Continuous Glucose Monitors (CGMs) have revolutionized the game of managing blood glucose levels. Real-time data shows how diet, exercise, and lifestyle influence glucose regulation. Fasting is one of the most sensitive times regarding your blood sugar level, especially for religious, health, or therapeutic purposes. That is why it is particularly important to have a CGM during this time.
In this blog, we’ll discuss how to analyze your CGM when fasting, what changes to expect, and how to interpret fluctuations in glucose levels to ensure a safe and effective fasting experience.

What is a Continuous Glucose Monitor (CGM)?

A Continuous Glucose Monitor, or CGM, is a tiny portable device used for recording the patient’s blood glucose levels continuously and producing real-time data on glucose fluctuation. It consists of an inserted sensor put underneath the skin; it typically wears underneath the abdomen or the arm and measures the glucose level in interstitial fluid every few minutes. It transmits this information to the smartphone or receiver, making it easy to keep track of glucose levels.
While many people with diabetes use CGMs, interest also extends to the general population interested in optimizing their health, such as those fasting to lose weight, maintain metabolism, or follow an intermittent fasting regimen.

How would fasting impact blood glucose levels?

Fasting triggers a variety of metabolic cycles in your body. The effect of fasting on glucose in the blood varies with the nature of the fast, how long you fast, and individual differences in sensitivity to insulin. Here are the key considerations:

Initial Phase (Hours 0-12):

Your body will use up any glucose in the bloodstream in the first few hours of fasting and break into the stored glycogen the body has in the form of short-term energy held in the liver and muscle tissues. Your blood sugar might drop a little through this phase, especially with not taking food, although your insulin secretion will likely fall as well, managing this.

Glycogen Depletion (12–24 hours)

As the fast continues, glycogen stores are depleted, and the body starts to use fat as fuel. At this point, blood glucose may be low but steady because of the glucagon the body has produced. This glucagon helps the liver release glucose into the blood.

Ketosis (After 24–48 Hours):

After 24–48 hours, the body enters ketosis. Ketones are broken down from fat to serve as an energy substitute for the brain and muscles. Blood glucose will start to level off at a new lower level, increasing the body’s responsiveness to insulin. In general, the CGM readings are depressed during this transition period.

Longer Fasts (48 Hours and Beyond):

Extended fasting will continue to drive blood glucose levels down as the body shifts almost entirely to ketones for energy. Depending on the patient, blood glucose will plateau at about 60–80 mg/dL. Still, they will never get low enough to become dangerous as the body becomes highly effective at holding onto glucose when fasting.

How to analyze your CGM when fasting

Using a CGM during fasting requires the identification of patterns in glucose levels and an understanding of what is expected by the body’s response to the fast. Some steps that would help interpret How to analyze your CGM when fasting are the following:

Baseline and Fasting Start

This means recording your baseline at the beginning of your fast. Under normal circumstances, CGM readings in the well-fed state should be within the normal range of 70–100 mg/dl (or 3.9–5.6 mmol/L). If baseline glucose levels are higher than this, it could point to insulin resistance or some other metabolic problem on your part, so these should always be taken into consideration when trying to make sense of your data.

• What to Expect: During the initial stages of fasting, you should expect a gradual drop in glucose levels because your body is switching from using readily available energy sources, like glucose, to using glycogen stores.

Variations and “Lows” When Glycogen Is Being Used

You will likely note dips in glucose on your CGM as glycogen stores fall and your body begins to rely more on fat for fuel. This is normal and reflects your body’s adaptation to the fast.

• What to Watch Out For: This might cause your CGM to slightly drop glucose levels from 90 mg/dL to 70 mg/dL due to some glucose release from your liver to make your glucose levels stable again. If it’s not long since you last had food, it should stay at most 60 mg/dL. But if it is trending to 60 mg/dL and continues there, then that could be hypoglycemia, and you should consult your doctor.

Stable Blood Glucose in Ketosis

Once in ketosis, blood glucose should level off lower. In general, one going into ketosis may also find that their readings on a CGM or with glucometer checks would be more stable but, of course, lower, as ketones, rather than glucose, are primarily used for energy and would show levels as low as 60–80 mg/dL. However, this is normal and not usually concerning for healthy individuals.

• What to Look For: During ketosis, you may experience a flatline or slow drop in blood glucose. You will likely also notice increased ketone production (if you use a separate ketone meter), showing that your body is switching from glucose to fat for fuel.

Avoiding Hypoglycemia (Low Blood Sugar)

For most people, especially if they are metabolically healthy, hypoglycemia during fasting is rare. However, if you experience a significant drop in glucose below 55 mg/dL (3 mmol/L), it may be a sign that you are fasting too long or have a problem with blood sugar regulation.

• What to Be Cautious About: If you are experiencing recurring glucose crashes or low levels with symptoms of dizziness, weakness, sweating, or confusion, this can be an indication of hypoglycemia, and you might want to consider breaking your fast or seek medical consultation.

Post-Fasting Glucose Spiking (Refeeding)

Right after breaking your fast, your CGM should show a spike in glucose. This is normal, as your body responds to the food rush into the system by releasing insulin to take that glucose in. This will vary with what you are eating afterward. The higher the carbohydrate intake tends to cause more incredible spikes in glucose, while balanced protein/fat/fiber-based meals are less spiky but still rising.

• What to Note: When you break your fast, note your glucose level for the next few hours. The rise in blood glucose levels should be slow, and a sharp peak, particularly if it hits 180 mg/dL or ten mmol/L, might suggest exaggerated insulin release, so it may require a change in the foods you introduce when refeeding.

Guidelines on How to Obtain an Optimal CGM During Fasting

• Track Your Fasting Duration: Pay attention to how long your fasting cycle is and understand which state you are in—glycogen depletion or ketosis. That should give you an idea of how you will look at these glucose swings.
• Don’t Overreact to Changes: Most changes in glucose are minute fluctuations that occur when one is fasting. Track the trend instead of overreacting to little variations.
• Stay Hydrated: Proper hydration will help stabilize glucose levels. Dehydration can cause artificially high, inconsistent glucose readings at other times.
• Know Your Baseline: Knowing your glucose levels before you begin fasting helps you understand whether your results are typical. If your glucose is above normal at the baseline level, it’s likely insulin resistance, possibly requiring lifestyle or dietary alterations.
• Be Aware of Exercise: If you exercise during fasting, your glucose level will temporarily decrease or stabilize according to the intensity and duration of the exercise. Note the impact of exercise on CGM readings.

Conclusion

It is a very powerful way to understand how your body reacts to fasting and monitor glucose fluctuations to optimize your fasting regimen and achieve better health outcomes. With the data of CGM, you can look at trends in the data and ensure that your body is in a healthy metabolic state, avoiding dangerous dips in glucose and making necessary adjustments to your fasting practices.
Whether fasting for weight reduction, metabolic health, or as part of your therapeutic program, a CGM can give you the information you need to make informed decisions about your health.

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