Introduction

A family history of diabetes is associated with a range of metabolic abnormalities [1] and is a strong risk factor for the development of type 2 diabetes [2–4]. It is likely that this elevated risk of type 2 diabetes is mediated, in part, by both genetic and shared environmental components among family members, but the precise factors accounting for this increase in risk are poorly understood. Anthropometric and lifestyle-related risk factors such as BMI, waist circumference and physical inactivity are major risk factors for type 2 diabetes [5–7], and the aggregation of such traits among families [3, 8] may account for a portion of the excess risk attributable to family history. However, adjustment for these factors in a previous study of women left most of the association between family history and type 2 diabetes risk unexplained [3].

A number of common genetic variants have recently been shown to be associated with type 2 diabetes [9], although the addition here of genetic risk scores for up to 20 variants associated with type 2 diabetes provided little improvement over the performance of type 2 diabetes prediction models already containing family history [10–12]. Although a recent study observed a weak association between the number of parents with diabetes and a genetic risk score [13], it still remains unknown whether common genetic variation associated with type 2 diabetes explains any of the family history-associated risk.

We therefore investigated the association of a family history of diabetes in different family members and age of familial diagnosis to risk of type 2 diabetes in a large prospective case-cohort study of European individuals. We examined the extent to which the increased risk associated with a family history was mediated by anthropometric, lifestyle and genetic risk factors. We also investigated the effect of interactions between these factors and family history on risk of type 2 diabetes to establish whether family history acts as an independent risk factor over and above these novel genetic and conventional risk factors.

Methods

Participants and study design

The InterAct study [14] is a large, prospective case-cohort study involving 27,779 individuals from eight European countries, which is nested within the European Prospective Investigation into Cancer and Nutrition (EPIC) [15]. The InterAct study, drawn from a total cohort of 340,234 individuals comprising 3.99 million person–years of follow-up, was designed to investigate the interplay between genetic and lifestyle factors and risk of type 2 diabetes. It comprises 12,403 incident cases of type 2 diabetes and a representative subcohort (n = 16,154) that also includes 778 of the 12,403 incident cases. Family history data were available for 13,869 individuals from six countries, including 6,168 incident cases, while 6,887 participants had a full availability of mediators and were included in mediation analyses. Written informed consent was obtained from all participants, and all study protocols were approved by local ethics committees.

Ascertainment and verification of cases of type 2 diabetes

Briefly, and as previously described in the paper outlining the InterAct study [14], all InterAct participants were free of known diabetes at baseline. The ascertainment of incident type 2 diabetes involved a review of existing EPIC datasets using multiple sources of evidence. Incident cases in Denmark and Sweden were identified via local and national diabetes and pharmaceutical registers. For other centres, we sought further verification from no fewer than two independent sources, including an individual medical records review at some centres. Information from any follow-up visit or external evidence with a date later than the baseline visit was used. Follow-up was censored at the date of diagnosis, 31 December 2007 or date of death, whichever occurred first. If date of diagnosis could not be ascertained from any of the sources listed above (n = 310), the midpoint between recruitment and censoring was used.

Baseline measurements in the EPIC-InterAct study

Briefly, height, weight, waist and hip circumferences were collected at baseline as previously described [16]. Physical activity was assessed using a valid brief questionnaire covering occupational and recreational activity levels [17]. Smoking status and level of education reached were also ascertained by questionnaire. Adherence to a Mediterranean diet pattern was assessed by food frequency questionnaire as previously described [18], and a blood sample was taken from which DNA could be extracted. Baseline characteristics from centres that ascertained family history were reported both overall and by country (see electronic supplementary material [ESM] Table 1).

Family history

A family history of diabetes was ascertained by questionnaire at follow-up, unless otherwise indicated, for six countries: France, the UK, the Netherlands, Germany, Sweden and Denmark. Family history data were not available in Italian or Spanish centres, nor at the Oxford or Heidelberg centres, and these were excluded from analyses.

