Depression - Explained

The World Health Organization defines depression as "a common mental disorder that presents with depressed mood, loss of interest or pleasure, feelings of guilt or low self-worth, disturbed sleep or appetite, low energy, and poor concentration". [1]

Changes in mood can be perfectly healthy. However, depression is set apart by a more pronounced change in mood, that lasts for a longer period of time, as well as additional symptoms such as lethargy.[2]

How common is depression?

Figures from the World Health Organization indicate that depression affects 121 million people all over the world. It is one of the commonest causes of disability worldwide.[1]

Around one in 7 people are affected by depression severe enough to need treatment during their lifetime.[3] It is an episodic condition, and recurrence is common.[4]

Despite the prevalence of depression, fewer than one in four people who have the condition are treated for it properly. There are concerns that this figure is even lower in some countries. Lack of sufficient resources and fear of social stigma are both barriers to effective treatment.[1]

What causes depression?

There has been much research over the underlying causes of depression over the years.

Possible theories that have been put forward suggest changes in:

There are also different types of depression that may affect symptoms and treatment.

Neurotransmitter levels

Treatments for depression tend to increase levels of neurotransmitters in the brain, that is, substances that are involved in signalling between nerveBundle of fibres that carries information in the form of electrical impulses. cells. This finding led to the belief that people with depression have low levels of the neurotransmitters 'serotonin', and 'norepinephrine'. However, these medications do not work for everyone; because of this, it is likely that other factors are also involved in the development of depression.[5,6]

Structure of parts of the brain

Specific parts of the brain are thought to regulate mood (examples include the 'prefrontal cortexThe outer region of an organ.', the 'amygdala' and the 'hippocampus'). Sometimes, changes in these parts of the brain are associated with depression - for example, scanning the brain has shown that some people with depression have a smaller amygdala or hippocampus.[4,5] In fact, psychological stressRelating to injury or concern. (which is associated with a higher risk of depression) can be associated with reductions in the number of nerveBundle of fibres that carries information in the form of electrical impulses. cells, or neurones, in the hippocampus.[6] However, not all people with depression will have changes in these areas.[4,5]

Blood flow to the brain

Changes in bloodA fluid that transports oxygen and other substances through the body, made up of blood cells suspended in a liquid. flow to the brain may also be associated with the development of depression, with either increased or decreased flow to certain areas. It is thought that these changes may underlie the symptoms of depression, including anxiety, apathy and memory loss.[4]

Hormone levels

Changes in hormones may also contribute: people with depression almost always have high levels of cortisolA steroid hormone important for helping to regulate carbohydrate metabolism and the stress response., a type of stressRelating to injury or concern. hormoneA substance produced by a gland in one part of the body and carried by the blood to the organs or tissues where it has an effect..[4,5]

Types of depression

There are various different types of depression:[3]

  • Major depressive disorder: Five or more symptoms of depression are experienced over a period of two weeks. These symptoms include depressed mood, changes in weight and sleep patterns, lack of interest and feelings of guilt. Read more about the symptoms of depression
  • Adjustment disorders: These tend to be mild, short-lived episodes of depression
  • Dysthymia: The occurrence of symptoms of depression that are persistent but are not severe enough to diagnose major depression
  • Bipolar disorder: Episodes of major depression associated with episodes of mania
  • Postpartum depression: Depression occurring a month or more after childbirth
  • Seasonal affective disorder: Recurrent major depression that occurs seasonally, usually in the winter months.[7]


  1. accessed January 5, 2010.
  2. accessed January 5, 2010.
  3. Last accessed July 14, 2010
  4. Maletic V, Robinson M, Oakes T et al. Neurobiology of depression: an integrated view of key findings. Int J Clin Pract 2007; 61: 2030-40.
  5. aan het Rot M, Mathew SJ, Charney DS. Neurobiological mechanisms in major depressive disorder. CMAJ 2009; 180: 305-13.
  6. Remick RA. Diagnosis and management of depression in primary care: a clinical update and review. CMAJ 2002; 167: 1253-60.
    1. accessed January 6, 2010.
    2. Pearlstein T, Howard M, Salisbury A et al. Postpartum depression. American Journal of Obstetrics & Gynecology 2009; 200: 357-64.
    3. Michalak EE, Murray G, Wilkinson C et al. A pilot study of adherence with light treatment for seasonal affective disorder. Psychiatry Research 2007; 149: 315-20.
    4. Enns MW, Cox BJ, Levitt AJ et al. Personality and seasonal affective disorder: results from the CAN-SAD study. J Affect Disord 2006; 93: 35-42.