How to lose weight using a continuous glucose monitor (CGM) and Data-Driven Fasting

With the explosion of continuous glucose monitoring (CGM) technology, a new wave of biohacking is on the rise.

Constant instantaneous access to your blood sugar can give you a powerful insight into your metabolism. 

You can even use your CGM data as an instantaneous fuel gauge that tells you if you need to eat NOW or you can wait a little longer.

The perils of paralysis by analysis

But not everyone does better with MORE data.  Too much data can be confusing and overwhelming for some people. 

It’s critical to understand how to interpret the data and leverage the full power of this exciting new technology while avoiding paralysis by analysis.

This article will teach you how to harness the power of your glucose monitor using only the minimum effective dose of testing, pain, and expense.   

First, we’ll look at how the different macronutrients in the food you eat affect your blood glucose.  Later, we’ll look at the different options for tracking your blood glucose, including cost comparisons to ensure you get the most bang for your buck.

How does food affect your blood glucose?

A CGM can provide powerful insights into how the food you eat affects your blood sugars. 

Many things affect your blood sugars (e.g., stress, sleep, exercise, that time of the month for women, etc.).  But by far, the most significant manageable factor is the food we eat.

Sadly, many people get confused and frustrated by the data and give up before they can tame the little dotted line on their CGM. 

To correctly interpret your blood glucose fluctuations, it’s crucial to understand how each macronutrient (carbs, fat, protein, and alcohol) influences your blood glucose. 

Carbs

High carb foods (e.g., rice, potato, vegetables, fruit, etc.) will raise your blood sugars quickly.  But it’s likely that they will also fall again soon (e.g. within a couple of hours). 

Because your body doesn’t have much room to store glucose, your pancreas sends out a sharp burst of insulin to slow the release of glucose from your liver until you have used up the carbs from your food and allow your blood glucose to drop to normal levels.  For more details, see What affects your blood sugar and insulin (other than carbs)?

You don’t need to be too concerned about a moderate rise in blood sugar within the normal healthy range that quickly returns to your baseline.  But if you see your blood sugar rise significantly (i.e. by more than 1.6 mmol/L or 30 mg/dL after eating) AND stay elevated for a long time, you should consider eating less of or avoiding that food in the future. 

Reducing the carbs in your diet will allow the glucose in your blood and your liver (glycogen) to be depleted.  This allows the fatty acids in your blood to be used up, and eventually, your body will turn to your body fat for fuel. 

oxidative priority

Fat

You are unlikely to see a significant increase in your blood glucose after a high-fat meal. 

Some people believe ‘fat is a free food’ because it doesn’t raise blood sugars or insulin quickly.  But the reality is, your body has plenty of room to store fat from your diet, and you also don’t require as much insulin to convert fat from your food to fat on your body. 

Fat in your diet will fill your onboard fuel tanks.  If your fat stores are overfull, it will also take longer for your blood glucose to fall.  When your body fat fuel tank is full, your blood glucose backs up and overflows into your bloodstream. You may also see your blood sugars higher the following day after a high-fat meal at night. 

A diet with fewer carbs and more fat will allow your glucose stores to be depleted, but may still keep your body fat stores locked away if it contains too much fat. 

Fat+carb combo foods

Rather than fearing short term blood glucose ‘spikes’, we want to avoid foods that will keep your blood sugar elevated for longer. 

You want to reduce the overall area under the curve of your CGM plot.  You can do this, not just by reducing the rise after meals but by also lowering the blood sugars before you eat and while you sleep.

Our satiety analysis of half a million days of MyFitnessPal data shows that it’s the foods that contain a similar mixture of fat and carbs together (usually with low protein) that we tend to overeat and leave our blood sugars and insulin elevated for longer. 

carbohydrate effects on satiety

When available, we naturally gravitate to fat+carb combo foods. 

fat and carb food combinations effects on satiety

The most effective way to reduce the area under the curve on your continuous glucose monitor is to:

  • reduce your intake of foods that are a mixture of refined oils, grains and sugars, and
  • prioritise nutrient-dense food that tends to have a higher protein percentage. 

Protein

Protein typically has a minimal effect on blood glucose. 

  • If you’re insulin sensitive, protein may cause your blood glucose to fall a little. 
  • If you’re insulin resistant, a high protein meal may cause your blood glucose to rise (due to uncontrolled gluconeogenesis). 

However, elevated glucose after a high protein meal is not a reason to avoid protein in your diet.  Because your body requires protein to survive, your appetite will make up for protein loss to glucose by increasing your hunger signals and cravings for food. 

