Objective
Insulin does not work as well as it should in type 1 diabetes (T1D). The increasing obesity rates are making this worse, all of which may be increasing heart and kidney disease risk in T1D. Therefore, there are increasing needs for testing insulin sensitivity (how well the body responds to insulin) in T1D, but the current method for measuring insulin sensitivity, called an intravenous (IV) hyperinsulinemic euglycemic clamp (HE-C), is difficult for study participants and investigators, requires frequent blood draws, and has limitations. The HE-C is also not ideal if the action of a medicine being tested for T1D involves stimulation of the digestive system, as it gives the glucose by IV. Our first objective is therefore to create a simpler way to measure insulin sensitivity in T1D that is easier for individuals with T1D to tolerate, but is still accurate. Our first hypothesis is that we can calculate insulin sensitivity from an easier to perform oral glucose tolerance test (OGTT), as well as we currently do with the HE-C, and that continuous glucose monitoring (CGM) could replace the frequent blood draws. We also hypothesize that we will be able to go beyond a whole-body estimate of insulin sensitivity, to testing the insulin sensitivity of the individual tissues that respond most to insulin (muscle, liver and fat). We will do this with the OGTT method, by 1) adding stable glucose isotope tracers [naturally occurring and safe forms of glucose that can be measured in the laboratory] to the OGTT drink and to the IV fluid to measure the amount of glucose the liver makes, 2) measuring free fatty acids and glycerol that are released when fat is broken down and released from fat cells, during the OGTT and 3) having the participant wear CGM during the procedure to gather more glucose data. To address this first hypothesis, insulin sensitivity will be measured using math-based modeling of the glucose, insulin, free fatty acids and stable isotope tracers, collected before and after an oral glucose drink in T1D, and CGM blood sugars during the OGTT will be assessed to see if they improve the modeling. Our second hypothesis is that we can improve the HE-C for studies where it remains the best test of insulin sensitivity. One current limitation of the HE-C is that having well-controlled blood glucose values the night before the HE-C makes the free fatty acid, glycerol and glucose tracers difficult to measure in the blood. We aim to overcome this limitation by testing several new targets for overnight blood glucose prior to the HE-C. We hypothesize that a small increase in overnight blood glucose will allow us to better measure the glucose tracer released from the liver and free fatty acids and glycerol released from fat tissue, and thus improve the ability of a HE-C to assess the insulin sensitivity of the fat and liver tissue. We will also test the accuracy of CGM blood sugars during the HE-C by comparing them to the frequently sampled blood sugar measurements taken from the IV during the HE-C, to see if the need for frequently sampled blood sugars during a HE-C can be reduced. Lastly, we will be able to look at potential reasons for why insulin does not work as well as it should in T1D (called insulin resistance) by measuring hormones and other blood tests during the HE-C and OGTT. Understanding insulin resistance in T1D would help create ideas to make insulin work better in T1D and help stop the heart and kidney disease that insulin resistance causes in T1D, to reduce diabetes complications and lengthen lifespan.
Background Rationale
Insulin resistance (IR) is associated with heart disease, kidney disease and early death in type 1 diabetes (T1D) and type 2 diabetes (T2D), but currently is only directly treated in T2D. IR in T1D is unique, as its features (pancreatic antibodies, normal triglycerides and HDL cholesterol, low liver, abdominal and muscle fat, insulin injected near the skin) are quite different than the IR of T2D (low HDL, high triglycerides, high liver, abdominal and muscle fat, the pancreas delivering insulin directly to liver). The result: too much insulin in the periphery and too little at the liver, causing IR and allowing the liver to produce new sugar when it shouldn’t. Pancreas transplant improves IR, but only when the transplant connects to the liver. Low liver insulin also lowers liver growth factors, causing the pituitary gland to make more growth hormone, which worsens IR and increases fat tissue release of free fatty acids (FFA). Puberty also increases growth hormone, worsening IR. High FFA cause muscle IR and blood vessel dysfunction. Abnormal pancreatic glucagon secretion in T1D may further increase liver sugar release. Lastly, the body’s hormonal response to low blood sugars worsens IR. More research is therefore needed into how to treat these unique features of IR in T1D.
Simple estimates of IR perform poorly in T1D because they require the pancreas to produce insulin. Simple estimates also only assess whole-body insulin sensitivity, not particular tissues. Thus, the gold standard measure of IR, the hyperinsulinemic euglycemic clamp (HE-C), where insulin is given by IV and the body’s response to it measured, is most accurate in T1D. To adapt to T1D, investigators add an overnight IV insulin infusion to first normalize the blood sugar. To assess IR in muscle, liver and fat, the HE-C can be paired with infusions of labeled glucose and/or fat to track these substances, and with different insulin doses to assess IR in each tissue (low-dose insulin for fat, medium for liver, high for muscle).
