Until recently the outlook for a youth or young adult diagnosed with diabetes, which
was almost universally type 1, was bleak. Indeed, using data from the National Health
Interview Survey as recent as from 1984 to 2000, it was estimated that U.S. children
diagnosed with diabetes at 10 years of age had a life expectancy approximately 19
years less than seen in the general population (1). However, more recent data from
the Pittsburgh Epidemiology of Diabetes Complications (EDC) study suggest those diagnosed
with childhood-onset diabetes between 1965 and 1980 have a life expectancy of almost
69 years, which is less than 4 years lower than the comparable U.S. population (2).
This good news has been accompanied by the observation from the Finnish Diabetic Nephropathy
(FinnDiane) study that virtually all of the excess mortality seen in type 1 diabetes
is related to the development of micro- or macroalbuminuria (3). This seminal observation
has been confirmed and extended for up to a 20-year period in the EDC population (4).
The improved prognosis, in terms of mortality, has been accompanied by a dramatic
reduction (5) or delay (6) in the incidence of end-stage renal disease. Interestingly,
the decline in cardiovascular disease (CVD), the leading cause of overall mortality
in diabetes, is less marked (5). One cautionary note, however, has to be made concerning
the improvement in mortality of patients with type 1 diabetes. In a recent analysis
of over 17,000 individuals in Finland, diagnosed between 1970 and 1999, Harjutsalo
et al. (7) compared the time trends of mortality for those diagnosed at an age less
than 15 years to those diagnosed at an age of 15 through 29 years. Although a very
significant fall was seen in mortality over time for the young-onset group, consistent
with the Pittsburgh EDC population (who were all diagnosed before the age of 17),
mortality for the older-onset group increased over time reflecting an increasing number
deaths related to alcohol, drugs, and acute complications (7). This raises the possibility
that type 1 diabetes mortality patterns may differ markedly by age of onset.
The picture becomes more confusing, and disturbing, when one considers the recent
increased incidence of apparent type 2 diabetes occurring in youth and young adults
(8). One major challenge is that of typology, or our ability to distinguish between
type 1 and type 2 diabetes, which is particularly difficult in an overweight or obese
young adult. The SEARCH for Diabetes in Youth (SEARCH) study has examined this issue
in some depth and described four groups based on the presence or absence of diabetes
autoantibodies and of insulin resistance (9). How well this schema would work in the
future in terms of predicting outcome remains to be seen but it is likely to be quite
relevant as a number of studies have suggested that even in clear type 1 individuals
it is those with evidence of insulin resistance or an insulin resistance/type 2 diabetes
family background that have increased cardiovascular and renal disease (10–15). The
complexity of this issue is further demonstrated by the observation that many classic
type 1 diabetic subjects may retain some residual β-cell function for many years after
diagnosis (16), which may partly relate to the benign natural history seen in many
of the patients from the Joslin 50-Year Medalist Study who have survived 50 years
of type 1 diabetes (17).
So what do we know about the prognosis of type 2 diabetes in youth and young adults?
A number of studies have suggested that individuals with type 2 diabetes have worse
cardiovascular risk factors than similarly aged individuals with type 1 diabetes.
Indeed, the SEARCH study has shown more adverse cardiovascular risk profiles, including
blood pressure (18) and lipid levels (19), and a higher prevalence of microalbuminuria
(20) in youth-onset type 2 diabetes compared with type 1. Up to now, however, there
have been few data on mortality or major outcomes of diabetes comparing type 1 and
type 2 diabetes where onset occurred in youth or young adulthood. Hillier and Pedula
(21) some years ago suggested that type 2 diabetes with an onset between age 18 and
44 years ran a more aggressive course than cases diagnosed later, particularly in
terms of relative impact compared with the age-matched general population. The results
of the Treatment Options for Type 2 Diabetes in Adolescents and Youth (TODAY) study
support such a conclusion in terms of metabolic deterioration and have been recently
reviewed (22).
In this issue, Constantino et al. (23) now provide further data concerning young adult–onset
type 2 diabetes. Using a diabetes clinical database, and matching to the Australian
National Death Index, these investigators were able to compare clinical and mortality
outcomes from 354 patients with type 2 diabetes and 470 with type 1 diabetes.
Strikingly there was a twofold greater mortality in the type 2 cohort predominantly
due to an excess of cardiovascular deaths. Although the clinical data were largely
collected through routine encounters, a standardized protocol was used and the data
quality is thus likely to be generally high. Likewise, the linkage with the Australian
National Death Index is validated and mortality ascertainment data are likely to be
complete. A significant weakness of the study, however, is the reliance purely on
death certificates alone for cause of death, which were only available for 72% of
deaths at the time of analysis. A number of studies have demonstrated the pathways
and contributors to death are quite complex in diabetes (24) and the study would be
greatly enhanced by the investigation and standardized recording of causes of death.
