4. Discussion
Diabetes and the associated comorbidities such as overweight and cardiac
diseases are lead causes of human morbidity and mortality. Overweight is
a metabolic disorder due to the accumulation of excess dietary calories
into visceral fat and the release of high concentrations of free fatty
acids into various organs. It represents a state of chronic oxidative
stress and low-grade inflammation that may progress to hyperglycemia and
type 2 diabetes (Hotamisligil, 2006). The status of the disease diabetes
produces vital changes at the intracellular levels (Rifkin and Porte,
1990; Parinandi et al., 1990). An increasing body of evidence has
reported that there is a link between vitamin A and diabetes. Moreover,
reported the increase in the oxidative stress status in diabetes
mellitus (Halliwell and Gutteridge,
2015, Low and Nickander,
1991, Singh et al., 2001). Few studies
used the retinoic acid and suggested that it can have a reno-protective
effect in the early stages of diabetic nephropathy through a unique
anti‐inflammatory pattern (Cha et al., 2007).
In the present study, streptozotocin induced diabetic mice demonstrated
the expected increases in body weight, plasma glucose and total lipids,
consisted with the metabolic abnormalities in diabetes patients (Manzato
et al., 1993; Merzouk et al., 2000). In the current experiment,
treatment with Vitamin A had no significant effects on neither fasting
blood glucose nor insulin levels. Nonetheless, previous clinical study
proved that Vitamin A caused a transient impairment of glucose
metabolism and insulin sensitivity, as indicated by the increases in
insulin levels and HOMA-IR indices (Corbetta et al., 2006).
Interestingly, Vitamin A in our study showed a significant improvement
in body weight, fat mass, and lipid profile. Three weeks after the
beginning of the experiment, the untreated diabetic mice significantly
gained more weight compared to either the normal control or vitamin A
treated group. Subsequently, the rate of weight and fat gain also
increased in the diabetic untreated mice while decreased significantly
in the treated group especially after week 12 (at the end of
experiment). Multiple studies have shown that adipose tissue act on the
metabolism and homeostasis of vitamin A. Our results was different than
the fact that chronic vitamin A enriched diet feeding can significantly
impacted the obesity development both in young and adult obese rats of
WNIN/Ob strain, possibly through thermogenic and glucocorticoid pathways
without eliciting any toxic symptoms
(Jeyakumar and
Vajreswari,
2015). The exact mechanism of this is unclear and need further
investigations. In accordance with our results, previous research
indicated that retinoids can reduce body weight, subcutaneous and
visceral fat of mice fed high fat diet (Geng et al., 2017). A possible
mechanism by which vitamin A reduces fat and body mass, as well as
increases energy expenditure, is through the activation of uncoupling
protein 1 located in the inner mitochondrial membrane of adipocytes
(Jeyakumar et al., 2008). Nevertheless, Ilić and his colleagues,2020,
stated that vitamin A caused increase in body weight in rats on high fat
diet while high dose of vitamin A lowered body weight in rats fed on
standard diet. The discrepancy between their results and our results may
be due to the difference in dose and duration of treatment as well as
the species and gender of animals.
To evaluate the effects of vitamin A on the antioxidant enzymes
activities in diabetic-induced mice, the levels of SOD,
CAT,
and GPO in the liver tissues of all groups were estimated. These
enzymes act as one unit with each other to decrease any abnormalities of
the reactive oxygen species (ROS), that can lead to oxidative stress
(Chelikani et al.,
2004, Finkel and Holbrook,
2000, Fridovich,
1986, Jang et al., 2000).
In the present research, we demonstrated a significant decrease in the
activity of SOD in diabetic induced mice compared with control normal
group. Meanwhile, vitamin A showed a significant amelioration in the
activity of SOD in the treated diabetic mice compared with untreated
diabetic ones. It is well known that second to SOD in defense is the CAT
and GPO. (Chelikani et al., 2004, Tiwari
et al., 2013). It has been shown that CAT is essential in a pancreatic
β-cells protection from oxidative stress. Any deficiency in this
activity will lead to ROS elevation, oxidative stress and finally
cellular dysfunction that is seen in diabetes mellitus
(Góth and Eaton, 2000). GPO works also
when the level of oxidative stress is increased, to protects damaging
the cells from free radicals (Birben et
al.,
2012, Tiwari
et al., 2013). In our experiment, we demonstrated that the activities
of CAT and GPO were significantly decreased in diabetic induced mice
compared with those in control normal mice. However, treatment with
vitamin A showed a significant increase in their activities compared
with untreated diabetic ones.
Coinciding with the previous results, other studies proved the
hypothesis that oxidative stress in diabetes mellitus can be caused by
impaired antioxidant system
(Ramakrishna
and Jailkhani,
2008, Sindhu
et al., 2004). The mechanism of vitamin A on the antioxidant enzymes
still understudied. However a possible explanation is that vitamin A can
produces protein
carriers, such as zinc and copper, that can lead to an increase in
their bioavailability inside the cell. These ions can play a role in the
activity of multiple enzymes, such as the antioxidant enzymes
(Claro da Silva et al., 2016).
To examine lipid peroxidation in our study we measured MDA levels in the
serum that serves as a biomarker of lipid damage and oxidative stress
(Halliwell and Chirico, 1993). Our
results demonstrated a significant elevation in MDA in diabetic induced
mice compared with controls. These results are in agreement with
previous studies that showed an elevated level of MDA in diabetes
mellitus in both human and animal studies
(Ramakrishna
and Jailkhani,
2008, Sindhu
et al., 2004). On the other hand, treatment with vitamin A in diabetic
induced mice demonstrated a significant decrease in MDA levels compared
with that in untreated diabetic mice. These Results can support the
hypothesis that vitamin A can have an antioxidant effect and protection
effect on the damage induced by lipid oxidation. Vitamin A’s antioxidant
properties have been frequently reported in vivo and in vitro
(Palace
et al., 1999). Retinol can react with peroxide radicals (ROO •),
thereby it interrupts the chain reaction of lipid peroxidation to form
hydroperoxides (ROOH). Furthermore, vitamin A is capable of directly
reacting with ROS to form a 5,6-epoxide retinoid
(Edge
et al., 1997).
The present research demonstrated that lipid profile was significantly
improved after 12 weeks of treatment with Vitamin A in diabetic induced
mice. This amelioration can be explained by the relationship of lipids
and the oxidative symptom, however, the definitive role of lipid
contribution to the oxidative system has yet to be fully elucidated. In
consistent with our results, a clinical study proved that patients with
cardiovascular diseases have elucidated Low levels of vitamin A.
Moreover, these patients had lower HDL-cholesterol levels indicating the
possible beneficial effects of vitamin A on serum lipids as well as
cardiovascular diseases
(Godala
et al., 2017) .
In conclusion, the results of the present study demonstrated some
beneficial effects of Vitamin A in streptozotocin-induced diabetic mice.
It caused significant improvement in body weight, lipid profile, lipid
peroxidation and oxidative stress. These data suggest that Vitamin A
supplementation might help in decrease the metabolic complications of
diabetes mellitus. However, further studies should be done to examine
the appropriate dose of Vitamin A to avoid side effects or toxicity.
Also, more clinical research is needed to determine how Vitamin A status
affects humans.