In France, participants were asked to report a history of diabetes in their mother and father separately, and the age category (<55, 55–59, 60–64, 65+ years or unknown age) at which it was diagnosed. At baseline in the UK, information concerning the presence of a family history and age at diagnosis for the mother, father or siblings was requested. At baseline in the Netherlands, family history and age at diagnosis for either the mother or the father was ascertained. At follow-up, further information was collected on family history and age at diagnosis for siblings. For the Netherlands and the UK, those with familial diabetes diagnosed after the age of 20 were considered to have a positive family history of type 2 diabetes. For individuals who reported family history information at baseline and at follow-up, we used the follow-up information. In Germany, the presence of diabetes in the mother, father or siblings and the age category at which it was diagnosed (<30 years, 30–60, older than 60 years) were ascertained. Those with a family member diagnosed after age 30 were considered to have a family history. Similarly, in Denmark the presence or absence of diabetes in the mother, father or siblings and whether it was diagnosed before or after age 35 was collected. Those with a familial diagnosis after age 35 were considered to have a family history. In Umeå, Sweden, participants were asked a question on the presence or absence of diabetes in their parents or siblings, which allowed us to establish only the presence of diabetes in a first-degree relative. Individuals from Umeå were therefore excluded from analyses including categorical family history information. In Malmö, the presence of a family history was ascertained for the mother, father and siblings separately. The availability of family history data by country is shown in ESM Table 2. In centres that ascertained the family history, we compared risk factors for type 2 diabetes among responders and non-responders to the question by linear or logistic regression adjusted for age, sex and centre. This analysis was performed overall (and not by country) given the low level of missing data in some countries.

From the above variables, the primary exposure variable of a family history in any first-degree relative was constructed. Where possible, we also classified individuals as having different, non-exclusive degrees of family history: parental, sibling, maternal, paternal or biparental. In countries and centres with data on both parents and siblings separately, a variable was also constructed to count the number of family members reported to have had diabetes.

Genotyping and genetic risk score

The genotyping of 35 single nucleotide polymorphisms (SNPs) associated with type 2 diabetes [9, 19–22] (ESM Table 3) was undertaken using a Sequenom iPLEX array (Sequenom, San Diego CA, USA) or TaqMan (Applied Biosystems, Carlsbad, CA, USA). Those homozygous for the risk allele at each locus were dummy-coded as 2, heterozygotes as 1, and those carrying no risk alleles as 0. A genetic risk score was constructed by summing the number of risk alleles. To maximise sample size, missing genotypes were imputed by assigning the mean genotype at each locus for cases and non-cases separately for individuals successfully genotyped for at least 30 of the 35 loci. In total, 2,272 individuals had a part of their genetic score imputed, although in the vast majority of these individuals (77%), only one of the 35 SNPs was imputed. No genotyping data were available for individuals from Denmark (n = 3,068).

Statistical analysis

To estimate the association between family history and incident type 2 diabetes, Prentice-weighted Cox regression models with age as the underlying timescale were fitted within countries, and HRs were combined using random effects meta-analysis [23, 24]. For interactions, effect estimates from the relevant product terms were also meta-analysed as above. Unless otherwise specified, all models were adjusted for sex and centre. When the number of affected family members was included in a model, we did not stratify by country owing to the low number with more than two affected family members. In analyses comparing the association of family history with type 2 diabetes by age of familial diagnosis, only the UK and the Netherlands had available information on the actual age at diagnosis. For analyses of the association of family history with type 2 diabetes by age of participant diagnosis, we compared the association between family history and type 2 diabetes using cases diagnosed at or before and after the age of 60 years (1,862 and 4,306 cases, respectively).

Comparisons of quantitative characteristics between subcohort participants with and without a family history were performed by linear regression, and for binary variables by logistic regression, adjusted for age, sex and centre.

