This study aimed to evaluate the effect of EE and macronutrient substrate utilization on severe hyperglycemia requiring multiple daily insulin injections and co-treatment with SGLT2i compared to the CRIC diet. We evaluated the effect of caloric restriction in the CRIC diet, in which carbohydrate loss was matched to the amount of urinary glucose loss in patients with type 2 diabetes and compared it with SGLT2i treatment. The restricted carbohydrate amount in the CRIC diet were matched to the amount of glucose eliminated in urine with renal SGLT2 inhibition and approximately 50 g/day of glucose3 was reduced from the CON without changing the amount of protein and fat content in the diet. The pharmacological inhibition of SGLT2 facilitates lipid oxidation10,18. Hyperglycemia impairs energy substrate oxidation in the fasting and postprandial states, in terms of metabolic inflexibility19 via the impaired utilization of intracellular energy sources.20 Thus, the increased elimination of glucose in the urine reduces excessive extracellular glucose, which is expected to improve the burden of substrate oxidation within intracellular energy availability, leading to the mitigation of excessive glucose toxicity. A previous study assessed the efficacy of basal/bolus insulin versus sliding-scale insulin in a tertiary hospital setting only for a period of one week.3 Therefore, we examined the effects of the co-administration of SGLT2i with intensified insulin therapy for treating severe hyperglycemia, and whether this combination was capable of resolving glucotoxicity and achieving euglycemia within a week, which was continued over a 3-month period. We hypothesized that the CRIC diet and SGLT2i may have an impact on the fasting and postprandial EE, substrate oxidation rate, and insulin requirement to the same extent.
In this prospective 12-week study, we compared the effect of the pharmacological inhibition of SGLT2i and the equivalent amount of restricted carbohydrate intake within the CRIC diet, both of which reduced the net glucose balance. Although the plasma glucose and FFA responses after the meal were comparable in the SGLT2i and CRIC groups, the total insulin requirement was significantly higher in CRIC compared to SGLT2i during the study period. The total body muscle mass and total daily dose of insulin were increased in the CRIC group compared to the CON group, implicating increased anabolic demands due to the relatively high proportion of the protein intake21,22 Data from a series of studies suggest that high protein intake could have detrimental metabolic effects; acute intravenous amino acid infusion or protein ingestion reduces insulin sensitivity23,24,25,26, and habitual high protein intake is associated with insulin resistance and increased risk of developing type 2 diabetes27,28,29.
The mechanisms responsible for the adverse effect of the CRIC diet on insulin action are unclear. The CRIC diet consists of relatively high protein and fat content compared to the CON. Studies conducted in cultured myotubes, and isolated skeletal muscles in rodents have demonstrated that amino acids, especially the branched-chain amino acid leucine, can impair insulin-mediated glucose uptake by adenosine monophosphate-activated protein kinase (AMPK)-mediated mammalian target of rapamycin (mTOR) phosphorylation and subsequent negative feedback inhibition of phosphatidylinositol 3-kinase (PI3K)-AKT signaling30,31.
Therefore, the discrepancy between the urinary loss of carbohydrate and equivalent reduction in dietary carbohydrate intake, with respect to the negative carbohydrate balance in the entire metabolic response in patients with type 2 diabetes, and a relatively high protein intake ratio nullify the beneficial effects of insulin and subsequently increases the required dose of insulin. Thus, adding an SGLT2 inhibitor to intensive insulin therapy may have some effect in overcoming glucotoxicity by reducing extracellular glucose availability and lowering the dose of insulin required to achieve glycemic control, irrespective of the relatively high protein and fat availability.
Impact of the method used to limit carbohydrate availability (low oral intake or high urinary output) on EE
SGLT2 inhibitors enhance urinary glucose excretion, and thus increase whole body energy loss32. The resting EE were similar in all groups in this study. We first assumed that SGLT2i group had a lower capacity to maintain negative energy balance (urinary elimination of glucose of approximately 50 g per day accounts for energy loss of 200 kcal/day) during urinary energy loss (i.e., glucosuria), which inhibits adaptive thermogenesis, leading to a suppression in the EE10,33. SGLT2 inhibitors are associated with a reduction in body weight, which appears to be mediated by increased energy loss as a common denominator19. The whole-body baseline EE did not differ in the CON, SGLT2i and CRIC groups.
However, energy consumption did not differ between the CON and SGLT2i treatment groups, suggesting that baseline EE was more likely to respond to ingested calories than the whole-body net energy; the SGLT2i group had extra urinary calorie loss, even if the calorie intake was the same as the CON group.
The degree of post-prandial EE depends on the proportion of macronutrient intake34. Underlining this process, the EE after protein ingestion is higher compared to the carbohydrate and fat intake35. We observed that increased post-prandial EE in the CRIC group than that associated with a relatively high proportion of protein intake would have an impact on the EE.
Impact of the route of limited carbohydrate availability (low oral intake or high urinary output) on the respiratory quotient
In the physiological state, the daily whole-body RQ is characterized by diurnal fluctuations, reflective of a metabolically flexible state in which the mitochondria switch the energy substrates (lipids and carbohydrates) based on the nutritional and physiological cues36. A high RQ is indicative of glucose oxidation, whereas a low RQ reflects predominant fat oxidation, where the role of amino acids as an oxidative substrate is only minimal. Similar to previous studies3,the basal RQ was lower in the SGLT2i group compared to the CON, owing to the reliance on enhanced fat oxidation during the fasting state in the SGLT2i group. Although not significant, the basal RQ in the CRIC group tended to be lower compared to the CON group. The basal RQ would be resulted by the whole-body carbohydrate availability, and not the net carbohydrate loss.
Progression toward the postprandial state is accompanied by increased carbohydrate oxidation and a corresponding increase in the RQ in individuals with normal physiology36. The differences in the fasting and post-prandial respiratory exchange ratio were similar in the SGLT2i and CRIC. This indicates that the amount of remaining carbohydrate in the body was essential for whole-body energy metabolism. The difference in the method of energy dissipation, i.e., decreased oral intake or increased urinary glucose loss, did not contribute to the whole-body energy expenditure.
There were several limitations to the present study. First, we only enrolled in-patients with type 2 diabetes; however, this was necessary because we needed to regularly obtain blood and urine samples to measure the metabolic substrates, EE and RQ at multiple times under controlled conditions. Second, the result of the fasting-to-fed state experiments were obtained from a single meal per day for 12 weeks and cannot be generalized to all patients with type 2 diabetes. Third, we could not evaluate the effects of ketone bodies or the activities of different glycolytic pathways using tracer methodologies. Finally, the short treatment period (12 weeks) might be insufficient to observe clinically meaningful changes in some variables; therefore, long-term studies are necessary to verify the present results.
In conclusion, we compared the effect of reduced carbohydrate availability using two different approaches (low intake or high urinary output) in order to assess the insulin requirement to achieve euglycemia, whole-body EE, and metabolic flexibility (RQ) in the fasting and postprandial states in patients with type 2 diabetes. The total daily dose of insulin was higher in the CRIC group compared to the SGLT2i group, probably due to increased anabolic demands due to the relatively high protein intake compared to the SGLT2i and CON groups. The baseline EE was similar in all groups; however, the postprandial EE increased with SGLT2i and the CRIC diet, which indicates that the proportionally high protein and fat availability altered the substrate preference, rendering it amenable to macronutrient oxidation. Furthermore, the decreased RQ in the fasting state and increased RQ in the postprandial state in both SGLT2i and CRIC groups indicates similar metabolic flexibility in response to similar macronutrient availability.