Understanding Blood Glucose
In everyone's blood there's a type of sugar called glucose, and it's absolutely essential for human life. Digested food is the source of this glucose, and blood carries it around the body to energize cells, keeping the muscles moving and the brain thinking. But balance is key: Blood glucose that's too high or too low can have serious consequences for your health. Here's a guide to how glucose behaves in the body, how you can monitor it, and what can be done to keep this vital substance working for you.
Glucose and Insulin
The human body has evolved an elaborate scheme to keep glucose high enough for cells to remain well nourished but not so high that it gets in the way of the biological processes that keep you healthy. Blood glucose regulation requires the cooperation of many body parts—including fat tissue, muscles, the brain, the liver, the pancreas, and the small intestine—as well as a host of hormones, including insulin. Diabetes develops when some members of this glucose-regulating team fail to do their job properly, leading to elevated blood glucose. Over time, high glucose levels can do damage to the eyes, kidneys, nerves, and blood vessels.
For most of the day, in a person who doesn't have diabetes, the body keeps blood glucose levels in an ideal range of 70 to 100 mg/dl. After a meal, as food is broken down for use by the body, blood glucose temporarily rises. That increase triggers the production of insulin, which signals to the cells in the body—particularly those in fat, muscles, and the liver—that they should absorb any extra blood glucose, either burning it as energy or storing it for later use. As cells absorb glucose, its concentration in the blood stops increasing and starts to coast back down toward the target range. As blood glucose falls, insulin production tapers off, signaling to cells that it's time to stop absorbing so much glucose. Thus, blood glucose levels stabilize.
Between meals or during exercise, the body still needs a continuous supply of glucose to function properly. The liver plays the part of glucose reservoir: It not only stores extra glucose from a meal but makes it from scratch as needed. Since insulin is the body's signal that you've just eaten and have plenty of glucose available, the liver recognizes the absence of insulin as a sign that food is scarce and that it should trickle out some of its glucose stores to keep blood glucose levels from dipping.
This orderly rise and fall of glucose, punctuated by periods of stability, is seriously distorted in diabetes, a disorder caused by a shortage of insulin. Without this hormone doing its job, blood glucose levels remain high after a meal. Not only that, but an insulin deficit signals to the liver that it should keep delivering glucose to the bloodstream, sending glucose levels even higher.
Measure by Measure
The first thing you need to know in order to keep blood glucose under control is, of course, what your blood glucose is doing. The gold-standard laboratory test of blood glucose is the hemoglobin A1C test. It measures what percentage of hemoglobin, a protein in the blood that carries oxygen around the body, is "glycated," or has a glucose molecule attached to it. The test measures the average concentration of glucose in the blood over the previous two to three months. An A1C of 7 percent means that 7 out of 100 hemoglobin molecules in the blood are tagged with a glucose molecule. (Because the A1C looks at a part of the red blood cell, some people—for example, those with sickle cell anemia—may get erroneous results and require an alternate form of testing.)
|Glucose by the Numbers|
|A1C and estimated average glucose (eAG) both describe average blood glucose levels over the previous two to three months. Here's how those numbers compare to make a diagnosis of pre-diabetes and diabetes.|
|A1C||5.7–6.4%||6.5% and above|
|eAG||117–139 mg/dl||140 mg/dl and abov|
Today, diabetes can be diagnosed when A1C is 6.5 percent or higher. But most people with diabetes have been diagnosed based on either of two other tests: the fasting plasma glucose test, which is done after not eating for at least eight hours so that it isn't influenced much by recent meals, or the oral glucose tolerance test, for which patients consume a standard sugary drink, wait two hours, and then have their blood glucose checked. Unlike the A1C, the results of both tests—like those of home tests on blood glucose meters—are given in real concentration units (mg/dl), not percentages. Recently, the way laboratories measure the A1C changed, making the test more accurate.
Another number you may see is estimated average glucose (eAG). Derived from the A1C, the eAG is used to give people an idea of their average blood glucose over two to three months, but it is expressed in mg/dl instead of percentages, so it looks more like what people get when they measure their own blood glucose.
While the A1C and eAG offer a big-picture view, it's home blood glucose testing that gives you the tools to understand how blood glucose reacts to each part of your day, from breakfast to bedtime. Most people measure their blood glucose with a meter and a test strip. These strips are coated with an enzyme that basically steals an electron from each molecule of glucose in a drop of blood. Through some fancy electron swapping, an electrical current is generated when the test strip is inserted into a meter that is proportional to the number of glucose molecules in a blood sample. The meter can convert that current into a blood glucose reading.
The most detailed picture of blood glucose that technology has to offer comes from continuous glucose monitors (CGMs), small devices worn on the body that offer round-the-clock updates. Instead of taking measurements from blood, a CGM has an enzyme-coated glucose sensor that slots under the skin into the fluid that permeates the body's tissues (interstitial fluid). While the sensor functions in much the same way as a test strip in a meter, the glucose levels in interstitial fluid are different from and lag a bit behind those in blood. That's why CGMs must be used in conjunction with a standard meter. The sheer volume of data coming out of a CGM (a glucose reading every five minutes or so) is astounding, and there are software programs that help sort through this information and provide meaningful analyses and trends. Recent studies found that using a CGM helps people with type 1 diabetes achieve better blood glucose control.
Learn and Live
Without blood glucose measurements, it would be hard to assess whether a new exercise routine, diet, or medication is making someone healthier or not. Exercise is helpful for people with diabetes because it makes the body more receptive to insulin, so a little can go a longer way. Choosing healthy foods—fruits, veggies, and whole grains over unhealthy fats and refined sugars—can also tame high blood glucose. Medications try to lower blood glucose by providing more insulin directly or by getting members of the body's blood glucose regulatory team to work more effectively.
Sometimes, however, medication can cause a problem by leaving the body with an excess of insulin. This may cause blood glucose levels to dip below the target range, a dangerous condition called hypoglycemia. If levels remain too low, the body and, most important, the brain are starved of energy. In severe cases, hypoglycemia can lead to coma or death.
It's virtually impossible for medication to regulate blood glucose as well as a healthy body does, but with good medical care and personal vigilance, it's possible to get in the ballpark. Proper blood glucose control, coupled with attention to blood pressure and cholesterol, can help prevent such severe diabetes complications as nerve damage, blindness, amputation, heart attack, stroke, and kidney failure. So even though tracking blood glucose and understanding its patterns may seem like a challenge, this is one undertaking that offers up healthy rewards.