Introduction

In the developed world today, the number of infectious causes of death is very small. Public health policy of the past has enabled a degree of protection from contagious pathogens through the advent of hygiene and sanitation improvements; vaccination programs and cultural practice shifts (i.e. hygiene) [1]. Life expectancy has risen over time in correlation with these improvements. Currently the biggest threats upon mortality include a range of non-communicable diseases (NCD), which normally present in later adult life [2]. These include cardiovascular disease (CVD) and type 2 diabetes mellitus (T2DM), amongst others. Total burden from NCDs worldwide has been estimated at 63 % of total deaths by the World Health Organisation in 2008 [2]. Additionally the global incidence of diabetes has been projected to increase exponentially over the next 20 years and beyond [2]. The situation is similar in developing regions. NCDs have long been known to contribute significantly to mortality and morbidity even against the backdrop of prevalent infectious disease [2, 3]. It is in the public interest to find ways of reducing the impact in both developed and developing regions. Research is currently being conducted into both preventions and treatments which may alleviate the burden. The aetiologies of these diseases remain the subject of ongoing research however some theories are gaining ground, including the early life origin theory. Public health provides an important context for consideration as the ability to prevent adult chronic disease has implications in the general spectrum of advancing life expectancy. In order to improve knowledge regarding the rising incidence of NCDs and to improve the forum for public health planning, this review aims to discuss the implication and limitations of the development origin hypothesis in a historical context.

The early life origin of health and disease theory

The general theory of the developmental origin of human health and disease is widely known. Published as a theory in 1995, the early life origin theory proposes the foetal and developmental origin of non-communicable disease risk [4]. Current public health policies aimed at CVD and T2DM tend to focus on providing individual lifestyle advice on diet and exercise to reduce the incidence of heart disease and diabetes. In addition there is a focus on medical care and therapeutic interventions such as that experienced in a clinical setting [2]. This relates to the theory of risk determined by adult life style. In comparison, the early life origin hypothesis suggests that the earliest stages of life convey significant CVD and T2DM risk. In particular focus, the theory has examined the correlations between low birth weight and adult onset of these diseases. Evidence produced has been compelling with a large number of studies across the globe revealing similar results, including the original studies in England [4, 5], comprehensive cohort studies in the Nordic countries [6–9], and more recent studies in non-western countries such as China [10] and also indigenous populations [11, 12]. Studies have generally correlated low birth weight, as a result of slowed foetal growth, to be an important indicator in the development of CVD and T2DM. More recent studies have aimed to address limitations in early studies, including the finding that birth weight alone may not be a significant marker in the development of adult onset disease, as well as the uncertainty in low cohort follow-up studies [9–11]. For example, the rate of foetal development was identified as a relatively better indicator of disease rather than birth weight alone [11]. Targeting the limitations of the research remains an ongoing factor in order to completely elucidate the importance of the developmental origin of health and disease hypothesis, and to determine its place within the risk spectrum [13].

Epigenetics

The early life origin hypothesis has been advanced by the field of epigenetic science. Epigenetics is the study of how changes in phenotype beyond the DNA sequence alone can be inherited and has been associated with an incredible range of adult chronic disease [14]. Epigenetic mechanisms generally describe regulators of gene expression such as DNA methylation status, histone post-translational modifications, and interference by miRNA (Fig. 1). The term ‘Epigenetics’ developed through the mid 20th century in response to the philosophical ideas surrounding genetic heritability combined with the Aristotelian idea of ‘Epigenesis’ [15].

Fig. 1
figure 1

Epigenetic mechanisms involves the modification of gene expression without modification to the DNA sequence. Epigenetic mechanisms include DNA methylation, histone post-translational modifications and interference by miRNA. The chromatin proteins associated with DNA or histones may be activated or silenced. Post translational modification of the amino acids that make up histone proteins include acetylation by histone acetyltransferases (HATs) or histone deacetylases (HDACs) or methylation at specific sites on the histone tails. DNA methylation occurs via DNA methyltransferses (DNMT) which utilises S-adenosyl l-methionine (SAM) as the methyl donor, mostly at CpG sites to convert cytosine to 5-methylcytosine

