The ‘connecting’ peptide or C-peptide is a polypeptide released from the pancreatic beta-cells during cleavage of insulin from proinsulin and thus represents the capacity of insulin secretion, as both are secreted in equimolar amounts by the beta cells. Directly measuring insulin levels may be difficult in T2DM patients especially in those who are receiving insulin therapy. Hence measuring the C-peptide level is a valuable test to measure endogenous insulin in diabetic patients [7–9]. Obesity especially central obesity is known to be associated with insulin resistance which in turn is associated with hyperinsulinemia. Hence these patients correspondingly have higher C-peptide levels. However, with advancing T2DM beta cells are progressively destroyed, with C-peptide levels gradually becoming low.

WJ. Lee et al., in his study reported elevated baseline C-peptide >4 ng/ml in 58 % of morbidly obese patients with T2DM with a mean baseline C-peptide of 5.3 ± 3.5 ng/ml. There was a mean reduction by 64.1 % in C-peptide levels after a significant weight reduction 1 year after surgery with a T2DM remission rate of 78 % corresponding to decreasing insulin resistance and reduction in hyperinsulinemia [9]. WJ lee et al. reported that patients with T2DM remission had higher baseline C-peptide levels than those without remission [8]. Ramos Levi et al. also reported that patients with T2DM remission had higher C-peptide levels than those without remission [10].

WJ. Lee et al. reported diabetes remission rates for those with preoperative C-peptide <3, 3–6, and >6 ng/ml to be 55.3 %, 82.0 % and 90.3 % respectively in morbidly obese T2DM [11]. Dixon et al. in a study in a Chinese population reported fasting C-peptide concentration >2.9 ng/mL to be a predictor of remission in morbidly obese T2DM [12]. In another study in Korean population Dixon et al. reported that a baseline C-peptide >2.4 ng/ml was associated with remission in morbidly obese T2DM [13]. Lakdawala et al. reported a fasting C-peptide levels more than or equal to 3 nmol/l to be a predictor of remission in morbidly obese T2DM [14]. Ramos Levi et al. reported a cutoff of C-peptide of 3.75 ng/ml for prediction of remission in morbidly obese T2DM [10]. WJ Lee et al. reported that a fasting C-peptide levels more than or equal to 3 nmol/l is the best prognostic marker of good remission after bariatric surgery even in the non-morbidly obese [15].

WJ Lee et al. reported low C-peptide levels <1 nmol/l in 1 % of morbidly obese patients with T2DM [9]. Both WJ Lee et al. and Aarts et al. reported that a pre-operative fasting C-peptide less than 1 ng/l in severely obese T2DM indicated partial beta cell failure and predicted a markedly reduced chance of resolution of T2DM [16] In the study in a Korean population Dixon et al. reported that a baseline C-peptide of <2.0 ng/ml predicted a poor glycaemic response in morbidly obese T2 DM [13]. However interpretation of low C-peptide levels must be made with a corresponding level of blood glucose as fasting levels of C-peptide may be suppressed in a hyperglycaemic status [16].

Body mass index (BMI) has been studied extensively as a predicting variable for outcomes of T2DM following bariatric surgery. Mingrone et al. reported that the baseline BMI was unrelated to diabetes remission in morbidly obese patients [6]. Panunzi et al. also reported that the baseline BMI was unrelated to diabetes remission. They reported similar diabetes remission rates in patients with BMI of more than 35 kg/m² versus BMI of less than 35 kg/m² [17] WJ Lee et al. in a multi- institutional study found similar diabetes remission rates in patients with BMI of 30 kg/m² versus BMI of less than 30 kg/m² [11].

Some studies identified BMI as predictor of T2DM remission. Lakdawala et al. identified BMI ≥35 kg/m² as a predictor of long term remission [14]. Dixon et al. in a study in Korean population also identified BMI ≥35 kg/m² as a predictor of long term remission [12]. Robert et al. identified that at 1 year BMI less than 35 was a positive predicting factor of diabetes remission [18]. Robert et al. paradoxically demonstrated that a BMI >50 kg/m² was a negative predictor of T2DM remission [18].

The duration of T2DM reflects the natural course of the disease of progressive deterioration of beta cell function. Hence more the duration of disease, the lower the residual islet cell amount is likely to be.

WJ Lee et al. reported that less than 4 years of duration of DM will have a better outcome than those with more than 8 years (only 4 % remission rate). Hence this was added as a component in his scoring system [8, 11, 21]. Lakdawala et al. reported patients with less than 5 years duration of DM had remission rate is nearing 100 % whereas those with 5–15 years the remission rate varied between 60 and 75 % [14]. Similarly, Dixon et al. also reported that longer the duration of DM, the remission rates were lower [12, 22]. Rosenthal et al. noted that the T2DM remission rate was very low in patients who had a duration of T2DM of greater than 5 years and received sleeve gastrectomies [23]. A multi-institutional study by Lee et al. showed that the duration of T2DM was the most important predictor of T2DM remission after bariatric surgery [11]. In a study by Robert et al., T2DM less than 4 years duration was reported as a positive predictor factor for remission at 1 year after bariatric surgery [18]. Blackstone et al. in T2DM after RYGB study also supported the shorter duration of T2DM less than 4 years as a positive predictive factor [24]. Yan et al. identified that patients with a complete remission had a significantly shorter history of diabetes [25].

Age reflects the general reserve of beta-cell function which gradually declines with increasing age. Hamza et al. reported younger age to be an independent predictor of T2DM remission. They also reported that each additional 12 years of age reduced the chance of T2DM remission by 20 % [26]. WJ Lee et al. had reported that patients less than 40 years of age showed better remission rate than patients over 40 years of age which later became a part of his scoring index [9, 21]. Lakdawala et al. reported better remission rates in younger patients, with patients over 60 years associated with significantly poor outcomes [14].