Minority ethnic groups - understanding diet, weight and type 2 diabetes: scoping review

Type 2 Diabetes and Ethnicity

General Context: Type 2 Diabetes as a Public Health Priority

The prevalence of T2D is increasing rapidly in Scotland, as in many other high-income countries. According to the latest Scottish Diabetes Survey (2020), 5.8% of the Scottish population of all ages are diagnosed with diabetes, and Type 2 Diabetes (T2D) is the most prevalent (87.7% of all cases). In addition to this, it is estimated that around 10% of cases of diabetes remain undiagnosed, (ScotPHO, 2022). Diabetes is an important cause of disability and increases the risk of coronary heart disease and other health problems (ScotPHO, 2022). Therefore, this significant increase of cases of diabetes in Scotland and the UK is a major public health priority (Goff, 2019).

The onset of T2D is primarily due to poor diet (specifically excess energy intake), low levels of physical activity and the resulting increase in levels of obesity. It is estimated that T2D is around ten times higher among those with a body mass index (BMI) over 30 compared with those with a BMI under 30. T2D is much more common at older ages, and the increase in the number of older people in Scotland also contributes to the increase in numbers of those with diabetes. Furthermore, T2D is more common in the families of those with type 2 diabetes, and in some ME groups (Scottish Diabetes Survey, 2020; ScotPHO, 2022).

The Higher Prevalence of T2D in Minority ethnic (ME) Groups

Evidence indicates that ME groups in the UK and in other high-income countries suffer disproportionately from diabetes compared with non-minority populations. The prevalence of T2D is estimated to be approximately 3 to 5 times higher in certain ME groups than in the white British population (Goff, 2019). This high incidence mainly affects two ME communities:

• South Asian groups – the susceptibility to T2D of people of South Asian descent has been known since 1985 (Bhopal et al. 2014). They have the highest rate of T2D. Among migrants in Europe, it was found to be up to six fold higher in Bangladeshis, fivefold higher in Pakistanis and fourfold higher in Indians (Agyemang et al., 2021);
• Black African and Caribbean groups - the incidence of type 2 diabetes in UK in these communities is estimated to be 3 times higher than in the general population (Moore et al., 2022).

The onset of T2D in these populations occurs 10-12 years younger, and a greater proportion of people develop T2D before the age of 40 years compared with white Europeans (Goff, 2019). text

The higher prevalence of T2D in ME groups is true also for childhood T2D: it has been estimated that South Asian children in the UK are 14 times more likely to develop T2D than white European children (Goff, 2019). Agyemang et al. (2021), in their review on T2D burden among migrants in Europe, report that also gestational diabetes is higher in most ME groups than in Europeans. On one side this might contribute to their high risk of T2D, but on the other it raises questions on why and whether it is treated effectively.

In certain ME groups there is also the problem that many cases remain undiagnosed. Anderson et al. (2021) analyse baseline UK Biobank data on plasma glycated haemoglobin (HbA1c) to compare the prevalence of pre-diabetes and undiagnosed diabetes mellitus in white, South Asian, black, and Chinese participants. They find that while 1 in 22 (4.7%) of individuals aged 40–70 years old in the UK have actionable HbA1c concentrations, 1 in 6–7 individuals of black or South Asian ethnicity have actionable values. This means that approximately 1 in 30 are living with undiagnosed diabetes.

Also the prevalence of diabetes-related complications differs between ME groups and white-British. For example, Tillin et al. (2013) find that the risk of stroke was almost twice as high in South Asians and over twofold higher in African-Caribbean individuals with diabetes compared to their white-European peers.

Why is T2D more prevalent in ME communities?

It is argued that the higher prevalence of T2D in ME communities is driven by a complex interplay of biological, socio-economic and lifestyle factors, both in developing and in the treatment of T2D. Therefore, genetic, physiological, socio-economic and lifestyle factors contribute to ethnic inequalities in T2D (Goff, 2019).