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Nature versus nurture has long been considered for their influence on NCD incidence rates. Genetic technologies have enhanced our ability to examine the underlying genetic causes but fail to adequately account for the observed incidence of CVD and T2DM alone. In addition, environmental consequences examined through epidemiology have supported the social and lifestyle effects of disease, but has also been subject to limitations due in part to the uncertainty between correlation and causation. The rapid changes in incidence of NCDs, particularly T2DM, produces an apparent disparity between genetic or environmental factors within population groups [16]. It is likely that this may be at least partially explained through epigenetics [17–20]. The epigenetic basis of disease has gained strength over the past two decades, with research expanding in this field to determine its significance in disease heritability. In particular the molecular basis for the involvement of epigenetics in disease has been evaluated in a number of animal models, including an extensive evaluation of the effects of methylation of the Agouti gene in mice, with studies revealing epigenetic methylation patterns can directly determine the risk of disease phenotype development [18, 21–23]. Therapeutic interventions aimed at favourably altering epigenetic profiles in this model have been promisingly explored [24, 25].

In an evolutionary context, adaptation responses to short term environmental cues has given rise to the adaptive developmental plasticity model that proposes a critical role for epigenetics in appropriating phenotypic development [26]. Within the scope of developmental biology, this model assists in bridging the gap between Darwinian evolution and individual phenotypic adaption. The concept of environment-phenotype mismatch as a result of this plasticity, representing the disparity between the energy environment of the foetus in utero and that experienced postnatally [27], has the potential to validate the mechanisms of the model. This would occur by highlighting the importance of the foetal environment in the determination of the overall gene expression profiles through to adulthood. This is in addition to the established body of evidence demonstrating how the maternal environment impacts the foetus’ birth weight, for example through nutrition and smoking [28–30] (Fig. 2).

Fig. 2
figure 2

The developmental origin of human health and disease. A summary of the early life origin of health and disease hypothesis which involves the role of maternal health and lifestyle in impaired foetal development leading to the increased risk of cardiovascular disease and type 2 diabetes mellitus in adulthood

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Influence on public health

In addition to the volume of biomedical research, the developmental origin theory has been well established and is generally accepted in the field of economic science as outlined in a review by Almond and Curri [31]. This has been an important area due to the financial burden of health, and the projected dominance of medical intervention costs in the future [2]. Economics has long been closely associated with public health interventions. Unfortunately the system is subject to uncertainty in expenditure due to the large variation in ideologies when it comes to intervention and treatments which affect mortality [31]. In the past, significant public health achievements have been found to correlate to increases in life expectancy [1]. These have often but not always been spurred by scientific research, with current public health interventions mostly aimed at altering diet and exercise at the individual level [2].

Overall, the evidence both from epidemiological and epigenetic studies has established the early life origin hypothesis as an important consideration when understanding the risk factors for CVD and T2DM [32]. Indeed, it has been proposed that epigenetic mechanisms have a likely involvement in the developmental origins of health and disease, with the requirement of epidemiology to explore and identify associations between early life exposures, changes in epigenetic regulation, and disease [33]. Research in both fields is ongoing in the hope of further elucidating the correlations and causations involved in what are now a significant cause of mortality and morbidity in our community. Acceptance of the early life origin theory has the potential to influence the management of disease risks associated with early life development via careful public health planning.

Maternal health

Maternal health and lifestyle choice is one area of public health which has been influenced by foetal and early life development theories. This has been more of a cultural change rather than a legislative change with negative social views of smoking, alcohol, stress and a positive view for nutritional supplementation during pregnancy [34]. Studies have also examined the role of both under and over nutrition in the context of foetal development, and have identified a large range of mechanisms correlating to birth weight which produces the effects of the developmental origins theory for both CVD and T2DM [8, 30, 35–37]. In addition, birth weight and diabetes risk within aboriginal populations have been examined and have highlighted the importance of nutritional intervention in influencing the foetal development risk [11, 12]. The role of maternal diabetes status has also been correlated to the risk of development of T2DM and CVD for both small and large for gestational age (SGA and LGA) children representing an important area for public health interventions [38]. Nutritional studies have previously examined data from the prominent Dutch Hunger Winter study of the negative impact of maternal famine on offspring’s adult disease profile [39]. A comparative study in Germany has examined the economic impact of a corresponding war environment on intergenerational economic outcomes [40]. Maternal obesity is also a major concern, with increased risk of both short-term and long-term factors involved in foetal and early life development [41]. For example, a recent study involving over 8,400 children in the USA reported that children born to obese mothers were twice as likely to be obese by 2 years of age [42]. These studies, amongst others [19, 43], highlight the importance of the maternal environment for later life disease as tracked through historical studies, and provide a potential point for intervention strategies to impact NCD incidence into the future. This is particularly intriguing considering the role of epigenetic determination in utero influencing later disease risk.

