Discussion
The basal characteristics of healthy participants and patients with T1DM were compared and both groups were found to be very similar in terms of their age, BMI and waist circumference.
In this study, for the first time alongside a serum specimen, C-peptide levels have been detected in hair and nail samples of healthy adults and patients with T1DM. The mean and median hair C-peptide levels of the control group are comparable with the previous study conducted on hair samples of healthy adults.27 It has shown that the median hair C-peptide level in healthy people was twofold greater than the level in patients with T1DM, which was statistically significant. Similarly, a significantly higher median nail C-peptide was recorded in healthy individuals compared with patients with T1DM. Small amounts of insulin secretion, reflected by fasting or stimulated C-peptide, were observed in patients with T1DM in the Diabetes Control and Complications Trial and other studies.13 29 30 However, the insulin secretion found in T1DM is insufficient to regulate its metabolic effects and it is much lower than the amount secreted by healthy people. Therefore, subjects with T1DM had significantly higher FPG and HbA1c than normal individuals.
In people with T1DM, the amount of insulin secretion can be affected by the proportion of the remaining β-cells, the blood glucose concentration and the duration of diabetes.13 31 32 Our subjects with T1DM were young adults with a short duration of diabetes (mean of 3.3 years), thus it is not surprising for them to secrete small amounts of insulin/C-peptide. Additionally, increased glucose concentration may further stimulate insulin secretion. Thus, the remaining β-cells in T1DM might be under a relatively stronger glycemia-induced β-cell stimulation compared with normal people with normoglycemia.
Among the studied participants, hair and nail C-peptide levels are similar between males and females both in healthy persons and persons with T1DM. A previous study showed that although urinary excretion of C-peptide was observed to be similar in both genders, urinary C-peptide creatinine ratio (UCPCR) was higher in females than in males, suggesting that the proportion of muscle mass and creatinine excretion can affect the UCPCR.18 Therefore, hair and nail C-peptide levels might not be affected by the difference in the muscle mass as found in UCPCR.
Interestingly, among subjects with T1DM, the serum, nail and hair C-peptide levels were found to be lower in those with a diabetes duration of ≥7 years compared with the levels in patients with a shorter duration. These results are consistent with the studies that concluded there is a progressive loss of the pancreatic β-cells in T1DM. Importantly, a recent study based on the measurement of UCPCR has suggested that there are two phases of β-cell destruction: exponential β-cells lost within the first 7 years of the disease and a subsequent slower decline in β-cells mass.32 33
In the current study, the level of hair C-peptide was found to be significantly higher than nail C-peptide in the control and T1DM groups. This is in line with previous studies that extracted other hormones and found a higher level of the substance in hair than in nail samples.34 35 The reason could be partly related to the difference in the incorporation rate of the circulatory C-peptide into hair and nail tissues and the ability of the methanol to extract the C-peptide from each. Another reason could be due to the difference in the rate of growth between hair and nails, which results in different duration of exposure of these tissues to the interstitial C-peptide.24 25
These results showed that the serum C-peptide levels have a significant negative correlation with the duration of diabetes; however, such correlation was not observed with the nail C-peptide or hair C-peptide. This may be caused by prolonged exposure of hair and nail tissues to a minimal circulation of C-peptide compared with the level of serum C-peptide. Serum C-peptide was measured in ng/mL while hair and nail C-peptide levels were detected in pg/mg of hair and nails, respectively. Therefore, a trace amount of endogenous C-peptide can be incorporated into hair and nail tissues and be detectable but might be measured in serum as ≤0.01 ng/mL, as seen in a quarter of cases with T1DM. This may explain why the hair and nail levels of the hormones may not correlate with that of the serum.
Furthermore, we found that hair and nail C-peptide levels had a significantly strong positive correlation. However, neither hair nor nail C-peptide levels were found to be correlated with the fasting serum C-peptide. It is not clear why but it may be due to the hypothesis that each of the hair and nail C-peptide levels represent both basal and prandial insulin secretion. The prandial insulin secretion is around 50% of the total daily insulin secretion, thus this may explain why hair and nail C-peptide levels are not correlated to that of the fasting serum C-peptide.36 Another reason may be due to the point that hair and nail C-peptide levels represent a long-term level compared with the momentary fasting serum C-peptide.
Among the control group, nail C-peptide had a significant negative correlation with age and positive correlation with BMI; hair C-peptide levels significantly decreased with age. Consistent with the previous studies, it is well established that aging-related beta-cell dysfunction can lead to a reduction in insulin production.37 Central fat deposition is linked to increased IR, and a healthy pancreas needs to produce larger amounts of insulin to overcome the IR.38
Our results also showed that participants having first-degree relatives with T2DM had significantly lower hair and nail C-peptide levels and higher FPG compared with participants without. This finding is interesting as these participants may have a natural metabolic tendency to develop diabetes; however, our data are not sufficient to suggest that.11
Although the range limits of the sample weight were appreciated during sampling, hair and nail C-peptide levels were negatively correlated with their corresponding sample weight. This is in line with the previous study and also necessitates that a similar sample weight of hair and nails should be taken.27
This study concludes that C-peptide can be extracted and measured in hair and nail samples of healthy persons and individuals with T1DM. The significantly lower hair and nail C-peptide levels in people with T1DM compared with healthy adults and a significantly lower serum, hair and nail C-peptide levels observed in patients with a diabetes duration of >7 years compared with a shorter diabetes duration support the feasibility of C-peptide measurement in hair and nails, in normal persons and in persons with T1DM.
A significantly greater C-peptide level in hair than nail specimens may indicate different incorporation and/or extraction rate of C-peptide into and from these tissues and their different growth rate. Interestingly, hair and nail C-peptide levels had a significantly positive correlation. However, neither hair nor nail C-peptide levels were found to be correlated with the fasting serum C-peptide, which might be related to hair and nail C-peptide levels that indicate long-term insulin secretion, including both basal and prandial insulin peaks when compared with fasting C-peptide. Among the control group, nail C-peptide has a significant negative correlation with age and a positive correlation with BMI. Our results suggest that hair and nail C-peptide levels can be a valid measurement of long-term insulin secretion in healthy persons and persons with T1DM.
Although the current study is, for the first time, a measurement of nail and hair C-peptide levels in healthy persons and subjects with T1DM and has found an interesting correlation between hair and nail C-peptide levels with other relevant parameters, there are few limitations. First, we did not obtain stimulated or urinary C-peptide to be compared with hair and nail C-peptide levels. Second, our study subjects were relatively young and has relatively short duration of diabetes.
More research is needed to validate hair and nail C-peptide levels and set the normal limits by including a large sample size with multiethnic and multinational diversity. Further studies are required to also include people with prediabetes, T2DM and the observation of long-term endogenous insulin secretion reflected by the hair and nail C-peptide levels and a prediction of future T2DM among those with prediabetes and future needs of insulin management in patients with overt T2DM. Studying the link between long-term C-peptide, glycemic variability and complications appear to be interesting and clinically implemented. Future studies can link nail and hair C-peptide levels to the diseases associated with IR, like metabolic syndrome, polycystic ovarian syndrome, cardiovascular disease and non-alcoholic steatohepatitis. Future work should maximize the preserved pancreatic β-cells and may use hair and/or nail C-peptide levels as additional measurements to indicate β-cell activity.