4. Discussion
The recent International Diabetes Federation reports have confirmed that persons of any age category could develop T1D [
11], and the exposure to prolonged T1D invariably should present with confirmed CVD. A high risk of mortality from vascular disease [
24] occurs in type-1 diabetics If not properly managed by daily monitoring of the blood glucose level, insulin treatment, medication, diet, physical exercise, and minimalized psychological stress [
11]. Another recent publication from the Australasian diabetes data network (ADDN) study group states that T1D patients less than 30 years of age carry a five-fold higher risk of mortality due to cardiovascular complications [
105]. The findings of the present meta-analysis which we have based on a comprehensive group of multivariate biomarkers associated with diabetes-induced vascular complications are consistent with the factual data reported by many authors including the ADDN study group, EURODIAB, FINNDANE, DCCT/EDIC, SEARCH CVD, ACCORD and ADVANCE clinical trials [
45,
106,
107,
108,
109]. There is a consensus that if children are diagnosed with T1D at the age of 10 years, may potentially lose 17 years of life [
11]. In our study, 46% of the participants with T1D were below the mean age of 15 years and as such are highly vulnerable to CVD-related mortality. Given these facts, it is of utmost importance to establish a set of reliable and easily accessible biomarkers and methods to evaluate childhood vascular dysfunction, before putting them on life-long medication, which would otherwise add organ damage to its sequelae.
The effect size is a valuable tool in determining the impact of an intervention between the diseased group and the healthy control group. According to the Cohen’s classification, an effect size of 0.2 is considered a small difference between the control and intervention, which is a difference of 0.2 standard deviations which is a negligible difference. In other words, the effect size is the magnitude of the difference between the comparators. It is a population measure and is not affected by the sample size. An effect size of 0.2-0.5 is considered a small effect, 0.5-0.8 is a medium effect and above 0.8 is considered a high effect that signifies a large difference between the control and the intervention cohorts [
110]. Therefore, in analysing the data of this study, out of the 28 biomarkers that were evaluated, the following categorisation reveals the small, medium and the large effects produced by the risk factors of vascular dysfunction in individuals diagnosed with T1D.
The hallmark of Type-1-diabetes is the patients are combinedly diagnosed with hyperglycaemia, hypertension, and hyperlipidaemia which are characteristic symptoms of the disease [
111] and have been confirmed by this study. Out of the six categories of the biomarkers that were evaluated in Figures 3–8, this meta-analysis has reaffirmed that in T1D, the blood pressure, glycaemic and the cholesterol metabolism are conspicuously impaired. There were significant differences between the healthy control and the baseline T1D populations in SBP (SMD: 0.41, p< 0.0001), DBP (SMD: 0.3, P<0.0001), HbA1c% (SMD: 3.42, p<0.0001), HDL (SMD: 0.17, p<0.0001), LDL (SMD: 2.09, p<0.0001), Apo A (SMD: 0.63, p<0.0001) and Apo B (SMD: 0.55, p<0.0001) that were elevated in the T1D, as evidenced by the standardized mean difference in the forest plots. In the student-t-distribution bar graphs, only SBP (p< 0.0001), MAP (p< 0.0245), HbA1c% (p<0.0001) and LDL (p< 0.0064) produced significant upregulation in their values.
However, when the risk factors identified in this study are arranged from having high to low CVD risk based on the effect size, the 6 most reliable biomarkers are HbA1c% (3.42), FBG (2.37), LDL-C (2.09), AI% (1.54), sICAM-1 (1.09), and FMD% (-0.88) which is a deviation from the already established traditional risk factor milieu. The risk factors evaluated in this study appear to have evolved from mainly lifestyle factors that are linked to modifiable eating habits and beverage intake, physical exercise, and medication, which along with socioeconomic conditions, had contributed to this change.
Prolonged hypertension is a predictor of CVD. In the present study, the augmentation index percentage (AI%) is the third most reliable biomarker of T1D-induced CVD, also known as AI75, which is based on the arterial pressure measured at 75 beats per minute [
112]. Irrespective of the number of studies that measured AI% in this analysis was very few with a total participant number of 620, the overall result was that it creates a remarkable difference between the control and T1D groups. Therefore, arterial pressure features as a better prognostic marker than the traditional markers of BP that comprise of SBP and DBP.
