What Happens on Diet When You Don't Eat Enough Calories?

Failing to eat enough calories or get proper nutrition can carry several medical repercussions. In developed countries, the problem of malnutrition is not so common, but it can still occur, especially among individuals who cannot afford food, such as the poor, especially the homeless, and people with physical or psychiatric disorders 1. Infants, young children and adolescents may be at risk for undernutrition because of their relatively high calorie and nutritional needs. Older people also may develop this condition due to altered metabolisms.

Is This an Emergency?

If you are experiencing serious medical symptoms, seek emergency treatment immediately.


Malnutrition and undernutrition are often used interchangeably to refer to a deficiency in caloric consumption 1. In truth, however, undernutrition denotes a specific type of malnutrition 1. Malnutrition is a state that exists when a patient is receiving an improper amount of calories and nutrients, whether too much or too little 1. Undernutrition denotes the condition in which the patient is not receiving enough calories and nutrients, according to Lab Tests Online.

In addition, although mineral and vitamin deficiencies are typically classed as separate, distinct disorders, they usually go hand-in-hand with caloric undernutrition. A lack of calories is often accompanied by lack of vitamins and minerals.


Several possible causes for undernutrition have been identified, as noted by Merck Manual Home Edition. These may include diseases or drugs that hinder the body’s capacity to absorb or metabolize nutrients, a lack of access to food or a radically increased need for additional calories.

Vices also affect caloric intake. Smoking stimulates the sympathetic nervous system, making the body use up more energy. It makes food less palatable by deadening the sense of taste and smell. Alcohol contains calories despite having little nutritional content; therefore, drinking decreases the appetite. Liver damage from alcohol also interferes with nutrient absorption.


Typically, the first outward sign of undernutrition is a drop in body fat, according to Medline Plus, a service of the National Institutes of Health, or NIH. In serious cases, bones protrude, cheeks appear hollow, hair falls out and skin becomes inelastic and thin.

Other symptoms of undernutrition include loss of appetite, irritability, unresponsiveness, fatigue and an inability to stay warm. Patients will find it hard to complete everyday tasks due to weakness. More serious complications can also occur, depending on the severity of the calorie deficiency.

Complications and Effects

Life-threatening conditions can develop when undernutrition reaches a severe level, according to the British Nutrition Foundation.

Undernutrition also weakens the immune system, leading to even more complications from infectious diseases. In cases of prolonged caloric deficiency, vital organs such as the liver, heart and lungs may fail. Total starvation, in which there is no food intake at all--is fatal in 8 to 12 weeks. Children under the age of 5 are especially vulnerable to the effects of starvation because they require a proportionally greater caloric intake than adults.


In most cases, undernutrition is treated with a gradual increase in calorie intake, as noted by Merck Manuals Home Edition. Patients at starvation level need to have their consumption carefully moderated. Six to eight small meals a day are the norm. Whenever possible, nutrients should be given by mouth, but those patients who are unable to digest solid food will require liquid supplements. These are administered either through the digestive tract or intravenously. Severe cases of undernutrition require hospitalization.