I treat patients for a variety of issues: thyroid problems, perimenopause, pituitary tumors, irregular periods. Even with this full variety, more often than not our conversations work their way around to talking about how our body processes carbohydrates, how blood glucose is created, and how that process can go wrong.
Sometimes the reason for this conversational turn is obvious. I treat a number of my patients for diabetes or polycystic ovarian syndrome, where issues of carbohydrate metabolism are front and center. Other times the connection is less obvious, such as when I am sharing the causes of perimenopausal weight gain.
Frequently my patients — both those who have diabetes and those who do not — will ask if they should avoid carbohydrates altogether or monitor their blood sugar at home. Today we are going to take a deep dive into carbohydrate metabolism to understand the answers to those questions.
Scientists’ understanding of carbohydrate metabolism and insulin has grown in recent years. This can help us address problems related to carbohydrate metabolism sooner and prevent people from developing Type 2 diabetes and the many complications that high blood sugar can cause.
What are carbohydrates?
Chemically, carbohydrates are any one of a number of sugars and starches that are broken down into glucose. Practically, carbohydrates are one of the macronutrients — along with protein and fat — that make up our diets. They include the obvious — sugar, bread, rice, pasta, potatoes, and fruit — and the not so obvious — dairy products, legumes, nuts and seeds all contain carbohydrates.
All carbohydrates are absorbed through the cells of the small intestine into the bloodstream. How easily a starch can be converted to glucose affects how quickly this happens. Pure glucose, like that found in glucose tablets used by diabetics in emergencies and found in some gels used by distance runners and other athletes, makes its way from your mouth through your stomach and into the small intestine quite quickly. This spikes the blood glucose up fast.
A carbohydrate high in fiber like quinoa that you eat as part of a meal with some protein and fat takes a long time to make its way through your gastrointestinal system to your small intestine and doesn’t all get absorbed at once. As a result, the blood glucose rises and falls more gradually and typically never gets as high as it would after the same amount of simple carbohydrates.
Our brain can only use glucose for fuel. Even though our brain is about 2% of our body weight, it utilizes 20% of the glucose in our body. Our brain cells are the only cells in our body that can use glucose without the help of the hormone insulin. For the rest of our body, the hormones insulin and glucagon are critical in regulating how glucose in our blood is used and stored.
Both insulin and glucagon are made in the pancreas. Insulin lowers the sugar level in the blood by helping muscle, fat, and liver cells take up and use or store glucose. Glucagon raises glucose levels in the blood by telling the liver and fat to release stored glucose. When we are fasting, the brain gets first dibs on that glucose.
But beyond that, insulin also acts on the brain, regulating how our body expends energy, maintains temperature, and our food intake. This is one way medicines like Wegovy and Zepbound affect appetite — by changing how much insulin is available to act on the brain.
What is insulin resistance, and how is that different from prediabetes and diabetes?
Body tissues respond to insulin in a predictable way. If glucose levels in the blood rise a certain amount, the pancreas makes a certain amount of insulin to bring glucose levels back down. Several factors affect this relationship between glucose and insulin, including time of day, stress levels, and menstrual cycle changes. We all need more insulin to keep our blood glucose at normal levels in the morning, when we are under stress, and in the week or so before our period — part of the reason many of us crave carbohydrates and have an increased appetite as symptoms of PMS.
Those changes are temporary, but some changes increase how much insulin is needed to keep blood glucose levels normal all the time. That is insulin resistance. The terminology can be confusing, but people with insulin resistance actually have too much insulin. The most common causes of insulin resistance are genetic predisposition and fat deposition around the organs in our abdomen and our liver — called visceral fat. Visceral fat can form at any time in life, but the hormonal changes of perimenopause are especially favorable for this kind of weight gain.
Insulin resistance leads to higher levels of insulin in the blood and the brain, which can in turn increase appetite and weight gain, which then causes more insulin resistance — a vicious cycle. Over time, the need for more and more insulin to keep blood glucose levels normal puts stress on the pancreas, and eventually the pancreas cannot make enough insulin to maintain normal blood glucose levels. The glucose levels in the blood start to creep up.
