Calorie Restriction

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Calorie Restriction





Parental concerns



The definition of calorie restriction is when the body does not receive an adequate intake of energy. This can result as a consequence of lack of availability of adequate volumes of food. However, sometimes the failure to consume an adequate calorie intake may be a conscious and deliberate decision, for example, in individuals seeking to lose weight or people who suffer with the eating disorder, anorexia nervosa . 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 the aforementioned tasks, energy is also 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.

Estimated daily calorie needs .

  Calorie range 
 Sedentary Active
2-3 years1,0001,400
4-8 years1,2001,800
4-8 years1,4002,000

Sedentary means a lifestyle that includes only the light physical activity associated with typical day-to-day life.

Active means a lifestyle that includes physical activity equivalent to walking more than 3 miles per day at 3 to 4 miles per hour, in addition to the light physical activity associated with typical day-to-day life.

source: Center for Nutrition Policy and Promotion, U.S.Department of Agriculture.

(Illustration by GGS Information Services/Thomson Gale.)

with guidelines on how to assess and determine an individuals BMR using specific calculations.


Individuals require a certain number of calories on a daily basis, which incidentally may vary subject to activity levels and physical status but overall is within a relatively stable range. When dietary intake remains stable weight too remains unchanged. Conversely, increases or decreases in intake can result in weight changes. To clarify,.

  • When energy intake equals energy expenditure, weight remains unchanged.
  • When energy intake exceeds expenditure, weight increases.
  • When energy intake is less than expenditure, weight will decrease.

It is when expenditure exceeds calorie intake that the body experiences a negative energy balance and weight loss ensues.

There is little research available to help us understand the impact of starvation on humans. Understandably, ethically it would not be appropriate to impose such situations on individuals, however, two major studies did impose such situations in 1919 and 1950. This 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 highlighted that resting metabolic rate (RMR) decreased rapidly at first and then more slowly with continuing weight loss. The second study, carried out by Keys and colleagues, again exposed agroupof male volunteers to a reduced calorie intake over a period of 24 weeks to achieve a weight loss of 25%. They 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 of the symptoms associated with starvation when individuals adopt restrictive eating patterns. The study highlighted that many of the signs and symptoms that people typically associate with people who suffer with an eating disorder are very likely to develop in non-eating disordered individuals if they were to be exposed to a similar extreme reduction in their dietary intake. These symptoms include an increased preoccupation with food, adoption of unusual eating patterns including concoctions of food, spending a long time consuming food, social withdrawal and a lowering of mood and an associated decrease in libido.


Physical effects of starvation

Malnutrition occurs when is there is a deficiency of one or more nutrients. It may be mild or severe. Severe Malnutrition is rare in developed countries but sadly 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, whilst 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 and is used as part of the process in determining an individuals’ 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 reached an expected height for weight ratio.

It is important to remember that individuals of a healthy body weight could still be classified as malnourished.


BMI— This relates weight to height and is used as part of the nutritional assessment of individuals. BMI is the weight (in kg) divided by the height (in meters) squared. The acceptable healthy range is 20–25.

Calorie— The amount of heat needed to raise 1g of water by 10C. For nutritional purposes the Calorie (or kilocalorie) is the amount of heat needed to raise 1,000g of water by 10. The modern unit is the joule. One calorie is a little over 4 joules.

Malnutrition— A disorder resulting from an inadequate diet or failure to absorb or assimilate nutrients.

Minerals— Elements that are essential for the body to function correctly including calcium, iron, phosphorous, magnesium, sodium, chloride, iodine, manganese, copper, and zinc.

Starvation— A long-term consequence of food deprivation.

Vitamins— Compounds required by the body in small amounts to assist in energy production and in cell growth and maintenance. They are essential for life and with the exception of vitamin D, cannot be made in the body. They should ideally be consumed from food. However, individuals who struggle to eat can obtain their vitamin requirements from dietary supplements.

if they are lacking in essential nutrients. Therefore, overall nutritional status of individuals cannot be assessed solely on body weight. Consequently, nutritional assessment screening tools are useful to help identify individuals who may be at risk of malnutrition in hospitals, care homes and primary care settings. In summary, malnutrition may develop owing to a number of factors including a decreased dietary intake, increased nutritional requirements or increased losses or an inability to absorb or utilize nutrients.

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). Overall 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 individuals dietary intake is inadequate there is likely to be an overall deficiency of many nutrients.

Below is a list of some of the general effects of starvation.

  • tiredness
  • lethargy
  • poor concentration
  • weakness
  • hair loss and sometimes the growth of additional hair on the arms and back. (The growth of additional hair called lanugo hair and is frequently evident on people who suffer with anorexia nervosa.)
  • decreased muscle mass and tone
  • weight loss
  • menstruation may stop if weight loss is serious
  • delayed puberty in adolescents
  • iron-deficiency anemia
  • poor immunity
  • decrease in bone mineral density and an increased risk of osteoporosis if weight loss is not reversed
  • constipation
  • deficiencies of various vitamins and minerals
  • delayed gastric emptying

Cognitive changes associated with starvation

As the body starts to lose weight as a 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.

The cognitive changes associated with starvation include.

  • increased risk of depression-related symptoms
  • poor decision making skills
  • irritable
  • anxiety
  • very reactive
  • poor concentration

Social and sexual changes associated with starvation

  • withdrawal from social activities
  • decreased libido
  • decreased enjoyment in previously enjoyed activities

Parental concerns

As mentioned above, children who have an insufficient energy intakes 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 registered doctor monitors growth rates regularly or a suitably qualified professional.



Garrow, J.S., James, W.P.T. and Ralph, A. Human Nutrition and Dietetics, 10th Edition. Churchill Livingstone.

Thomas, B. Manual of Dietetic Practice, 3rd Edition. Blackwell Science Ltd.


Benedict, F.G., Miles, W.R., Roth, P. and Smith, M. “Human vitality and efficiency under prolonged restricted diet.” Cited in Garrow, J.S., James, W.P.T. and Ralph, A. Human Nutrition and Dietetics 10th Edition. Churchill Livingstone.

Keys, A., Brozek, J., Hanschel, A., Mickelson, O. and Taylor, H.L. “The biology of human starvation.”The Journal of Nutrition 135, 1347(6).




Annette Laura Dunne, BSc (Hons) MSc RD