We included BMI, waist and hip circumference, diet, physical activity, smoking status, level of education and a genetic risk score as potential mediators of the family history-associated risk of type 2 diabetes. In mediation analyses, the sample was restricted to those with a full availability for each mediator. In full case-cohort analyses, the proportion of the family history association mediated was calculated as \( {1} - \left[ {{\text{lo}}{{\text{g}}_{\text{e}}}\left( {{\text{H}}{{\text{R}}_{\text{adjusted model}}}} \right)/{\text{lo}}{{\text{g}}_{\text{e}}}\left( {{\text{H}}{{\text{R}}_{\text{crude model}}}} \right)} \right] \) [25]. We performed additional mediation analyses including only individuals in the subcohort by logistic regression, as previously described [26]. Briefly, this method allowed an estimation of the indirect effect of family history (equal to the product of the family history-mediator and mediator-diabetes coefficients), the direct effect (i.e. that mediated directly by family history, independent of the mediator[s]) and the total effect of family history (i.e. unadjusted for any potential mediators).

We also investigated the added value of including the family history or the genetic score to a basic model containing age, sex and BMI to predict the incidence of diabetes using logistic regression. The effect of adding further variables to the basic model was assessed by comparison of the respective AUCs [27]. The inclusion of a categorical family history variable (individuals having zero, one, two or three family members with type 2 diabetes) was compared with the conventional family history classification of having either no or any family history of the disease. We also investigated the effect on predictive performance of adding either the genetic score or a single question on the presence or absence of family history to the model containing age, sex and BMI.

Results

Individuals with a family history of diabetes in any first-degree family member were at higher risk of type 2 diabetes (HR 2.72, 95% CI 2.48, 2.99), I 2 = 15.7 (p heterogeneity = 0.31) (Fig. 1, ESM Fig. 1). Although the proportion of individuals answering questions on family history and reporting a positive family history differed between countries (ESM Table 4), between-country heterogeneity for the association of family history with type 2 diabetes was low for all degrees of family history (I 2 ≤ 20%). The presence of diabetes in different family members was associated with a similar HR of type 2 diabetes (Fig. 1), although having a biparental family history was associated with higher risk (HR 5.14, 95% CI 3.74, 7.07). Having any one family member with type 2 diabetes was associated with a 2.5-fold increase in risk of type 2 diabetes (HR 2.56, 95% CI 2.41, 2.72), whereas having two (HR 3.99, 95% CI 3.58, 4.43) or three family members (HR 5.73, 95% CI 4.28, 7.67) with type 2 diabetes was associated with an even higher risk. We observed no difference in the association between family history and type 2 diabetes when it was ascertained at baseline or at follow-up (p heterogeneity = 0.44). In centres that ascertained family history, individuals with that data missing were younger and more likely to be male. After adjustment for age, sex and centre, non-responders also had a higher BMI (difference in means 0.53 kg/m2, 95% CI 0.32, 0.74) and hip (0.47 cm, 95% CI 0.05, 0.88) and waist circumference (1.54 cm, 95% CI 1.00, 2.09). However, we observed no difference in incidence of type 2 diabetes between responders and non-responders after adjustment for BMI (HR 1.04, 95% CI 0.98, 1.11); (p = 0.23).

Fig. 1
figure 1

Association of degrees of family history with risk of type 2 diabetes in men, in women and overall: the EPIC-InterAct study. Referent groups were individuals with no reported family history of diabetes. aOnly the UK and the Netherlands had sufficient data available on age at diagnosis of parental diabetes, so these analyses are restricted to those two countries

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Parental diagnosis at or before age 50 years was associated with a higher risk of type 2 diabetes (HR 4.69, 95% CI 3.35, 6.58) than parental diagnosis after age 50 (HR 2.61, 95% CI 2.17, 3.14; p heterogeneity = 0.003) (Fig. 1). This effect was largely confined to a maternal family history: maternal diabetes diagnosed at or before the age of 50 (median age of diagnosis 45 years) was associated with a higher risk (HR 6.00, 95% CI 3.94, 9.12) than maternal diagnosis after age 50 (HR 2.67, 95% CI 2.16, 3.30; p heterogeneity = 0.001). There was a suggestion that family history had a greater association with diabetes risk in InterAct participants diagnosed before 60 years of age (HR 3.14, 95% CI 2.57, 3.84) than those diagnosed after age 60 (HR 2.53, 95% CI 2.28, 2. 80; p heterogeneity = 0.058).

