Diabetes is a disease of the pancreas characterised by multiple defects in its pathophysiology¹ and abnormalities in protein, fat and carbohydrate metabolism.^2,3 It is evident that this disease leads to hyperglycaemia and to many other complications such as hyperlipidaemia, hypertension, atherosclerosis, retinopathy, neuropathy and nephropathy.⁴ The World Health Organization (WHO) projects that diabetes will be the seventh leading cause of death in 2030.⁵ Statistics also show the disease will affect more than the current 347 million people with diabetes by 2030.^6,7 The Diabetes Association of Nigeria had put the diabetic population in Nigeria at about 10 million as of 2004.⁸ The high cost of diabetic medications, their unavailability in poor developing countries and the high cost, where the majority of people in developing countries can barely afford such costs, coupled with the side effects from these drugs, have led to a search for a number of plants used to manage diabetic patients with folk medicines, and the innovation will result in isolating plant active secondary metabolites that can be cost-effective and safer than synthetic products.

Though the mechanism of these plants is not well documented, the majority of studies are being conducted to uncover the mechanism of action of these plants and their isolates. Hence, more attention is being paid to study the proper mechanism of these plants’ actions along with the bioactive compounds responsible for these activities.

Tannins, a secondary metabolite from plants, have been described as developing glucose uptake through mediators of the insulin-signalling pathways, such as phosphoinositide 3-kinase and p38 mitogen-activated protein kinase activation, GLUT-4 translocation^9,10,11 and antihyperglycaemic agent in diabetic rats.¹² Iweala et al.¹³ have shown that the seeds of Spondias mombin possess hypoglycaemic effects in alloxan-induced diabetic rats, while Nkanu et al.¹⁴ have shown that the leaves of S. mombin produce hypoglycaemic and hypolipidemic effects in alloxan-induced diabetic rats. However, little or no work is available on the effect of the tannin extract of S. mombin leaves on streptozotocin-induced diabetic rats. The present study seeks to evaluate the serum amylase activity and changes in organ and body weight of diabetic rats treated with tannin extract from S. mombin leaves.

Table 1 shows the effect of tannins on the body weight of the rats. A significant decrease in body weight was seen in the diabetic control group, which continued throughout the experimental period. On the other hand, administration of tannin extract to the diabetic rats led to a significant (p < 0.05) increase in body weight at Week 3. For the glibenclamide-treated group, significant increase was seen at Weeks 2 and 3.

Table 2 shows the organ weights of rats treated with tannin extract for 3 weeks. A significant (p < 0.05) increase and a significant (p < 0.05) decrease in the weights of the liver and pancreas, respectively, were observed in the diabetic control rats when compared to the normal control. Administration of tannin extracts reverts these changes seen in the diabetic control groups, whereas non-significant changes (p > 0.05) were seen for the hearts and kidneys of all STZ-induced rats when compared with the control.

Table 3 shows the effects of tannins on blood glucose level of control and tannins-treated diabetic rats.

A significant increase (p < 0.05) in blood glucose level was seen in the diabetic rats at Day 3 after STZ administration when compared with the normal control. Administration of tannin extracts led to a significant (p < 0.05) decrease in blood glucose level at Day 10 and Day 21 when compared with the diabetic control groups but was not significantly reduced when compared with the normal control.

Table 4 shows the effects of tannins on serum amylase level of control and diabetic rats.

The serum amylase results showed a significant decrease in serum amylase activity in the diabetic control rats. A significant increase in serum amylase activity was seen in the tannin-treated diabetic rats when compared with the diabetic control groups.

