To our knowledge, this is the first study to explore the effect of age and sex on the relationship between diabetes risk factors and T2DM and IFG in West African countries. As expected, we found that the associations between all risk factors, and both T2DM and IFG were significant, even after adjusting for age, sex, occupation, and education. The general findings on the risk factors were concordant with those of several previous studies among different population groups, including those from Nigeria [10], Australia [22], Asia, and European countries [7, 8, 23, 24]. There was no evidence that these associations varied according to either age or sex in the current study.
In the present study, all obesity markers measured by BMI, WC, WHR, and WHtR were strongly associated with both T2DM and IFG in both sexes and among all age groups, confirming previous studies among Africans [10, 25], and other populations including Asians, Americans, and Europeans [7, 15, 26]. However, our findings are inconsistent with those from the Lasky et al. [9] study among Ugandan subjects. In that study a strong, direct relationship was observed between obesity defined by BMI and the development of T2DM among women only. This difference could be because in the Lasky
et al. study [9], male subjects were primarily lean (defined as BMI < 20 kg/m2) whereas in the current study, male participants ranged from normal BMI to obese.
In the present study, in both sexes, the strongest association between WC and WHtR with T2DM (compared to overall body fat as measured by BMI) in the adjusted models reinforces the importance of abdominal adiposity as an independent risk factor for the development of T2DM [27, 28]. Although obesity, as defined by BMI, has the strongest association with IFG, this is a transition state before T2DM, and it may be the case that those classified as obese based on markers of central obesity spend less time in the IFG category [19]. Of note, glycaemic profiles have been shown to differ by sex [13, 29], with studies among populations from Mauritius and Australia finding impaired glucose tolerance (IGT) to be more common in women (due to the greater glucose load taken relative to body size) and IFG more common in men [13, 29]. The fact that an oral glucose tolerance test (OGTT) was not used for diagnosis of T2DM or pre-diabetes in the current study means that the comparison of results with other studies that did use an OGTT should be interpreted with caution. Moreover, the thresholds used for defining obesity markers are not consistent across studies.
Overall physical activity in the present study was found to be associated with around a two-fold higher risk of both T2DM and IFG, among both sexes and age groups.Previous studies among African populations have reported similar findings independent of BMI [30], as have studies from Portugal [31], the United Kingdom, Canada, Australia, and Finland [26, 32]. In a study among European participants, however, low levels of leisure-time physical activity (e.g swimming, jogging) was associated with incident diabetes among European women only [7]. Although the GPAQ used in this study did include assessment of leisure-time physical activity, levels of leisure-time physical activity are consistently low across African countries [30].
Our findings that hypertension was associated with T2DM and IFG among sexes are in direct agreement with earlier studies in Kenya [33] and Europe [7]. An association between systolic blood pressure and diabetes was observed among men only, however, in various European prospective cohort studies [7, 34]. This difference was ascribed to the fact that women with hypertension controlled their level of blood pressure better than men [7], implying the importance of awareness and management of HBP among the West African populations [35]. However, our findings that age modified the association between hypertension and T2DM can be ascribe to the evidence that hypertension increases with age in some populations [36, 37].
Heavy alcohol consumption has been found to be associated with T2DM in both sexes and all age groups in previous reviews and meta-analyses [38, 39]. However, sex differences in the association between heavy alcohol users and insulin sensitivity are reported only in studies among predominantly European populations [7, 40], with the incident diabetes among men only [40]. The significance of heavy alcohol consumptions highlights the importance of the low percentage of alcohol consumption in the current study as it may be insufficient to detect any significant difference. Almost 70% of women in the current sample have never drunk alcohol, with this being due to religious and cultural factors in West Africa. Mali, for example, is predominately a Muslim country, and alcohol consumption is prohibited [41]. Additionally, socio-culturally, heavy alcohol drinkers and smokers are highly stigmatised in most African countries and this may contribute to the low rate of alcohol consumption [41]. In the current study, drinking alcohol was found to be protective for T2DM and IFG, which is consistent with previous studies [42].
Smoking in this study was associated with T2DM and IFG and confirms earlier studies among South Africans and other populations [43]. Though the association between smoking and T2DM and IFG did not vary by sex or age in this study, some previous studies among European populations [6, 7] showed positive associations between cigarette smoking and incident diabetes in men only. An association was evident between cigarette smoking and incident diabetes in women however in the large American Nurses' Health Study [16]. The difference in the prevalence of smoking among women in these two studies (much higher in the Nurses’ Health Study) may explain these findings [6, 7]. As such, the low percentage of female smokers (1.4% of women and 19.9% of men) in the present study means it is challenging to assess sex-specific differences in associations with T2DM and pre-diabetes.
Finally, earlier studies on dietary patterns conducted in urban Ghana [44] and Senegal [45] showed that inadequate fruit and vegetable consumption was associated with an increased risk of T2DM. In this study, lower fruit intake was associated with increased prevalence of T2DM but the association with IFG was not statistically significant. Lower vegetable intake had opposite associations with T2DM and IFG although none were statistically significant. The simple diet recall questions used limit the ability to generalise from these findings, but the results are consistent with previous evidence that fruit consumption alone is protective against T2DM [46].
The current study has several strengths, including the large sample size from five different West African countries. This ensured greater statistical power to detect age and sex interaction effects between potentially modifiable risk factors and both T2DM and IFG. The study, however, has several limitations, which need to be considered when interpreting the results. First, because the study is cross-sectional in design, results do not imply causal relationships between risk factors and T2DM. Secondly, only risk factors that were assessed in all the five countries were assessed. Therefore, important risk factors, such as high-density lipoprotein cholesterol levels, could not be included in this analysis, though studies have shown this factor to interact with sex [8]. Thirdly, since an OGTT was not used to define diabetes and pre-diabetes, the results may differ from those where this method was used, especially given the sex-specific impact of a glucose load due to differences in body size between males and females [29]. Fourthly, fruit and vegetable consumption measures were not coded as per the WHO guidelines of five servings per day due to the low level of fruit and vegetable consumption in this sub-population and may constitute a limitation when comparing to other studies. Lastly, the different years that the survey data were collected may have introduced some bias, however, this is not an important limitation for this study given the focus on associations between risk factors and health outcomes. These limitations notwithstanding, the findings from the current study have important policy implications.
Policy Implications
Since the association between T2DM risk factors and both T2DM and IFG do not appear to vary with age or sex, policies and interventions may be similar among adults of both sexes in West African populations. This is particularly advantageous given the low-income context in West Africa, with interventions likely to be both more cost-effective and simpler. While, in general, smoking and alcohol are more prevalent among men, obesity and physical inactivity are more prevalent among women in West Africa. Any policies targeting these risk factors should consider socio-cultural factors and beliefs [47]. These may include i) the commonly held belief in much of Africa that overweight is an outward manifestation of high socioeconomic standing, prosperity, and beauty, as well as good health among females and the preference for central obesity among some affluent men [48, 49]; ii) the fact that physical activity among women is often discouraged in most countries as it is culturally considered to be undesirable and unattractive and associated with a masculine physique [47], and iii) the fact that physical activity is usually not viewed through a health lens for men, but through the lens of sports [50]. Policies should target the persistently low levels of awareness regarding the importance of fruit and vegetable consumption, as well as the globalisation of food markets, particularly concerning alcohol and tobacco industries. Globalisation has been shown to exacerbate the use and increased the ease of access to alcohol and tobacco use among young adults in Africa