Counterintuitively, if you see your blood glucose rise after meals, it likely means you need to focus on a diet with a higher percentage of protein.  For more details, see Protein and insulin. Why do my blood sugars rise after a high protein meal?

A diet with a higher percentage of protein (i.e. fewer carbs and fat) tends to be more nutrient-dense and provides greater satiety.   In time, this will allow both the glucose and fat in your blood to be depleted and allow your body to draw down on your stored body fat.  For more details, see Oxidative Priority: THE SECRET to effective fat loss.

glucose effect high protein diet

Alcohol

Alcohol by itself (e.g. vodka, not beer or cocktails) can drive blood glucose lower because your body holds back the glucose and fat in storage until you have burned off the alcohol in your system. 

While this may appear to be a good thing, keep in mind that alcohol is essentially rocket fuel for your metabolism and does not typically come packaged with a significant amount of nutrients.  It can also lead to poorer food choices and other adverse effects on your body.

What is a healthy blood sugar range? 

People who are metabolically healthy tend to have lower and more stable blood sugars. 

But, while swapping carbs for fat may help you achieve flatline blood sugars, it’s unlikely to help you lose body fat or improve your metabolic health. 

At the other extreme, roller-coaster blood glucose and insulin levels can drive hunger.  We don’t feel good when our blood sugar is falling quickly, and we want to eat NOW!  In our Data-Driven Fasting Challenge, we guide people to wait until their blood glucose is a little lower than normal for them. But waiting for too long (e.g. extended fasting or multi-day fasting) increases the probability that you will make poorer food choices and overeat when you allow yourself to eat again.

glucose and hunger chart

However, you don’t need to worry too much if you have blood sugars in the normal healthy range (as shown in the table below).   In our Data-Driven Fasting Challenge and Nutritional Optimisation Masterclass, we suggest that you only need to be concerned about your carb intake if your blood sugars rise by more than 1.6 mmol/L or 30 mg/dL after meals. 

fasting and after meal glucose limits

Most people interested in fasting are already following a lower-carb diet and see a  minimal rise in their blood sugars after meals.  The charts below, created from data from people using our Data-Driven Fasting system, show no correlation between waist:height ratio or BMI and their blood sugar levels after they eat. 

glucose rise after meal vs waist to height ratio

There is no correlation between the amount your blood glucose rises after you eat and your waking blood glucose (another critical marker of metabolic health).  While stable blood sugars in the healthy range are good, flatline blood glucose levels are not better.  Again, we need to manage the area under the curve by focusing more on our blood sugars before we eat and while we sleep. 

after meal glucose tracking

Stable and lower blood glucose levels are a positive marker of good metabolic health, but merely treating the symptom (elevated blood sugar) rather than addressing the cause (energy toxicity and excess body fat) doesn’t help. 

For more details on this topic, see Want to lose fat? DON’T aim for stable blood sugars! (Why your CGM could be making you fat).

It’s your blood glucose BEFORE you eat that matters

If your goal is to lose body fat and manage your risk of all the diseases related to energy toxicity, then the power of measuring your blood sugars lies in managing your blood sugar levels BEFORE you eat. 

Your blood sugar is a precise and instantaneous fuel gauge that you can use to validate your hunger and optimise your eating schedule.  As you learn to wait until you are truly hungry and need to eat, your waking blood glucose and all the other important health markers will fall into line. 

The most effective thing you can do to reduce the area under the curve of your CGM plot is to wait until you can validate your hunger and know that you need to refuel.  As shown in the chart below, there is a solid correlation between blood glucose before we eat (which we can manage by waiting until we need to refuel) and our waking glucose (which is a crucial indicator of metabolic health).

pre-meal glucose

The real power of measuring your blood glucose is that it can help you understand if you:

  • actually needed to eat now,
  • are just craving those yummy leftovers in the fridge,
  • are eating to soothe your emotions or because you are bored,
  • are eating out of habit, or
  • just because it’s ‘breakfast time’?

Over time, as you observe the relationship between your blood sugar when you feel hungry and your physical sensations of hunger, you can learn to train your hunger. 

For more details, see Hunger Training… how to use your blood glucose meter as a fuel gauge to train your appetite for sustainable long term fat loss.

CGM comparison

There are a number of ways you can measure your glucose that come with different features and price tags. 