After adjusting for age, sex and centre, those with a family history of type 2 diabetes had higher BMI, hip and waist circumferences, had reached lower levels of education, and had a higher genetic risk score (Table 1). The genetic score was strongly associated with the incidence of type 2 diabetes (β per-allele = 1.08, 95% CI 1.06, 1.10; p = 1.5 × 10–17). These risk factors for type 2 diabetes were subsequently considered to be potential mediators of risk associated with family history. Restricting the analyses to those with full availability of data on the above risk factors had little effect on the risk associated with family history (Fig. 2). Adjustment for waist circumference had the largest effect of any single risk factor but explained only 11% of the family history association with risk of type 2 diabetes. Furthermore, adjustment for BMI, waist and hip circumference, smoking, diet, physical activity, education level and genetic risk score together explained only 13% of the risk of type 2 diabetes associated with family history (Fig. 2). In subcohort analyses where we estimated the indirect effect of family history via each mediator, only BMI and waist and hip circumference accounted for a statistically significant portion of the association with family history (Table 2). A larger proportion of the maternal family history association with type 2 diabetes was explained by these risk factors than it was for paternal family history (17% vs <1%; Fig. 2, ESM Table 5). Although the mean genetic score was higher in individuals with a family history (p = 0.002; Table 1), the genetic score alone explained only 2% of the association with family history (Fig. 2, Table 2), and a similarly small proportion of the association with all degrees of family history (ESM Table 5). Despite being a prominent and established risk factor for type 2 diabetes, physical activity level was not associated with family history (Table 1) and as such explained less than 1% of the risk of type 2 diabetes associated with family history (Fig. 2, Table 2).

Table 1 Baseline characteristics by family history status in the subcohort: the EPIC-InterAct study
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Fig. 2
figure 2

Association of family history with type 2 diabetes risk after adjustment for potential mediators in individuals with full availability of mediators. All analyses were adjusted for sex and centre. Models were then individually adjusted for each specified risk factor, and all mediators where specified. The proportion of the family history effect explained by each mediator was calculated as \( {1} - \left[ {{\text{lo}}{{\text{g}}_{\text{e}}}\left( {{\text{H}}{{\text{R}}_{\text{adjusted model}}}} \right)/{\text{lo}}{{\text{g}}_{\text{e}}}\left( {{\text{H}}{{\text{R}}_{\text{crude model}}}} \right)} \right] \)

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Table 2 Mediation analyses in the InterAct subcohort participants with full availability of mediators
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A nominally significant interaction was observed between family history and physical activity (p = 0.048), although inactivity was associated with a higher incidence of type 2 diabetes, regardless of family history (ESM Table 6). Given the absence of interaction, family history in combination with physical activity, genetic risk score and particularly BMI identified individuals at high risk of type 2 diabetes (ESM Table 6). For example, obese individuals with a family history had a 22-fold increased incidence of type 2 diabetes relative to those with no family history and a normal BMI.

Including information on the number of family members with type 2 diabetes and comparing it with a model containing only a yes/no classification of family history, we observed only a very marginal increase in AUC (0.791 vs 0.790; p = 0.039). Inclusion of the genetic risk score in the model again resulted in a very marginal increase in AUC (0.818 vs 0.809; p < 0.001) compared with a model including age, sex, centre, BMI and family history. In addition, in comparisons between adding either a single question on family history information or the genetic risk score to a prediction model (containing age, sex and BMI), the single question resulted in a greater improvement in AUC (0.810 vs 0.792; p < 0.001) than was seen with the genetic risk score (AUC 0.803 vs 0.792; p < 0.001).