The blood pressure readings are considered to be hypertensive when it is greater than the 95th percentile for T1D individuals less than 18 years of age, and the ideal SBP and DBP should fall below its 95th percentile [
105]. In this study, the 95th percentile of SBP/DBP for the healthy control was 119/72 and for the T1D subjects it was 120/74 Hg mm respectively. In this cohort, the SBP was higher by 10.8% in the control group and 21.7% in the T1D group. The DBP was higher by 18% in the control group and 13.6% in the T1D group. The children and young adults usually carry an excess risk of premature mortality from vascular complications specially if they are co-morbid with hypertension [
113]. The CVD risk in T1D is further increased if uncontrolled hypertension had persisted for a longer duration with the additional risk of developing cardiac failure, limb amputation, stroke, and sudden death [
114]. Published reports have highlighted that 16% of young adults having T1D also have hypertension and up to 50% of the T1D youth develop hypertension worldwide [
115]. Studies have confirmed that a high glycated haemoglobin percentage positively correlates with hypertension and its risk factors are hyperglycaemia, male gender, higher BMI and the frequency and dosage of insulin treatment [
116].
Diabetes is an independent risk factor for childhood-onset CVD [
117]. Elevated blood pressure is common in the T1D and the healthy control population is also vulnerable to having increased SBP, influenced by epigenetic factors [
118]. SBP is also influenced by high pulse pressure which brings forth arterial stiffness [
119]. Arterial stiffness is a common occurrence among the T1D population and an early predictor of subclinical atherosclerosis [
120]. It is a surrogate marker of hyperglycaemia and ensuing vascular complications [
121]. Arterial stiffness sets in long before the signs of CVD emerge. The central arterial pressure wave consists of a forward-travelling wave generated by the contraction of the left ventricle and a reflected wave that arrives combinedly from the arteries in the periphery [
122]. With arterial stiffness the reflected wave from the periphery arrives too early during the systolic phase, thereby increasing the aortic pulse pressure, the left ventricular afterload, left ventricular mass, stroke volume and the oxygen demand [
123]. These events subsequently initiates atherosclerosis and left-ventricular hypertrophy. The carotid-femoral pulse wave velocity and the AI% are considered as the gold-standard to measure arterial stiffness [
124].
Intensive insulin treatment has shown to regulate the SBP, although a HbA1c greater than 7.5% is a strong risk factor for the plaque build-up in arteries despite being asymptomatic to coronary artery disease [
125]. Although there is no elevation in blood pressure in some of the T1D children, they show high increases in SBP during the night, who are without the usual nocturnal dip in blood pressure as seen in the healthy subjects [
126]. The loss of the nocturnal dip of SBP is considered a predictor of CIMT in children [
127]. These patients are considered at a higher risk for vascular dysfunction and hence are exposed to early-onset CVD. Hypertension may lead to diabetic nephropathy with an increased thickness of the basement membrane and a declining estimated glomerular filtration rate [
128]. However, the methods of determining hypertension may be outdated in many previous studies with the ambulatory BP measurements not reflecting the true status. An ultrasound -guided echocardiogram would add more weight to its readings which require the establishment of a fool-proof method that is reliable for all the ages of T1D. The FinnDiane laboratories in Helsinki have introduced de novo methodology to assess vascular health, leaving aside the obsolete instruments such as the sphygmomanometer [
129]. They include applanation tonometry to measure arterial stiffness, peripheral tonometry to assess endothelial deregulation and the ultrasound to evaluate CIMT [
130,
131]. The finger-pressure waves are measured by a plethysmograph to assess the autonomic functions [
132].
There are some noteworthy facts that have emerged from this study, being consistent with previous findings on the influence of T1D on vascular disease and subsequent exposure to early onset of CVD. The flow-mediated dilatation percentage (FMD%), which is a direct method of determining vascular dysfunction is significantly decreased (SMD: -0.88) in the T1D compared to the healthy control group [
74], and the pulse-wave velocity (PWV; SMD: 0.6) in the brachial or femoral artery and the carotid intima media thickness (CIMT; SMD: 0.62) significantly elevated in the T1D cohort [
133]. These findings are supported by both the forest plots and the paired student t-test bar graphs. Therefore, the low FMD% [
108] high PWV [
108] and the CIMT [
134] markers indicate that the T1D population with a mean age of 20 years is highly vulnerable to developing vascular disease as well as CVD quite early-on.