Dexcom

The Dexcom is the gold standard for CGM monitoring.  You can calibrate it to ensure accurate readings that align with the glucose in your blood (not just interstitial fluid).  The transmitter battery lasts for three months.  Sensors last for seven days, although you can stretch this a little longer if you use third party readers like xDrip

Dexcom - continuous glucose monitor

We use the Dexcom system to manage Monica’s Type 1 Diabetes combined with AndroidAPS, XDrip and Nightscout.  The blue bar along the top is the changes in her basal insulin rate, updated every five minutes to keep her glucose stable.  When her blood glucose is elevated, the system will increase the basal rate or dose with more insulin to bring her glucose down. 

CGM tracking

It’s brilliant but also comes with a hefty price tag.  We are able to reduce this cost by stretching the sensors out to 10-12 day before the sensors start to give junk readings.  Some people also manage to replace the battery in the transmitter (which is the most expensive part of the system, but this is not easy to do!  I’ve destroyed a few expensive transmitters trying to do it myself. 

Freestyle Libre

The Freestyle Libre is a less costly consumer-targeted option that is a little cheaper than the Dexcom.  Unfortunately, you can’t calibrate the Freestyle, and it is known to give a lower blood glucose.  This may make you feel better about your diabetes or metabolic health, but it is just not as accurate, so it is recommended you check with a finger prick before making any decisions about insulin dosing.

CGM - continuous glucose monitor

Depending on where you wear the sensor, people often see ‘compression lows’ at night when they put weight on the sensor and limit the circulation of the interstitial fluid just under your skin.  So, if you are concerned about lows through the night, it’s good to have a boring old blood sugar meter on hand that can verify your blood glucose. 

It’s also worth noting that because the CGM measures your interstitial fluid (not your blood), there is a delay of about 15 minutes between your blood sugars and your CGM readings.  This is particularly relevant if your blood sugars are changing quickly due to exercise or a high carb meal. 

The Freestyle Libre reader and sensors can be purchased separately, but you can also use your phone as a reader.  A number of companies like Nutrisense are selling the Freestyle Libre as a bundle with their own app for a monthly deal. 

Lumen

Lumen is not technically a blood sugar meter.  It measures the CO2 on your breath (i.e. respiratory quotient) and tells you whether you are burning more carbs vs fat.  This information provides an indication of whether you should skip the carbs or top up your stores before a heavy workout.  

If your blood sugars are high, you will be mainly burning glucose, and it will be wise to skip the carbs.  However, if your glucose is depleted and you’re about to work out, it might be wise to top up with some carbs. 

metabolism tracker

On the upside, there are no ongoing costs with Lumen.  On the downside, the initial price point is high, and most of the information could be ascertained (potentially more accurately) by just checking your blood sugar. 

Knowing whether you are burning fat or glucose is interesting, but (similar to measuring ketones) there is no way to tell if you are burning stored energy or the energy from your diet.  So it’s unlikely to be a helpful guide if your primary goal is to lose weight. 

Glucose metre

If you only need to check your glucose before you eat to verify your hunger, a boring old glucose meter will be fine. 

In our Data-Driven Fasting Challenge, we recommend the Contour Next One meter due to its accuracy and features.  However, there are plenty of cheaper options available if you’re on a tight budget. 

If you’re going to be using your glucose meter for a few months to reach your goal weight and reverse your diabetes, then we recommend investing in an accurate meter that you enjoy using.  As you will see in the cost comparison table below, the CGM makes any single point blood glucose metre look cheap!

The study Teaching people to eat according to appetite – Does the method of glucose measurement matter? by Dr Michelle Jospe and colleagues demonstrated that Hunger Training works with a CGM or single point blood glucose meter.  However, one drawback of a CGM is that many people see their glucose below their trigger and think they need to eat, even if they are not really hungry. 

The great thing about the simple blood glucose machine is that you don’t have anything attached to your body, and you only have to check two or three times a day before you eat.

We love data, but after working with thousands of people in our Data-Driven Fasting Challenge and Nutritional Optimisation Masterclass, we have realised that most people get overwhelmed with too much tracking and data.  Any system that works needs to be as simple as possible to achieve long-term results. 

glucose monitor

You can buy the Contour Next One meter for less than AUD$20 in Australia or USD$7 on Amazon.com.  If you are part of a diabetes scheme (like NDSS in Australia), they will give you the meter for free.  Full price strips are USD$29.99 for 100 test strips, AUD$30 (or half that if you are covered by a health scheme). 

Cost comparison

The table below shows an indicative cost comparison of the device options discussed above. 