Discussion

Family history was associated with incidence of type 2 diabetes with no evidence of heterogeneity between European countries. Individuals with more than one relative with diabetes or with a younger age of maternal diagnosis had an even higher risk. Most of the risk associated with family history was unexplained by major risk factors including BMI and physical inactivity. Although individuals with a positive family history had a mean BMI of almost 1 kg/m2 higher than those without (Table 1), the variation in BMI explained less than 9% of the association between family history and risk of type 2 diabetes, a figure that is less than the 21.1% reported in a previous study involving only women [3]. Our observation that risks associated with maternal and paternal family history were explained to a different extent by established risk factors also hints at a different aetiology of the family history-associated risk in different family members. Overall, adjustment for multiple anthropometric, lifestyle and genetic risk factors explained only 13% of the type 2 diabetes risk associated with family history (Fig. 2), the genetic risk score alone explaining only 2%.

Heritable lifestyle behaviours and anthropometric characteristics such as physical activity [28] and BMI [29] are strong risk factors for type 2 diabetes [5, 6] and good candidates to mediate the family history-associated risk of diabetes. Admittedly, the precision of measurement of physical activity is limited by the large scale of the EPIC project, but even for more easily measured risk factors such as BMI, these factors mediate a small proportion of the association (Fig. 2, Table 2). Although the important role of shared environmental factors is supported by findings that diabetes in a spouse is associated with increased risk [30], the only other study to investigate the mediation of family history-associated risk in a large prospective setting also found that major lifestyle and anthropometric risk factors explained only a minority of the associated risk in women [3]. Previous studies of adoptees found that they had no increased risk of type 2 diabetes with a family history of diabetes in their adoptive parents but showed a sustained increase in risk when their biological parents had diabetes [31], supporting the notion that genetic and/or intrauterine influences may mediate a significant proportion of the family history association. However, the genetic risk score comprising 35 variants associated with type 2 diabetes explained only 2% of the association between family history and risk of type 2 diabetes (Table 1). Admittedly, these common variants explain little of the overall variation in risk of type 2 diabetes [9], which may explain the small proportion of the family history association they appear to mediate. It is likely that currently unknown genetic variation and gene–environment and epistatic interactions account for a proportion of this association. Although practical and computational limitations have to date constrained the large-scale investigation of such effects, overlooking them may contribute to the inability to explain complex traits with apparently large familial components [32]. Ongoing sequence-based efforts to identify rare and low-frequency type 2 diabetes risk alleles should help to clarify this question.

Parental diabetes diagnosed at a younger age was associated with a higher risk of type 2 diabetes (Fig. 1), an effect largely confined to those with a younger maternal age at diagnosis of type 2 diabetes. In mothers diagnosed before the age of 50 years, the median age of diagnosis was 45, which suggests that a minority of mothers had overt diabetes during pregnancy, but may indicate the presence of perinatal dysglycaemia. A family history of diabetes is associated with metabolic abnormalities [1], the extent of which was found to be greater in those with a family history diagnosed at a younger age [33]. It has also been reported that individuals with a young maternal age at diagnosis of type 2 diabetes had a greatly increased odds of impaired glucose tolerance [2], an effect not observed for younger paternal diagnosis. Furthermore, studies in high-risk populations suggest that maternal dysglycaemia during pregnancy even below the threshold for diabetes or impaired glucose tolerance is associated with a higher risk of type 2 diabetes in the offspring [34]. An accumulating body of evidence from animal models also supports the existence of epigenetic effects conferred by the intrauterine environment on disease risk [35, 36]. These findings also suggest that not only information on the extent of family history (i.e. how many family members have the condition), but also the nature of the family history and the age at diagnosis, particularly maternal diagnosis, may provide further insight into individual risk of type 2 diabetes. We noted that there is difference in the extent to which risk of diabetes associated with a maternal and or a paternal family history is explained by our proposed mediators. Overall, we observe that 17% of the maternal and less than 1% of the paternal family history associated-risk is explained by the same mediators (ESM Table 5). This finding suggests that the risk of type 2 diabetes attributable to family history may have a distinct aetiology depending on the family member affected, and is supported by observations of distinct metabolic perturbations dependent on which parent is affected [1].