We can measure this upward creep, and indeed glucose levels in general, in three different ways:
- Directly measuring glucose in the blood with a blood test or finger stick
- Measuring the percentage of red blood cells that have glucose attached to them, giving us a rough average for the past three months called hemoglobin A1c
- Measuring the flow of glucose in the fluid between our cells with a continuous glucose monitor and using an algorithm to convert that to estimated blood glucose
When blood glucose levels reach a certain threshold (blood glucose greater than 100 mg/dl fasting or a hemoglobin A1c or average blood sugar of 5.7%) for a sustained period of time, we call that prediabetes. If this process is left unchecked and glucose levels rise further (fasting blood glucose of greater than 126 mg/dl, a random blood glucose of 200 mg/dl, or a hemoglobin A1c of 6.5%), we call this Type 2 diabetes. Disordered glucose metabolism exists on a spectrum, with insulin resistance at the mild end of the spectrum progressing to Type 2 diabetes at the severe end of the spectrum.
Insulin resistance, prediabetes, and early-stage Type 2 diabetes are typically reversible with lifestyle changes and a modest amount of weight loss. Evidence for a particular diet is hard to come by. In most cases I will recommend a balanced diet high in fiber, lean protein, and healthy fats and low in simple carbohydrates — not a ketogenic diet, which would eliminate all carbohydrates. The goal is to make nutrition changes that are sustainable, and most of us struggle to maintain highly restrictive diets like a ketogenic diet.
Should I have my blood sugar checked or wear a glucose monitor?
If you are at average risk for diabetes — you have no family history of Type 2 diabetes and you were never diagnosed with gestational diabetes — routine screening with a blood glucose and hemoglobin A1c test at your annual physical is adequate. If you have an increased risk of developing Type 2 diabetes, you may want to check your hemoglobin A1c more frequently.
One of the most impactful developments in diabetes management has been technology that allows us to monitor patients’ glucose 24 hours a day with a continuous glucose monitor. Early monitors were expensive and bulky. They had to be calibrated routinely with a finger stick.
In the past five years, continuous glucose monitors have gotten smaller, cheaper, and easier to use. As a result, interest in using glucose monitors for purposes other than treating diabetes patients has flourished. People are using them to make diet changes in an effort to lose weight and to help with optimizing carbohydrate intake while exercising.
While there may be value in using this technology for weight loss or athletic performance, FDA-approved devices have largely been tested in diabetic populations. Our targets for blood glucose levels, including what is considered low blood glucose, are different in patients with diabetes compared with those who do not have diabetes. The ranges and cutoffs at which continuous glucose monitors are most accurate are tailored to the diabetic population.
Additionally, continuous glucose monitors do not directly measure glucose in the blood. They measure the flow of glucose in the fluid around our cells and use algorithms to convert that information into an estimate of blood glucose. Those algorithms were also developed based on data from diabetic patients.
Studies on the use of continuous glucose monitors in non-diabetic populations, like this one, show how little we know about how to interpret data from continuous glucose monitors outside of diabetes. For now, continuous glucose monitors are not ready for use in non-diabetics. Claims about using a continuous glucose monitor to support weight loss or optimize athletic performance are not backed by data.
The bottom line
- Healthy carbohydrate metabolism is critical to brain functioning.
- Insulin resistance is needing more insulin to keep blood glucose levels normal. Prediabetes and diabetes are when the pancreas can no longer make enough insulin and glucose levels begin to rise.
- Continuous glucose monitors are an attractive technology for non-diabetics, but the monitors available are tested and optimized for people with diabetes. They are not ready for use in people without diabetes.
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What about if you are already prediabetic? It’s not exactly the “diabetic” category, but it’s definitely not the weight loss/athletic category (in my case).
What do you generally recommend for folks with a family history of type 2 diabetes and history of gestational diabetes? More frequent monitoring of A1c? Home monitoring of blood sugars? Is it worth keeping a closer watch for the higher risk population who has not been diagnosed with diabetes?