Your Fasting Blood Sugar Level Doesn't Really Tell Me How Healthy You Eat
Let's talk about the tests that better describe your metabolic health
In today’s newsletter:
What a fasting blood sugar (glucose) tells me (and what it doesn’t)
The tests that better describe metabolic health and risk factors
Links and resources: one of my favorite nutrition tools, what I’m reading, and a couple things that make life in the kitchen easier! (for paid subscribers)
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I use lab testing with nearly all of my clients. Labs are a helpful and more objective way to assess progress. And, while blood sugar (glucose) levels are a good screener, I find that some people become overly concerned about a single blood glucose test. A fasting glucose is limited in terms of what it actually tells me about someone’s metabolic health, especially if we only have one or two measurements to go from. Your blood glucose level (assuming it was drawn after a 12-hour fast) tells me a number of different things, and not all of them have to do with your diet:
Stress: Cortisol is a steroid hormone and classified as a glucocorticoid. “Gluco” means sugar; it’s a sugar-regulating hormone. When cortisol is released, especially due to stress, a cascade of events lead to the release of sugar from the liver (via glycogen stores). So, even if you didn’t eat anything the morning of your blood draw, your glucose could be elevated because you had a more frantic or stressful morning. Your body is trying to make sure you have plenty of energy to get through whatever stressor you’re facing.
Illness: When you’re sick, your body tries to make sure you have the fuel needed to fight infections and heal. Thus, when you’re sick, your fasting blood sugar may be higher than I would typically like to see it (over 90 or 95 in my book).
Your activity: Did you exercise (do weight lifting) the day before the draw? Did you walk or workout that morning? Did you decide not to exercise like you typically would because of the blood draw? This could make the fasting glucose higher or lower than it might typically be.
When you last ate. A fasting blood glucose level could be very different if the last time you ate before the draw was 6:00 pm versus 10:00 pm, or if you waited until 1:00 pm to have your blood drawn, and you haven’t eaten since the evening before. In a later-day blood draw scenario, your body would likely release sugar from glycogen stores in your liver to ensure your brain and body had the fuel they need to operate until your blood draw. Thus, you might have a higher-than-usual blood sugar level.
Hormonal changes: Transitioning through perimenopause and menopause can increase insulin resistance. Estrogen is an insulin-sensitizing hormone. That means estrogen helps the body use sugar and other carbohydrates more efficiently. As women transition through perimenopause and menopause, estrogen eventually reaches very low levels. This can be one reason that blood sugar and A1c can increase as women age.
Certain medications: Steroid medications (e.g. Prednisone) can increase blood sugar significantly. It’s called “steroid-induced hyperglycemia.” Other medications that can raise blood sugar include statins, blood pressure meds, protease inhibitors, diuretics, and antipsychotics.
What you last ate: Of course, your blood sugar is affected by what you eat. Did you have a burger, fries, and shake the night before your blood draw? Or, did you have a big green salad with chicken, avocado, and pecans?
More to learn: Based on this paper, there are several other hormones besides insulin that impact blood sugar regulation. I often have to remind myself that we know a lot compared to 25 years ago, but we still have so much yet to learn.
What’s really tricky about interpreting fasting blood sugar is how much any one (or several) of these variables is playing a role in your blood sugar levels.
Here’s a list of what I would suggest you get tested to have a better idea about your level of insulin resistance or blood sugar control. And, before we get to the list, if you decide to complete any of the following tests, please work with someone trained to interpret and guide any diet, supplement, or other lifestyle changes.
Hemoglobin A1c (“A1c”): This is a ~3 month average marker for blood sugar control. Like fasting glucose, it’s not perfect, but it does give us another marker to track.
Cholesterol panel: particularly your triglycerides (TGs), your HDL (“healthy” cholesterol level) and your TG-to-HDL ratio. A higher ratio may be a marker to predict insulin resistance/metabolic syndrome, coronary, and/or cardiovascular disease.
Fasting insulin: This tells a story about your level of insulin resistance (the metabolic pattern of diabetes). I’m picky about these levels (typically, the acceptable range is anything <20, which to me is much too high), but my goal is to help clients prevent or prolong the diagnosis of chronic diseases and keep them well. I like to see fasting insulin ~5 or lower. Our insulin rises in response to the presence of food in our bodies, so if you’ve been fasting for 12+ hours, insulin should be low.
Continuous glucose monitor (CGM): This is a monitor that you wear all day, every day for about 10 days at a time. You can sleep, exercise, and shower while wearing it. CGMs can show us how our blood sugar levels respond to various meals, workouts, stressors, sleep, and even how it changes throughout a menstrual cycle.
Other (more obscure?) tests that measure insulin resistance/sensitivity, cardiometabolic risk and health:
Fructosamine: this is an average marker of blood sugar control for ~2-3 weeks prior to the draw.
LADA Antibodies: Latent Autoimmune Diabetes of Adulthood is sometimes nicknamed “Type 1.5” diabetes. It’s an unusual form of diabetes that appears in adulthood that is kind of a combination of types 1 and 2 diabetes.
Advanced lipid panel: This measures the number and size of cholesterol particles: larger cholesterol is less risky than smaller, denser cholesterol.
Lp(a): This is more predictive of cardiovascular risk, however there hasn’t been a lot of research that suggests one’s lifestyle can influence Lp(a) levels.
While any lab test can tell us something, the significance, accuracy, and validity of any single test is variable. A fasting blood sugar on its own is one data point, but if you combine that with an A1c and a cholesterol panel and ideally a fasting insulin, then we start to paint a picture of your metabolic health and risk factors.