Diabetes is the leading cause of mortality worldwide and STZ, which is a naturally occurring nitrosourea, has been widely used to induce insulin-dependent diabetes mellitus in experimental animals because of its toxic effects on islet beta cells.¹⁷

In the present study, induction of rats with STZ led to a significant decrease in body weight in all administered groups when compared with the normal control (Table 1). However, a significant increase in body weight was seen in the tannin-treated diabetic rats when compared with the control at Week 3. Significant decrease in body weight was seen in the diabetic control groups, which continued until the end of the experimental period. The significant reduction in total body weight seen in the diabetic control rats despite their increased feed intake is attributed to the fact that STZ-induced diabetes is characterised by severe loss of body weight,¹⁸ resulting from loss of fat from adipose tissue and catabolism of amino acids in muscle tissue. Moreover, there is loss or degeneration of structural proteins that are known to be a major contributor to body weight. Again, it is also an indication that the food being taken in is not being transformed into weight gain or growth attainment.

The findings from organ weight measurement in this study showed a significant increase in liver weight for the diabetic control rats and a non-significant change in the liver of diabetic tannin-treated rats when compared to the control liver (Table 2). The result also shows a non-significant change in the weight of the heart and kidney of treated and diabetic control rats. Moreover, a significant decrease was observed in the weight of the pancreas in the diabetic control rats when compared with the treated rats. The decrease in the weight of the pancreas could be attributed to the disruption and disappearance of pancreatic islets and selective destruction of insulin-producing cells.^19,20 The significant increase in the weight of the liver seen in this study could be attributed to increased triglyceride accumulation leading to enlarged liver, which could be a result of the increased influx of fatty acids into the liver induced by hypoinsulinaemia. This finding is in agreement with previous work by Habibuddin et al,²¹ Lee et al.²² and Ohno et al.²³

The destruction of the insulin-secreting β-cells starts 3 days post-STZ administration, reaching its peak at 3–4 weeks in rats, leaving less active cells, which results in a diabetic state.^24,25 This explains the increased glucose concentration seen in the study on Day 3 after STZ administration (Table 3). Treatment with tannin extract from S. mombin leaves led to a significant decrease in blood glucose level on days 10 and 21; this decrease was comparable to that seen for the glibenclamide-treated groups but not comparable to the normal control. This result is in line with work conducted by Velayutham et al.²⁶ on the antihyperglycaemic effect of tannins from Ficus racemosa. Moreover, Babby et al.²⁷ showed that tannic acid reduces blood glucose levels in STZ-induced rats; Praveen et al.²⁸ showed that tannins from Jatropha gossypifolia exert antihyperglycaemic effects. Studies have demonstrated that tannic acid induces glucose transport through activation of the insulin-mediated signalling pathway in adipocytes, in human diabetic patients.²⁹ A study conducted by Xueqing et al.³⁰ showed that tannic acid may have the potential to become the lead compound in the development of new types of antidiabetic pharmaceuticals that are able to reduce blood glucose levels without increasing adiposity.

Amylases are enzymes that hydrolyse the carbohydrates into small glucose particles.³¹ All the alpha amylase enzyme molecules act on alpha 1-4, 1-6 links of glucose residues.³² The enzyme alpha amylase is one of the most important in the human body, responsible for hydrolysis of starch into small sugar molecules.³³ Amylase is one of the main enzymes produced in exocrine pancreatic cells and may be recognised as an adequate indicator of organ activity, both in physiological and pathological states. The significant decrease in serum amylase activity seen in the diabetic control rats could have resulted from the induction of the animals with STZ (Table 4). STZ has been proposed to cause a reduction in amylase activity by interfering with calcium and magnesium homeostasis and amylase gene expression.³⁴ Administration of tannins extract to the diabetic rats led to a significant (p < 0.05) increase in serum amylase levels when compared to the diabetic control groups. This therefore shows that the tannins had a stimulant effect on this enzyme and probably exert a similar influence on other pancreatic hydrolases. It has been clearly demonstrated that a significant number of type I diabetes mellitus patients have impaired exocrine pancreatic function, as shown by a reduced duodenal output of enzymes in response to endogenous (meal, nutrients) and exogenous stimuli.

Tannin extract of S. mombin leaves has shown to be a potential agent in the treatment of hyperglycaemia resulting from diabetes as well as restoration of body weight and serum amylase level in STZ-induced diabetic rats.