There is a large body of evidence gathered from a plethora of clinical studies which confirm that the T1D subjects invariably develop vascular defects from having hypertension, hyperglycaemia, and dyslipidaemia either alone or in combination and are at high risk of developing much-debilitating CVD or ED.
The mean HbA1c% in this study was 5.07 + 0.34 in the control group and 8.56 + 1.08 in the T1D group. There was a 68% increase in the mean value of HbA1c% in the T1D compared to the healthy control. The 95th percentile for the HbA1c % in this T1D study was 10.2 and 53.5% of the readings were above the mean value that was computed, indicating that rigorous glycaemic control is needed for this T1D population. These results are consistent with the readings published in previous literature and correlates positively with the vascular parameters such as CIMT, sICAM-1, AI% and FMD% (Figure 6) with which it can be concluded that these vascular functionality measures are not dependent on the age, BP and other traditional biomarkers of CVD, but are extremely dependent on the level of hyperglycaemia in the T1D.
Intensive insulin therapy is not without its advantages. When a rigorous treatment regimen is followed at the beginning, there seem to be retention of metabolic memory in the T1D subjects [
108] . A previous study that exerted intensive glycaemic control at the onset of T1D, had their CVD risk reduced by 57% even after 17 years of disease duration [
134] . Insulin therapy and the regulation of the glucose titres in plasma identified by a declining HbA1c% has reconfirmed that glycaemic control is extremely important to children and the youth with T1D. Early optimal plasma glucose control was shown to be highly effective in regulating the risk of CVD evidenced by the diabetes control and complications trial (DCCT) which had the longest duration of longitudinal evaluation of the traditional CVD risk factors spanning 30 years [
135]. Prolonged and mismanaged hyperglycaemia takes a heavy toll on the vasculature, one of the best examples being the increasing CIMT [
136]. Secondarily, it causes arterial stiffness which will steadily progress towards atherosclerosis and other CVD complications. A child diagnosed with T1D during the first year from birth will invariably have developed CVD by 20 years and will be highly prone to death by 55 years of age [
137].
The mean LDL-c value in this study was 2.5+0.28 mmol/L (100.24 + 15 mg/dL) in the T1D group and 2.45+0.3 mmol/L (90.42 + 19.88 mg/dL) in the control group. The 95th percentile for the LDL-c was 2.7 mmol/L (104 mg/dL) in the present analysis. Up to 47% (in the mmol/L subgroup; n=3782) and 42.8% (in mg/dL subgroup; n=2298) of the total readings were above the respective mean values. There are published studies which have yielded similar results as this present study [
137]. In our lipid profile, the total cholesterol (TC; SMD: 0.02), and total triglycerides (TG; SMD: 0.12), HDL-c (SMD: 0.17, p<0.01) and LDL-c (SMD: 2.09, p<0.0006) markers of dyslipidaemia showed only a very small upregulation in the T1D group. There are clinical studies that have reported strong positive correlation between high HbA1c% and LDL- cholesterol level [
138], which has significant associations with plaque-build-up and obliterating the vasculature causing blockages that induce cerebrovascular infarcts or stroke, which is somewhat controversial given the results of this study. Apo A (SMD: 0.63, p < 0.001) and Apo B (SMD: 0.55, p < 0.00001) have shown significant elevations in the T1D subjects in the forest plots in comparison to the healthy control participants. Out of the lipid markers that were evaluated, LDL cholesterol and Apo B are classified as atherogenic lipids that favour CVD [
139] which is disproven for LDL-c by this study which produced non-significant results in the student-t -test bar graphs as well. The ratio of Apo B/Apo A-1 (has been suggested as a better biomarker of CVD prognosis, in the recent decades [
140]. However, Mazanderani et al (2009) have reported that Apo B is a better biomarker than LDL-c in diagnosing T1D and the decision to initiate lipid-lowering therapy in T1D patients should not be based upon the LDL-c levels alone because T1D is aggravated by poor glycaemic control and not from dyslipidaemia per se [
141]. These lipids are also risk factors of obesity that is prevalent among the children, owing to their excessive consumption of fatty and oily junk food [
142]. The notion that a higher BMI (SMD: 0.17, p < 0.003) and increased body weight (SMD: 0.22, p < 0.0005) and having a higher level of fasting lipids in sera, particularly LDL titres greater than 3.4 mmol/L or 130-159 mg/dL are strong contenders for CVD [
143], is disputed by the effect sizes of BMI and the BW of the present study. The Apo B molecule comprises of atherogenic lipids: LDL and very low-density lipoproteins (VLDL) cholesterol which is frequently high in T1D patients and contributes to the development of arterial stiffness [
144]. Although LDL is a surrogate marker for vascular disease, and an independent risk factor for CIMT [
136], LDL-c (SMD: 2.09) has a very small effect on developing CVD.