Biological Factors

There are a number of biological reasons that seem to drive the higher susceptibility of South Asians to T2D. Narayan & Kanaya (2020) find that these are: lower ability to secrete insulin, and thus may have less compensatory reserves when challenged with unhealthy lifestyles; lower muscle mass, specific propensity to ectopic hepatic fat accumulation and for intramyocellular fat deposition. The hypothesis is that this susceptibility is primarily driven by poor metabolic capacity (i.e. reduced beta cell mass and/or function, which impairs insulin secretion), coupled with low lean muscle mass, which may be responsible for reduced insulin action. This is accentuated by a propensity to ectopic fat deposition in the liver and muscle, which further impairs insulin action.

Regarding the biological basis to increased risk, it has also been observed that minority ethnic groups experience a higher risk of T2D at lower levels of obesity than white Europeans (Ntuk et al., 2014). This depends on the levels of adiposity and how fat is stored. Fat distribution, particularly abdominal/visceral fat, are more sensitive predictor of insulin sensitivity than BMI. South Asians may be more prone to visceral fat deposition (Goff, 2019). This means that conventional clinical definitions for obesity, that are derived from populations of white European descendent, are not appropriate for screening diabetes risk in non-white groups and lower BMI. This is reflected in NICE (2014) recommendations, previously discussed in section 4 on page 12.

Recent research has shown that T2D seems to develop differently in black African people. Bello et al. 2019 (research funded by Diabetes UK) has revealed that insulin resistance is not the chief cause of type 2 diabetes in black African people, as is the case for white populations. Their research suggests that type 2 diabetes may develop in 'reverse order' in black populations: high levels of insulin leading to insulin resistance. But the research team doesn't yet know why insulin levels are high to begin with and they are planning to carry out further work to confirm this and explore why.

While these biological factors are much more in the realm of clinical practices rather than policy, they point out the need for more high quality research in this area (Bello et al., 2019; Narayan & Kanaya, 2020). As stated by Dr Louise Goff, lead researcher for Diabetes UK:

"[p]eople of [ME groups] might be getting poorer care because we're applying what we've learnt from white populations to everyone. [For example] many approaches to treating or preventing type 2 diabetes focus on tackling insulin resistance, but if this isn't a driving factor for black African people, then targeting this isn't going to be as effective."

Therefore more research is needed and specific prevention and treatment programmes might be needed for T2D across different ME groups.

Non-biological Factors

Regarding non-genetic factors, epidemiological and migration studies have observed different rates of T2D within the same ethnic group living in different environments or countries. For example, Agyemang et al. (2016) show that the prevalence of T2D (adjusted by age and education) among Ghanaian migrant men in London is almost three times higher than their peers in rural Ghana. Mbanya et al. (1999) also consider rural/urban differences in the same country of origin and find that the prevalence of T2D increases from rural Cameroon (0.8%) to urban Cameron (2%), to reach up to 14.60% in Manchester. Similar differences have been observed in the Asian population: the prevalence of T2D is 2.4% in rural areas of India, compared to 8.2% among Asian Indians living in urban India (Ramachandran et al., 1992). Asian Indians living in the UK or other European countries have about four times higher prevalence (Goff, 2019).

This shows that non-genetic factors play a significant role in the higher prevalence of T2D among ME groups. Among these, obesity represents one of the strongest contributors to the development of T2D. As shown above, it is known that obesity rates are higher in some ethnic groups than the general population, for example among Black African and Caribbean women. This can therefore explain in part higher prevalence of diabetes in some populations T2D (Goff, 2019).

In addition to this, Agyemang et al. (2021) identify pre-migration and post-migration factors as underlying the high risk of T2D within ME groups in Europe. Among the former, they identify adverse early life factors, such as low birth and malnutrition, that might lead to T2D in adulthood. The risk is further increased by a rapid gain in fatness (catch-up growth) in early life (Leung & Stanner, 2011). Furthermore, the difficulties that lead to the decision to migrate, such as poverty and wars, and the stress to the migration process itself, might subsequently have a negative impact on people's health, directly or indirectly through the adoption of risky behaviours. Among the post-migration factors, changes in dietary habits, as mentioned above, changes in lifestyles, living in deprived neighbourhoods and discrimination are all associated with a high risk of developing T2D. While the focus of Agyemang et al. (2021) is on migrants, some of the risk factors they identify are true also for established ME groups, such as discrimination, the difficulties related to integration, and socio-economic inequalities.