Fatigue (Low Energy Levels) Meaning and Causes

Low energy levels is one of the most common symptoms that we all experience in life, sometimes without finding any possible cause. It often starts just before an illness, continues during the course of an illness and even lingers after the illness resolves. Sometimes low energy levels can continue for weeks, months and even years and in certain cases the lack of energy is the only symptom that is present. Needless to say, this can be a confusing symptom to diagnose at times.

What is low energy?

Low energy simply means that we feel tired or fatigued. Normally we associate this with a physical state. The term is often used loosely to describe certain emotional and psychological states, from feeling demotivated and unenthusiastic to depression. A lack of energy can be relative and it is difficult to measure energy levels or lack thereof in an objective manner, like with blood tests.

Energy and Metabolism

Energy is associated with metabolism. A major part of metabolism is to break down nutrients to yield energy which is needed for every cell to sustain life. There are various other and equally necessary processes that occur within a cell to sustain life and therefore fall within the term ‘metabolism’ even if energy is not directly yielded. However, on a broader level our metabolism alone does not account entirely for our energy levels.

Defining energy levels as we feel it can be difficult. We say that we feel energetic when we are functioning at our peak without any tiredness. We also use the term energetic to describe our psychological state especially when we are excited. Therefore being energetic is a heightened state of functioning where as low energy levels is a diminished state. Apart from the metabolic rate, a host of other factors can contribute to this heightened state.

Causes of Low Energy Levels

It is important to understand that any physical or psychological condition can lead to low energy levels. Momentary lack of energy is not a problem. In fact we all experience low energy levels at the end of a day especially after a very active day. However, this tiredness is easily remedied with good nutrition and sleep. Therefore low energy levels is the diminished state that is not overcome with these simple measures.


Lifestyle factors are among the more common causes of unusual tiredness and fatigue. While overactivity can lead to tiredness, leading a sedentary lifestyle can also contribute to low energy levels. Even reduced exposure to the outdoor environment during daylight hours can be a factor as sunlight is required for vitamin D synthesis. People who are overweight or obese and consume large amounts of alcohol and tobacco are also more likely to experience low energy levels.


Almost any nutritional deficiency can lead to low energy levels depending on the severity and duration of the deficiency. Malnutrition may not always be prolonged which leads to deficiencies. Even one day of not eating properly will lead to low energy levels. Therefore people who are on strict diets, fasting or missing meals will often experience tiredness and fatigue. It is also a major symptom in eating disorders.


Sleep, or the lack thereof, is another major cause of low energy levels. Insufficient sleep or disturbed sleep prevents the body from recuperating. Naturally a person will tend to feel less energetic the next day but the energy levels will restore with proper sleep cycles. In sleep apnea, a person ceases breathing for short periods of time while asleep. Despite what seems like a full night’s sleep, a person tends to feel very tired the next day.


Fatigue is a common symptom of infections. Often it precedes the onset of symptoms, as is the case with the flu where a person may feel a lack of energy despite having no ther symptoms. The low energy levels may persist for a while after the infection resolves as the body recuperates from the illness. Symptoms like fatigue and fever can occur for short periods after exposure to vaccines but this reaction is not an actual infection.


Cardiopulmonary simply refers to the heart (cardio) and the airways and lungs (pulmonary). It plays an integral role in metabolism. The airways and lungs facilitate oxygen intake and carbon dioxide expulsion. The heart circulates nutrients and oxygen throughout the body. Therefore fatigue may be prominent in conditions such as:

  • Heart failure where the heart is weakened and cannot circulate blood properly.
  • Heart attack where a portion of the heart wall dies and the heart is weakened.
  • Heart valve disease where the proper flow of blood is impaired by diseases valves in the heart.
  • Pneumonia which is an infection of the lung(s) and may be associated with airway infections like bronchitis.
  • Asthma where the bronchi become narrowed due to spasms and impeding airflow.
  • COPD (chronic obstructive pulmonary disease) where airflow is impaired often as a result of long term cigarette smoking.

The blood is an important component in allowing heart and lung activity to have an effect throughout the body. Therefore low energy levels are common in conditions like anemia where the blood’s capacity to carry oxygen is diminished.


The hormones and glands (endocrine system) have a number of different roles in the body. Several hormones play a role in metabolism, including the digestion, absorption and utilization of nutrients and energy prodution by cells. The main hormonal conditions where low energy levels are prominent include:

  • Hypothyroidism where the thyroid hormones which regulate energy production are low.
  • Diabetes mellitus where the blood glucose levels are high since the cells cannot properly take glucose from the blood into the cells for energy production.
  • Cushing syndrome and Addison’s disease where there are abnormalities of hormones produced by the adrenal glands that sit at the top of the kidneys.


Due to the vast number of possible causes of low energy levels, it is difficult to mention all the causative conditions. Therefore some of the other conditions where low energy levels are prominent includes:

  • Autoimmune conditions
  • Cancer
  • Chronic fatigue syndrome
  • Dehydration
  • Depression
  • Hemorrhage (bleeding)
  • Kidney disease
  • Medication side effects
  • Menstrual abnormalities
  • Poisoning
  • Substance abuse
  • Trauma, including surgery

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