New integrated stress research – the concept of allostatic overload
Long-term overtaxation of the physiological adaptability of human beings may lead to health impairment. This phenomenon, called «allostatic overload» (31) – (33), is a consequence of physiological «wear and tear» due to strong and/or persistent threats to an individual’s existence or integrity (the word encompasses both mental and physical aspects). The human body’s reaction to stressors, which in our culture can be classified as physical (e.g. undernourishment, overfeeding, malnutrition, pollution, lack of sleep, lack of exercise, infections, noise) prove to converge at the same biological «level» as stressors we would classify as psychosocial (e.g. a life characterised by threats, neglect, abuse, poverty or overwhelming caregiving burdens): Both categories of stress can contribute over time to the development of autonomic dysfunction, changes in the immune system, chronic low-grade inflammation, endocrine disruptions and accelerated cell aging, measured as telomere shortening (Box 1).
Allostatic overload may be quantified by measurement of autonomous, hormonal, immunological and genetic variables:
Cardiovascular readings (blood pressure, pulse rate etc.)
Lung function tests (PEF, spirometry)
Hormone levels (cortisol, DHEA-S, A, NA etc.)
Inflammation markers (CRP, IL-6, TNF, fibrinogen etc.)
Glucose metabolism (blood sugar, HbA1c)
Lipid metabolism (TG, HDL, LDL, etc.)
Obesity measures (WHR, BMI etc.)
Chromosome-related variables: telomere length and telomerase level
Biological parameters that form part of an evaluation of allostatic load in the scientific models and studies that have been discussed (32). Abbreviations: PEF – peak expiratory flow; DHEA-S – dehydroepiandrosterone sulphate; A – adrenaline; NA – noradrenaline; CRP – C-reactive protein; IL-6 – interleukin 6; TNF – tumour necrosis factor; HbA1c – glycosylated haemoglobin; TG – triglycerides; HDL – high density lipoprotein; LDL – low density lipoprotein; WHR – waist-hip ratio; BMI – body mass index
Chronic metabolic disturbances related to allostatic overload appear to contribute, at least partly through epigenetic mechanisms, to the development of the major, complex diseases of our time – cardiovascular disease, overweight, diabetes, chronic obstructive pulmonary disease, other immunological and inflammation-driven diseases, osteoporosis, depression, anxiety, so-called behavioural disorders, substance dependence and other complaints classified as psychiatric, cognitive complaints, adverse pregnancy outcomes in the form of premature births and low birth weight, pain syndromes and chronic fatigue etc. (31) – (37). The concept allostatic overload thus enables an entirely new understanding of composite pictures of biological defects and failures which have traditionally been perceived as concomitant, but different disorders and diseases; in other words so-called co- or multimorbidity (38). The overall degree of health hazardous stress, expressed in terms of loss of control and lack of mastery of one’s own life (39), can well be considered in light of the individual’s rank in the social hierarchy. In combination, epigenetics and the allostasis concept may contribute substantially to a deeper understanding of the social gradients in morbidity and mortality (40). The extent to which social gradients impact people’s health is well documented, for instance by the epidemiological Whitehall studies in England (41). Through the concepts of epigenetics and allostasis, the well-established epidemiological concept «social gradients in health», has acquired a pathophysiological counterpart, expressed in the term «the biology of disadvantage» (42).
Recent trauma research provides insight into how detrimental childhood experiences may lead to toxic stress (36) which is reflected in disturbed maturation of brain structure in the developing individual. Neuroimaging studies have linked such structural changes (gray and/or white matter alterations) to experiences of verbal abuse, harsh corporal punishment and sexual abuse (43). The plasticity and vulnerability of the brain appear to vary with the individual’s age; the detrimental potential of adverse experiences will depend on when they occur and how long they last (43).
The brain appears to remain receptive to relational effects throughout life. Fortunately, favourable brain changes can also occur, in response to experiences which promote healing and health (salutogenesis). A number of studies have documented structural changes in the brain associated with successful psychotherapy (44). On a more general level, a recent meta-analysis demonstrated that strong social relations provide as good protection against premature death as smoking cessation, and this statistical protection is numerically greater than the risk associated with, for example, obesity and physical inactivity (45). This protective effect is independent of age, gender and a number of other factors, and is therefore unlikely to be limited to sub-groups.