We performed the HE-C in T1D youth and adults and found that due to high muscle IR, the overnight insulin required to keep blood sugars normal lowered fatty acid release from fat cells and glucose release from the liver before starting the HE-C, so much that they couldn’t be measured well. This happened even in normal-weight adults, but was worst in overweight teenagers. We still detected the large differences seen between people with and without T1D, but not the smaller differences needed to decide if and how interventions work in T1D. For example, we performed a study of overweight teenagers with T1D using the typical HE-C. We importantly demonstrated that 3 months of metformin improved muscle IR, but were unable to assess its impact on fat or liver IR. As a result, we don’t yet know if metformin impacts fat or liver metabolism in T1D, or if metformin directly changes muscle metabolism or changes muscle IR secondary to changes in the fat and/or liver. The HE-C is also difficult (requires blood sugar checks every 5 minutes) and ignores the digestive system’s impact on IR by bypassing the stomach with IV glucose.
The JDRF Metabolic Control Program goals include “understanding physiology in T1D and assess the benefit:risk of drugs to complement insulin action”. Current methods drawbacks block these goals, as researchers cannot currently carefully measure how treatments impact insulin action in T1D. For example, liver-targeted insulin delivery using pumps connected to portal vessels (pancreas to liver vessels) showed remarkable improvements in blood sugars and hypoglycemia, with liver-targeted insulins under development. However, to understand the full impact of these approaches, improved methods in T1D are needed.
Description of Project
Type 1 diabetes (T1D) increases the risk of kidney and heart disease, which cause early death. Increasing evidence points to a reduction in the body’s ability to respond to insulin, called insulin resistance (IR), as a cause of the heart and kidney disease in T1D. We previously found that fat, liver, and muscle tissues are affected by IR in both youth and adults with T1D, but for unclear reasons. Obesity is now increasingly a problem in T1D, which worsens IR. The best way to measure IR is called the hyperinsulinemic euglycemic clamp (HE-C). To use HE-C’s in T1D, investigators usually begin with an overnight hospital stay to maintain a normal blood sugar (approximately 95mg/dL) with IV insulin prior to starting the HE-C. However, the high insulin doses required to keep blood sugars normal overnight interfere with measuring how the fat and liver tissue respond to insulin, especially in teenagers, and with obesity. As a result, it is still unclear why IR is happening in the liver and fat in T1D, and investigators currently struggle with how to test how new T1D treatments affect IR. The HE-C is also expensive, difficult for study participants and staff, and by giving glucose IV, ignores the digestive system, the usual route of nutrient delivery. Thus, we need better methods for measuring IR in T1D to help reduce diabetes complications and increase lifespan.
We first hypothesize that muscle, fat and liver IR can be more easily measured in T1D using math-based modeling of blood glucose, insulin, and fatty acids collected during an oral glucose tolerance test (OGTT) with “labeled” glucose and glycerol. This test creates a simpler way to assess muscle, fat and liver IR for situations where HE-C’s are too difficult, or when a new intervention to be tested involves the digestive system. We will add an OGTT with continuous glucose monitoring (CGM) to 15 adults and 15 youth from the JDRF-funded TULIP and SUNDIAL studies who are already undergoing a HE-C. Muscle, adipose and hepatic IR calculated from math-based modeling from the OGTT will be compared to corresponding results from the HE-C from TULIP and SUNDIAL.
We secondly hypothesize that allowing higher overnight blood sugars before a HE-C will decrease suppression of fat release from fat cells and glucose release from the liver, improving the ability of a HE-C to assess fat and liver IR in T1D, as well as investigate why IR in T1D happens. To accomplish this, we will determine the effect of higher overnight glucose targets (120 and 140 mg/dL) prior to a HE-C with labeled glucose and glycerol tracer while wearing CGM in 10 adults and 10 youth from Aim 1 to assess the impact of higher overnight glucose on release of fatty acids, glycerol, liver glucose production and muscle IR. We will compare this data to the HE-C data from the TULIP (adult) and SUNDIAL (adolescent) studies. Our grant from leaders in the field of IR in T1D is ground-breaking in 1) improving methods for measuring liver and fat IR in T1D; 2) testing a simpler OGTT method that includes the digestive system and labeled glucose models to all together measure muscle, liver and fat IR; 3) testing whether CGM can replace or enhance the frequent blood draws currently required for a HE-C or OGTT 4) creating an understanding of why IR occurs in T1D to direct future interventions to improve IR and reduce diabetes complications. We are the ideal group to perform this study with our existing JDRF-funded pediatric and adult studies, collaborations between endocrinologists, basic scientists, statisticians, and mathematicians and expertise in all the proposed methods.
Anticipated Outcome
We anticipate that increasing the overnight blood glucose target will allow us to better detect fat cell release of fatty acids (fat insulin sensitivity) and liver release of glucose (liver insulin sensitivity) during a hyperinsulinemic euglycemic clamp (HE-C). In addition, we predict that this change will not harm the quality of our measurement of muscle insulin sensitivity, which is the amount of glucose required to maintain a normal blood sugar during the insulin infusion, called glucose infusion rate (GIR). Therefore, we expect to be able to use this new method to continue to accurately measure muscle insulin sensitivity, but also to improve our ability to measure fat and liver insulin sensitivity in type 1 diabetes (T1D). We also expect that a continuous glucose monitor (CGM) may be able to produce accurate enough blood sugar measurements during the HE-C to replace the current requirement of measuring blood sugar every 5 minutes.