Device set up (USD) ongoing (monthly) 3 months
Dexcom  $692  $474  $2,115
Freestyle Libre  $122  $237  $833
Lumen  $447    $447
Contour Next One  $9  $23  $77

The Contour Next One price shown at the bottom of the table assumes you are not paying out of pocket for the meter.  However, this would be if you chose a cheaper meter or have health coverage that subsidises the cost of the strips, and the meter will be free. 

Data-Driven Fasting

In summary, your blood sugar is the most powerful and yet underutilised biometric that you can track to optimise your weight and metabolic health.  But there are a lot of moving parts.  Too much data often leads to overwhelm and giving up before you get the results you want. 

In the Data-Driven Fasting Challenge, we guide people through a structured process to harness the powerful insights gained from their blood sugars and have consistently seen excellent results

The DDF Challenge guides people through the following phases to leverage the powerful insights from their blood glucose:

  • Baselining Phaseestablish the blood sugar level at which you tend to feel hungry and typically choose to eat and find Your Current Trigger to identify genuine hunger and when you need to eat to refuel.
  • Hunger Training – learn to delay eating until your blood sugar is below Your Personal Trigger to ensure you eat only when you need to refuel.   
  • Optimal Foods and Meals – learn how the foods you currently eat affect your blood sugar.  Which ones leave you satisfied? Which ones lead you to eat more than you need to?
  • Curb your late-night binging – use your waking blood sugar to identify if you are eating too late in the day.
  • Main Meal vs Discretionary Meals – identify the Main Meal that you will use to anchor your eating routine and ensure you get enough nutrients.
  • Optimise your eating routine – lock in your new eating routine to guarantee you are moving towards your goals.

To learn more about how you can use Data-Driven Fasting to leverage the insights from your blood sugar monitoring to guide your journey towards optimal weight and improved metabolic health, you can:

10 thoughts on “How to lose weight using a continuous glucose monitor (CGM) and Data-Driven Fasting”

  1. This was a very good article, I in the process of buying a glucose monitor and I wanted my insurance to pay for it. I was shocked on how much every thing cost, especially the strips! This article has put a lot of information together that is going to help me. Thanks

  2. Hi Marty, my son is an insulin- dependant diabetic (Type 1) recently diagnosed at 27 years old. Is this information relevant to him or only those with Type 2?

    • exogenous insulin is one thing that can disrupt the DDF process. it still works, but it’s sort of ‘cheating’. it makes sense not to eat carbs when your blood sugar is high, but dosing with extra insulin to achieve lower BGs would not necessarily lead to a better metabolic outcome. suggest the low carb and blood sugar recipe book would be a good place to start to stabilise blood sugars first. see https://optimisingnutrition.com/which-nutribooster-recipe-book-is-right-for-me/

  3. I have had a CGM since March 2021 and I have weekly fasting periods, either 2x 36-hour periods or a once-a-month 84-hour period. The CGM is very useful for observing the effect of the fasts, exercise and sleep on the metabolism. My normal diet usually shows a BG in the 80-90 mg/dL range – in the fed state. I do tinker with carbs on occasion to see what quantities and types matter the most; and typically the carbs+fats foods are the most reactive. In the fasted state, my BG will drift down about 10 mg/dL per day of fasting (median). I am sure it reaches a baseline limit, but I don’t know what that limit is. The delayed thermogenetic affect of dietary fats and protein confounds the BG average change because it takes 12-14’ish hours for the fats and protein to process. My biggest take away is that once my BG is tracking below 70 mg/dL, the changes in BG are no longer related to food, but instead are due to good or bad sleep, amount and intensity of exercise (tho, it can take a hour of zone 2 exercise for my BG to rise 10 mg/dL), and stress (cortisol). Surprisingly, I get a moderate spike in BG from my normal warm-to-hot showers. The relationship between BG and thermogenics in the body seems really strong.

    I have a bone to pick about the point of fasting inducing overeating. As a practical matter this really is a non-issue with extended fasting. The reason is that my gut microbiome goes thru a big cycle with my 36-hr or 84-hr fasts. Grabbing a bunch of food and stuffing one’s face is an excellent way to generate copious GI distress because the gut microbiome is not ready for that food. I typically break my long fast will 3-4 eggs (high in protein) cooked in a small amount of ghee, and then a moderate amount of mixed food 3-4 hours after the break of the fast. Whatever hunger signal I may be experiencing as I start refeeding is overwhelmed by memories of painful GI distress.

    • glad you have the refeed sorted. DDF is just designed for people who struggle to achieve their long term goals with more extended types of fasting.

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