**4. The mechanism of CR of treating T2DM**

In recent years, a number of studies have expounded the possible mechanisms from different sides of the CR studies.

#### **4.1. Weight loss**

It was thought that the effect of CR on blood glucose was similar to bariatric surgery, because both of them base on weight loss. Insulin sensitivity and blood glucose are improved after weight loss. However, it is still uncertain. An important issue is that blood glucose of patients fell before weight loss during 40 days VLCD [19]. Studies haven't been consistent with the effect of CR on obese patients with type 2 diabetes either. Bergman et al. [20] had compared the effect of CR on patients with normal BMI (22.8±0.42kg/m2 ) and obesity (36.1±1.548kg/m2 ), and found that both groups had a drop of hepatic glucose production volume after 12-48 hours CR, with a better improvement of the insulin sensitivity in normal BMI group. Recently, Unick et al. [14] divided the participators into overweight group, mildly obese group, moderately obese group and severely obese group according to their body weight, then they were treated with CR. The result showed that the overweight group lose less weight than other groups, but the change of other metabolic indicators was similar. This may show the change of body weight and metabolic indicators is not always parallel. So whether the effect of CR on the control of blood glucose was dependent on the weight loss is unclear now.

#### **4.2. Glycometabolism**

Firstly, CR inhibits gluconeogenesis, leading to the decrease of hepatic glucose output, thus reducing the glucose source; it increased fatty acid oxidation in the liver, and then produces ketones, which can improve the tolerance of hungry by inhibiting the appetite impulsion from hypothalamus; glucose metabolism and consumption in liver and muscles also increase after CR [21]. Secondly, CR increases the insulin sensitivity and reduces insulin resistance. When patients were recruited to evaluate the effect of VCLD, insulin resistance index significantly decreased eventually [22, 23]. However, it is still controversial. No contribution of CR on insulin sensitivity was found after 14 severely obese patients with T2DM were treated with VLCD for 7 days [10]. The short duration of VLCD and special population chosen may be responsible for this unsatisfactory result. Finally, CR improves pancreas islet function. The insulin secretion and the area under the insulin curve of OGTT increased after CR [24]. The first phase insulin secretion, which represents acute insulin response, increased after CR as well. Therefore, CR was thought to improve pancreas islet function.

the effects of 4 years of intensive lifestyle intervention in older and younger individuals [18]. Both groups were respectively divided into two subgroups and given either lifestyle inter‐ vention (include CR and exercise) or health education. After 4 years follow-up, lifestyle intervention was favourable to a better control of blood glucose, blood pressure. Surprisingly, the elderly group gained more benefits than the younger group. Therefore, CR may be

In recent years, a number of studies have expounded the possible mechanisms from different

It was thought that the effect of CR on blood glucose was similar to bariatric surgery, because both of them base on weight loss. Insulin sensitivity and blood glucose are improved after weight loss. However, it is still uncertain. An important issue is that blood glucose of patients fell before weight loss during 40 days VLCD [19]. Studies haven't been consistent with the effect of CR on obese patients with type 2 diabetes either. Bergman et al. [20] had compared

and found that both groups had a drop of hepatic glucose production volume after 12-48 hours CR, with a better improvement of the insulin sensitivity in normal BMI group. Recently, Unick et al. [14] divided the participators into overweight group, mildly obese group, moderately obese group and severely obese group according to their body weight, then they were treated with CR. The result showed that the overweight group lose less weight than other groups, but the change of other metabolic indicators was similar. This may show the change of body weight and metabolic indicators is not always parallel. So whether the effect of CR on the control of

Firstly, CR inhibits gluconeogenesis, leading to the decrease of hepatic glucose output, thus reducing the glucose source; it increased fatty acid oxidation in the liver, and then produces ketones, which can improve the tolerance of hungry by inhibiting the appetite impulsion from hypothalamus; glucose metabolism and consumption in liver and muscles also increase after CR [21]. Secondly, CR increases the insulin sensitivity and reduces insulin resistance. When patients were recruited to evaluate the effect of VCLD, insulin resistance index significantly decreased eventually [22, 23]. However, it is still controversial. No contribution of CR on insulin sensitivity was found after 14 severely obese patients with T2DM were treated with VLCD for 7 days [10]. The short duration of VLCD and special population chosen may be responsible for this unsatisfactory result. Finally, CR improves pancreas islet function. The insulin secretion and the area under the insulin curve of OGTT increased after CR [24]. The

) and obesity (36.1±1.548kg/m2

),

considered to be a treatment to metabolic disorders.

**4. The mechanism of CR of treating T2DM**

the effect of CR on patients with normal BMI (22.8±0.42kg/m2

blood glucose was dependent on the weight loss is unclear now.

sides of the CR studies.

**4.2. Glycometabolism**

**4.1. Weight loss**

188 Treatment of Type 2 Diabetes

Figure 1 shows the effects of CR on glycometabolism. CR leads to glycogen depletion in muscle and liver, and restriction of carbohydrate leads to lipolysis and the formation of ketone bodies by the liver. Together, hepatic glucose output is reduced via inhibition of gluconeogenesis and glycogenolysis. Meanwhile, high protein stimulates insulin secretion and increases satiety. Circulating ketone bodies probably contribute to tolerability of the diet by suppressing appetite in the hypothalamus. Weight loss and diminution of fat depots in the liver, muscle and peri-visceral space lead to reductions in insulin resistance. Improved insulin sensitivity, dynamic insulin secretion and reduced hepatic glucose output lead to reductions in blood glucose levels.

**Figure 1.** The effects of calorie restriction on glycometabolism. VLCD, very low calorie diet; CHO, carbohydrate.

#### **4.3. Inflammatory response and oxidative stress**

CR is "a new environment" to the human body. It causes lower blood glucose, insulin level, fat content and body weight. This helps human more tolerant to stress, thus some chronic diseases (T2DM, et al) could be prevented or treated [25]. Figure 2 has shown the effects of CR on oxidative stress. CR results in an increase in the level and activation of \_adenine nucleotide translocase (ANT) and uncoupling protein (UCPs) to reduce the mitochondrial membrane potential, which induces a decrease in superoxide radical production at complex 1 of the electron transport chain. Less damage to the lipids in the mitochondrial membrane is further reduced by increases in the membrane lipid saturation. Increases in superoxide dismutase convert superoxide into hydrogen peroxide and increased levels of glutathione peroxidase (GPX) and catalase convert this to water reducing the product of the toxic hydroxyl radical (OH-) [26]. Lower levels of OH reduce the oxidative damage to proteins and DNA, which is further ameliorated by enhanced levels of degradation and base excision repair respectively. Animal experiments also confirmed that the oxygen free radicals in mice reduced and the βhydroxybutyrate, which could act as an antioxidant, was increased after CR. In some clinical studies, some inflammatory factors (such as tumor necrosis factor-α, interleukin-6, interleu‐ kin-8) decreased significantly after CR.

**Figure 2.** The effects of calorie restriction on oxidative stress.

#### **4.4. Adiponectin**

In 2011, Qiao et al. [27] found adiponectin gene expression increased in CR mice. Adiponectin in circulation could regulate various metabolic processes, including anti-inflammatory, insulin sensitivity and resistance, et al. It becomes another possible mechanism.
