https://pmc.ncbi.nlm.nih.gov/articles/PMC1121241/

Biological and social systems are inherently complex, so it is hardly surprising that few if any human illnesses can be said to have a single “cause” or “cure.”1 This article applies the principles introduced in the introductory article in this series2 to three specific clinical areas: the control of blood glucose levels in diabetes, the management of diagnostic uncertainty, and health promotion.

A complex adaptive system is a collection of individual agents with freedom to act in ways that are not always totally predictable, and whose actions are interconnected so that the action of one part changes the context for other agents.2 In relation to human health and illness there are several levels of such systems.

• The human body is composed of multiple interacting and self regulating physiological systems including biochemical and neuroendocrine feedback loops
• The behaviour of any individual is determined partly by an internal set of rules based on past experience and partly by unique and adaptive responses to new stimuli from the environment
• The web of relationships in which individuals exist contains many varied and powerful determinants of their beliefs, expectations, and behaviour
• Individuals and their immediate social relationships are further embedded within wider social, political, and cultural systems which can influence outcomes in entirely novel and unpredictable ways
• All these interacting systems are dynamic and fluid
• A small change in one part of this web of interacting systems may lead to a much larger change in another part through amplification effects.

For all these reasons neither illness nor human behaviour is predictable and neither can safely be “modelled” in a simple cause and effect system.3 The human body is not a machine and its malfunctioning cannot be adequately analysed by breaking the system down into its component parts and considering each in isolation. Despite this fact, cause and effect modelling underpins much of the problem solving we attempt in clinical encounters; this perhaps explains why we so often fail.4

Summary points

• Human beings can be viewed as composed of and operating within multiple interacting and self adjusting systems (including biochemical, cellular, physiological, psychological, and social systems)
• Illness arises from dynamic interaction within and between these systems, not from failure of a single component
• Health can only be maintained (or re-established) through a holistic approach that accepts unpredictability and builds on subtle emergent forces within the overall system
• Three examples of complex situations (glycaemic control in diabetes, uncertainty in clinical diagnosis, and health promotion) illustrate that these principles can provide the key to practical solutions in clinical care

Glycaemic control in diabetes

Figure 1 shows a page from the diary of a man with type 1 diabetes. It includes biomedical details (blood glucose concentrations, insulin doses given), physiological inputs (meals and snacks) and outputs (exercise), social events (a party), pathological states (vomiting), and clinical encounters with health professionals (an appointment with his general practitioner and a phone call to the nurse). It gives a flavour of the complex interplay between physiology and behaviour and of the huge distance between the health professional in the clinic and the patient's experience of day to day control of his blood glucose. Even though this record is more detailed than most, it is still a woefully incomplete source of data from which to attempt to predict the course of the patient's blood glucose level and to advise on insulin dosage or dietary modification.

Figure 1.