Inflammation is another common characteristic in the T1D population and is linked to autoimmunity which induces the pancreatic beta cell destruction through immune mediation [
145]. High glucose variability was shown to increase the inflammatory biomarker, C-reactive protein (CRP; SMD: 0.33 ) which in this analysis has displayed a small increase in the T1D cohort, compared to the healthy control [
146]. The cytokines are prominent immunomodulatory substances which promote inflammation and the sensitisation of the distinctive T helper cell repertoires, macrophages and dendritic cells that leads to the destruction of the islets of Langerhans, in T1D [
147]. Studies have shown that the blockage of the pro-inflammatory cytokines results in a decrease in autoimmunity directed at the pancreas which limits its insulin production and secretion [
148]. This study has shown a small risk with CRP titres in blood in the T1D cohort compared to its healthy control (SMD: 0.19, p < 0.002) alongside significantly elevated IL-6 (SMD: 0.5, p < 0.0001) and TNF-alpha (SMD: 0.54, p < 0.0008) titres in the T1D evidenced by the forest plots. These are potential immunotherapeutic targets in the treatment of T1D which hold promise for future pharmacotherapy. The slight elevation in the CRP titres was also confirmed by the t-test bar graph (p < 0.0053). The ALT and GGT are biochemical markers of liver function and hepatic insulin sensitivity and resistance [
149]. In this study, the ALT (SMD: 0.19) and GGT (SMD: -0.11) levels in TID were not significantly different from the healthy control possibly owing to the fewer number of studies. However, ALT is a biomarker for the liver fat, being an epidemiologic marker for the non-alcoholic fatty liver syndrome, while GGT is a biomarker of cellular oxidative stress [
150]. The pancreatic beta cells are highly sensitive to oxidative stress which results in decreased secretion of insulin [
151].
The body-mass index (BMI) and the body weight (BW) are already known risk factors of CVD in T1D which increase proportionately with rising hyperglycaemic status [
152]. Intense insulin treatment causes weight gain [
153] and hence an increase in these two parameters. It also depends on the type, dose and frequency of the insulin intake given the fact that insulin is a growth hormone [
154]. These rapid increases in body weight and the BMI could be controlled by regular physical exercise, diet, and other behavioural practices . In this study, there were negligible differences in the BMI between the control and T1D groups (SMD: 0.17, p=0.003) although the T1D cohort displayed a significant BW gain (SMD: 0.22, p=0.0005) in comparison to the healthy control. The excess glucose is converted to fat and stored in the muscles and the adipose tissue but in T1D in which the glucose metabolism is impaired, these individuals could experience hypoglycaemic attacks as well during binge-eating episodes [
155]. The BMI and body weight parameters were not identified as potential biomarkers for T1D in the present study as it indicated a negligible risk of CVD although the forest plots and the paired t-test results indicated a significant difference between the HC and T1D groups.
The elevation of the soluble intracellular adhesion molecule (sICAM-1) titres is a common occurrence in T1D [
156] . The sICAM is well-known for its role in mediating inflammatory and immune-modulatory responses, particularly in downregulating the TH1 type immune responses that are responsible for the autoimmune destruction of the beta cells in the pancreatic islets [
157]. Its immune-protective functions include leucocyte recruitment and adhesion, triggering signal transduction in the islet cells and regulating the endothelial and epithelial barrier functions [
158]. The forest plot of sICAM indicated a clear mandate for a high CVD risk with an effect size or SMD of 1.09 (p< 0.0001) and additional confirmation from the student’s t-test which reported a significant difference between the healthy control and the T1D population (p= 0.02). Similarly, soluble vascular cell adhesion molecule (sVCAM-1; SMD 0.51) is a strong biomarker of endothelial dysfunction and hence a high-risk factor for CVD in both young and the old that are diagnosed with T1D [
159]. Both types of CAM are surrogate markers that display an increase in pulse pressure and hypertension, leading to progressive arterial stiffness [
159]. In this study, the sVCAM-1 showed a remarkably significant elevation in the T1D population compared to the healthy control by 20% evidenced by both the forest plot (SMD: 0.74; p < 0.00001) and the paired student t-test (p < 0.0001).
