; Malnutrition ; Metabolism .