The inclusion of categorical information on the extent of family history marginally improved prediction relative to a yes/no classification of family history. Although the addition of the 35 SNP genetic risk score also marginally improved prediction, extra family history information is more easily ascertained in the clinical setting, and we noted that even a single question on the presence of familial type 2 diabetes performed better than the genetic risk score in predicting type 2 diabetes.

Despite previous suggestions of potential interactions of family history with lifestyle factors [8, 37], the evidence for interactions in our study was weak (ESM Table 6). In groups already at high risk of diabetes by virtue of their obesity or physical inactivity, family history is associated with an even higher risk: obese individuals with a positive family history had a more than 20-fold higher risk of type 2 diabetes compared with lean individuals without a family history (ESM Table 6). Importantly, however, lower BMI and higher levels of physical activity were associated with a lower risk of type 2 diabetes, regardless of family history. Despite fatalistic perceptions held by some of those with a family history of type 2 diabetes [38], individuals with a family history of diabetes are likely to adopt ‘healthy’ behaviours when advised to do so by a physician [39]. Our findings suggest that individuals with a family history of diabetes have much to gain from such lifestyle interventions.

Strengths and limitations

We used a large sample of verified incident cases of type 2 diabetes nested within a very large multinational cohort study with standardised assessments of exposure and outcomes. The diversity of the cohort in terms of lifestyle, anthropometric and social characteristics allowed a robust assessment of the mediation of family history by these factors. Given the absence of DNA samples for some participants and the incomplete availability of other mediating variables, the sample size in the mediation analyses is smaller than that of the overall analyses. However, we observed near-identical associations between family history and diabetes in both the overall and the restricted sample, and therefore believe that the reduced sample size had a minimal impact on the generalisability of our mediation estimates.

Although methods of ascertainment of cases in InterAct were heterogeneous across countries, we observed little heterogeneity in associations between countries. InterAct cases in the study were clinically diagnosed, and it is possible that those reporting a family history undergo an increased frequency of testing for diabetes, which may serve to strengthen the observed association. However, we observed no interaction between prominent risk factors such as BMI and family history on risk of type 2 diabetes (although one might expect an even higher frequency of testing when these risk factors are present in combination). Family history information was not always available at baseline, but we found no heterogeneity in the association between family history and risk of type 2 diabetes, whether ascertained at baseline or at follow-up.

Although we tried to be specific for a family history of type 2 diabetes, it is possible that we may have included people who in fact had a family history of type 1 or other forms of diabetes. However, the potential level of misclassification is low, given the lower prevalence of type 1 diabetes. In addition, the direction of any bias in our estimate of the association between family history of diabetes and incident diabetes would be such as to underestimate the true association if familial type 1 diabetes is not associated with risk of type 2 diabetes. However, familial type 1 diabetes has also been suggested to be associated with risk of type 2 diabetes [40], in which case any misclassification would have little impact on our estimates.

No information was available on family size for analyses of a multiple family history of diabetes or a family history in siblings.

Conclusions

We found that, even after accounting for prominent type 2 diabetes risk factors such as physical activity, BMI, waist circumference and a multi-SNP genetic risk score, family history was strongly associated with future risk of type 2 diabetes, and that the majority of this risk remained unexplained. The independence of family history as a risk factor, and the relative ease with which it can be ascertained, means that establishing a family history of disease remains of primary clinical importance in this genomic era: a single family history question was shown to outweigh the added predictive power of the genetic risk score. Furthermore, given the modest mediation of family history-associated risk of diabetes by major risk factors, genetic and otherwise, a greater insight into how family history contributes to risk of type 2 diabetes would represent an important advance in our understanding of the aetiology of the disease.