Nevertheless, these data are unique and extremely valuable and support the growing
concern that type 2 diabetes with a youth/young adult–onset has a particularly high
risk of adverse vascular outcomes. Some of the figures from Constantino et al. (23)
are quite concerning with prevalence rates of ischemic heart disease reaching as high
as 13% at an age of 40 years compared with only 3% in the comparable group with type
1 diabetes whose mean age was 39 years.
In an interesting further analysis, the authors looked at the prevalence of risk factors
2–5 years after diabetes diagnosis when mean age was 28 years. Significant differences
between the two types of diabetes were seen with the type 2 subjects having significantly
higher blood pressures, lipids, and greater albuminuria. In contrast, smoking rates
were marginally lower in those with type 2 diabetes. Finally, it should be noted that
although the blood pressures and lipids were generally higher in type 2 diabetes than
type 1 diabetes, they were only moderately elevated (e.g., mean blood pressures were
120/78 mmHg and total cholesterol was 210 mg/dL).
These data therefore raise very significant clinical questions that need urgent answers.
First and foremost, it is important that we do not adopt the narrow “glucocentric”
approach that for so many years dominated our approach to diabetes management and
CVD prevention in type 2 diabetes. It should be noted these very divergent vascular
outcomes in the current study’s data occurred with an identical updated HbA1c of 8.1%
in both groups of subjects.
Second, we need to know more about the relative contribution of predictors of adverse
outcomes in young-onset type 2 diabetes. Unfortunately the data from Constantino et
al. on risk factors measured early on in the course of diabetes were available for
only 29% of subjects thus precluding prospective, definitive multivariable risk modeling.
Third, we need to address the lack of guidelines and evidence-based goals on which
to base cardiovascular intervention. This has been a long-standing problem in type
1 diabetes because, with the exception of the Heart Protection Study (HPS) (25), there
are no cardiovascular risk factor intervention trials in young-onset type 1 diabetes
with clinical outcomes on which to base treatment goals and strategy. While clearly
the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions
and Complications (DCCT/EDIC) studies intensive insulin therapy intervention in early-onset
type 1 is of great benefit, CVD still develops in the intervention group at a high
rate (26) and, as noted earlier, CVD rates do not seem to be declining as rapidly
as renal disease rates (5). It is thus quite possible that lower blood pressure and
lipid goals may be more appropriate in type 1 diabetic subjects than now appear to
be the case in older type 2 diabetic subjects, the group on which guidelines are loosely
based. In the light of the recent Constantino et al. and TODAY (22) studies, data
current guidelines and goals maybe even more out of tune for those with young-onset
type 2 diabetes. Fourth, an implication of the results in Constantino et al. is the
need to continue the search for other avenues to reduce the mortality and cardiovascular
morbidity seen in diabetes in general. Clearly, the enhanced risk in type 2 diabetes
may largely relate to insulin resistance itself, and as noted this is also an important
risk factor in type 1 diabetes.
A further focus should be to better identify and target those with a genetic predisposition.
Recent data concerning the combination of haptoglobin genotype 2–2 and diabetes (either
type 1 or type 2) leading to enhanced coronary artery disease risk (27,28) and renal
risk (in type 1 diabetes) (29) offers some hope in this regard. This is particularly
encouraging as the CVD risk may be ameliorated by vitamin E therapy (so far tested
only in type 2) (30). This is unlikely, however, to explain the differential risk
between type 1 and type 2 diabetes.
So where do we go from here? While guidelines and CVD risk factor goals clearly need
to be revisited in terms of their applicability to both young-onset type 1 and type
2 diabetes, they would be best based on clinical trial evidence. Thus, a CVD prevention
trial evaluating both intensive blood pressure and lipid control versus current management
would be helpful. The outcomes could also include renal disease while further randomized
arms might address new approaches (e.g., insulin sensitization and/or vitamin E therapy
in those with haptoglobin susceptibility). The target population should comprise young
adults with either type 1 or type 2 diabetes though the former should have longer
diabetes duration to provide comparable and sufficient event rates.
Constantino et al. (23) should serve not only as an alarm bell for the development
of appropriate management strategies for young-onset type 2 diabetes but also—especially
given the disappointing results of the TODAY study (22) of management of adolescent
type 2 diabetes—a call to further our prevention efforts in terms of type 2 diabetes
and insulin resistance in general. While we can probably still conclude that those
with type 1 diabetes and an onset in youth may have a normal life expectancy, particularly
if micro- or macroalbuminuria is avoided, it seems doubtful that the same optimism
can be extended to those developing type 2 diabetes at a similarly young age.