We also expect to be able to develop an oral model of insulin sensitivity in T1D by adding glucose tracers and math-based modeling to an oral glucose tolerance test (OGTT). We expect this approach to allow us to estimate GIR (muscle insulin sensitivity), fat cell release of fatty acids and glycerol (fat insulin sensitivity) and liver release of glucose (liver insulin sensitivity), all from one test. This modified OGTT would be simpler for the participant and study staff than the current HE-C clamp method. The modified OGTT could replace the HE-C when it is not practical, reduce the participant and staff burden and allow inclusion of the normal stimulation of the digestive track by nutrients. We also expect that a CGM may be able to allow more frequent blood sugar measurements during the modified OGTT than typically done with an OGTT, without increasing burden to the participant or staff.
Further, we expect to identify potential underlying mechanisms of insulin resistance in T1D by examining hormone levels and other potential underlying contributors to insulin resistance in T1D during the HE-C and OGTT, and examining their influence on muscle, fat and liver insulin sensitivity.
Our strengths include our proposal to both attempt to improve current methods and work to create new approaches to measuring insulin sensitivity in T1D, our expertise in insulin sensitivity in T1D, the opportunity to add our improved HE-C measures to the study outcomes already proposed in the SUNDIAL and TULIP studies focused on insulin resistance in T1D, and our team which brings together T1D experts with basic scientists, statisticians and mathematical modelers to improve care in T1D. While the number of participants we propose is not large, we will reduce the variability by using gold-standard tests, controlling the diet and physical activity conditions just prior to the study measurements and by performing the procedures within the same person as close together as is safe to do. We do not expect to have difficulty recruiting participants because recruiting is limited to the SUNDIAL and TULIP studies, which already include participants who have already agreed to participate in a HE-C. Therefore, in our experience these participants are likely so agree to a second HE-C. However, if our trial failed to recruit adequate numbers from SUNDIAL and TULIP, new participants could be recruited and the budget re-evaluated.
Relevance to T1D
Type 1 diabetes (T1D) is well known to increase risk of early-onset kidney and heart disease and therefore early death, despite new developments in insulins, and new methods for giving insulin and monitoring glucose. Average blood sugars are the worst in teenagers with T1D and recent information from the national T1D Exchange network showed the HbA1c may have actually worsened recently in teenagers with T1D. Therefore, we urgently need new ways to improve the care of people with T1D, to decrease the risk of diabetes complications and early death. Insulin resistance (IR) is a situation where the body does not respond as well as usual to insulin, and IR is now known to be very common in T1D. For example, using the gold-standard HE-C technique, we and others have demonstrated that fat, liver, and muscle IR are substantial in both normal-weight adolescents and adults with T1D. Obesity also worsens IR and is now increasing in T1D (40% of T1D adolescents in the national T1D Exchange are overweight or obese).
IR strongly increases the risk of kidney, heart and blood vessel disease in T1D, but is not currently being directly treated in most people with T1D. For example, in our recent adolescent JDRF-funded T1D studies, T1D youth of both normal and high body weight were more IR than youth of the same weight without diabetes. IR was related to lower exercise capacity, blood vessel dysfunction, a worse blood lipid profile and more cardiovascular risk factors in the T1D youth, and with greater risk of high urine protein, and worsening in kidney function and atherosclerosis in T1D adults. IR is worse in adolescents than adults due to the impact of puberty and is also worse in women and girls than in men and boys, which may contribute to the greater increase in abnormal blood lipids, calcium deposits in blood vessels and heart disease seen when women develop diabetes vs. men. In addition, IR in T1D has very different features than in type 2 diabetes, and therefore it is yet unclear why IR happens in T1D, requiring research on the causes in order to develop treatments.
Our proposal is relevant to T1D in 1) improving methods for assessing IR in T1D; 2) being the first study to include an oral glucose tolerance test paired with labeled glucose and glycerol and mathematical modeling, which is simpler for the participant and staff, yet still allows assessment of muscle, liver and fat IR; 3) testing whether continuous glucose monitoring can replace the need for frequent blood draws during IR testing; 4) generating new information on the causes of IR in T1D to create future treatments for IR in T1D, to reduce kidney and heart disease risk 5) challenging the traditional risk factors for diabetes complications by moving past just glucose, blood pressure and LDL cholesterol, and focusing on IR.
Future directions of this work include using the information we learn about potential causes of IR in T1D to help direct researchers working on potential new treatments for IR in T1D, such as an additional medication or new ways to deliver insulin. Another future direction is using the improved methods we develop to test whether current T1D treatments improve or worsen IR in the muscle, liver and/or fat, to help better understand the impact of diabetes treatments. Our methods could also help personalize treatments by being able to measure whether muscle, liver and/or fat was more affected by IR in a particular individual, and targeting them accordingly.