Diabetic nephropathy and renal microangiopathy are classical symptoms of progressive deterioration of the kidney functions, which is a grave consequence of T1D [
160]. The estimated glomerular filtration rate is a reliable parameter of renal disease accompanied by increased serum albuminuria, which is also a biomarker of vascular dysfunction [
161]. An eGFR below 60 mL/min/1.73 m2 signifies impaired kidneys [
162]. This study has identified the eGFR as a low-risk factor of CVD, although it was not confirmed by the paired t-distribution that showed no significant difference between the healthy control and the T1D. The mean eGFR value in this study was 122 mL/min/1.73 m2 which did not indicate impairment of renal functions probably because the mean age of the T1D cohort was less than 40 years.
Interestingly, in this study, the risk of bias which was evaluated using the ROBINS-I tool displayed an overall low estimate. The eligible studies that were included, apparently were meticulously planned, and carried out (Figures 9 and 10). The publication bias was assessed by funnel plots that are included in Figure 10. In most of the included studies, the authors measured the traditional risk factors of T1D, but not many studies reported the markers of vascular function or the inflammatory biomarkers. For this reason, the number of studies that were more were the ones of BP, glycaemic, lipidemic and obesity targets which are apparent from the respective funnel plots. However, in the other funnel plots, even though the number of studies are very few, the participant number mostly exceeded 100.
The data published by the Australasian Diabetes Data Network (ADDN) study in its 2023 publication states that most adolescents and young adults are diagnosed with hypertension, and it imposes an additive effect on the other modifiable risk factors of diabetes such as hyperglycaemia and BMI. They recommended careful revision of the existing healthcare models to reflect the true nature of diabetes-induced vascular dysfunction in order to establish an effective treatment strategy for the younger population, also stating that the diagnosis of elevated BP is often masked, and white-coat hypertension could become misleading. However, our findings do not flag hypertension as a high- risk factor of CVD which may be due to effective management of the disease with effective pharmacotherapies although the outcome of the ADDN study cannot be ignored due to the very large sample size [
105]. The data published by the SEARCH CVD study, conducted a decade ago, measured arterial stiffness with PWV that increased proportionately to the duration of diabetes in the youth. The PWV in the present study was identified as a medium risk factor of vascular remodelling, which could still be counted as a reliable measure of arterial stiffness in the youth diagnosed with T1D [
106]. A large observational cohort study that utilized data from the Swedish National Diabetes Registry on limb amputation in the T1D patients had hyperglycaemia and impaired renal functions as its strongest contenders, but not LDL-c or BMI which even in the present study displayed a low or negligible effect on T1D-associated CVD. Our findings have strong implications in improving the quality of life and health economics related to vascular disease in T1D patients that introduces a trend shift from the traditional risk factors as better markers of disease prognosis.
This study has several limitations, such as some risk factors were reported by a few studies. Contrastingly, the traditional risk factors were reported by almost all the included studies. This disparity will be reflected in the final outcome resulting in high and/or low standard deviations or standard error of the mean that impacts the effect size. Another limitation is that the study populations are distributed worldwide, belonging to different ethnicities, which hugely impacts the final outcome, given their differences in genetic, lifestyle and environmental factors. T1D is highly influenced by the socioeconomic conditions, educational and financial status, which varies widely among the participant populations. Thus it is difficult to maintain consistency in sampling sizes, sampling methods and data analysis methods. However, these drawbacks are compensated for by the large numbers of participants included per study. There are limitations also in the review process such as in retrieving full-text articles of eligible research studies from electronic databases. In some instances the authors do not respond to our requests for full-text reports or data that are missing such as the standard deviation.
The goal of this meta-analysis was to underpin the most reliable of the biomarkers of early vascular remodelling in a T1D population of children and youth below the age of 40 years because T1D features as a high-risk factor for paediatric CVD-related mortality. Our results agree with certain previous publications to date, but the primary outcome of this study has taken a new direction, with most of the risk factors that displayed a medium to large effect size being effective biomarkers of robust vascular function. Hence the clinicians and the diagnostic laboratories could further evaluate these biomarkers that we have presented in our analysis in future analytical research.