Beyond this, developing regions may be an important consideration for the developmental origin hypothesis in public health planning. This arises from the situations where by poor and vulnerable populations currently experiencing, or recently experienced maternal nutritional inadequacies will be paired in the future with the incorporation of westernised diets [30]. It has been predicted that this could underlie an increase in NCDs in the future following the foetal/postnatal energy environment mismatch theory [44, 45]. The knowledge that the mother’s life situation can impact her offspring’s life expectancy highlights the importance for consideration of the developmental origin of disease in public health policy.

Criticism in context

The early life origin theory is just one of the many factors thought to contribute to the development of cardiovascular disease. As yet however, the developmental origin hypothesis remains subject to criticism and overall influence on public health remains low. The most prevalent risk factor associated with disease involves the consumption of dietary fats and cholesterol, as well as obesity in the development of CVD, and as a result many public health campaigns exist which encourage individuals to reduce their dietary fat intake. This can be seen in the advent of low fat and low cholesterol products in the food industry. The evaluation of these risk factors has itself been reviewed with additional aspects now revealing significant involvement. These include the role of smoking, stress, and salt and sugar intake in raising individual risk which are all included in the WHO reports [2]. These factors are of public interest considering the evidence for both correlation and causation with NCDs. The difference between the developmental origin hypothesis and the lifestyle factors of CVD aetiology is that the latter are well established risks, and are more easily applied to the current intervention methodology on a case-by-case basis. In contrast, the early life origin hypothesis involves thinking for the future and targeting a particular group of people (future mothers) who may not show obvious symptoms of disease themselves. This may explain some of the criticism, and one reason why this hypothesis is not as prominent as other factors studied for intervention. The role of maternal diet, stress, and smoking however have all been related to restricting foetal growth, and thus birth weight, which is the major focus within the early life origin theory [23, 28]. In addition, maternal obesity or overnutrition before or during pregnancy may also result in foetal growth restriction, and increased risk of neonatal mortality and morbidity in humans [30, 46]. One other prominent criticism is that the hypothesis should not be an individual theory but rather form part of the overall lifetime “weathering” hypothesis [19, 43]. This purports that all life stages contribute significant cumulative risk for disease outcomes in later life.

Further considerations

By far the most criticism of the early life origin hypothesis comes from the correlating effect of birth weight with low socioeconomic status (SES). Here the idea of causation is removed from birth weight alone and instead placed upon other factors associated with low income. How can we be sure that slowed foetal development increases the risk of heart disease and diabetes when in general these particular groups of people are already more vulnerable? This effect has been reviewed with varying results, but generally appears to support the developmental origin hypothesis [6, 31]. Factors associated with low SES have been independently linked to causing low birth weight including the previous examples of smoking and nutrition [11, 28, 35]. The direct influence of the early life origin theory within this social context is still being elucidated regarding the underlying mechanisms of perceived correlation in vulnerable groups with NCD risk. Studies in both epidemiology and epigenetics, with an interdisciplinary approach, will enable a greater understanding of the causation of risk factors independent to SES.

Conclusion

The rising burden of Non-Communicable Disease upon life expectancy and life quality has contributed to the current research aims of evaluating effective prevention. The early life origin of health and disease theory is continuing to emerge as an important consideration amongst the current prevailing contributing factors to adult disease, with the potential for important implications in future policy planning and public health interventions. This has been considered most important for the world’s poorest and most vulnerable populations due to associated risks. It is possible that additional public health interventions assisting maternal health may help alleviate the burden of Non-Communicable Disease for subsequent generations. If we can better understand the causes of death then perhaps we can find better prevention strategies. The theory of the developmental origins of adult chronic disease, through the combination of epidemiology and epigenetics, may be an important contribution to the advancement of current life expectancy. To date, the developmental origin theory of human health and disease remains as only one part in the overall perceived spectrum of risk.