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All About Nutrition & CGMs

Continuous glucose monitors (CGMs) are devices that measure your blood glucose levels in real-time by sensing the glucose levels found in the fluid in tissues (interstitial fluid). They are worn on the skin for an average of 10-14 days or 90 days if implanted. Rather than having to prick your fingers throughout the day to get a blood glucose (BG) reading, a CGM provides a BG reading every 5 minutes — 288 times each day! You also can see the direction and speed of the BG in real-time by reading the receiver or smartphone or by scanning the sensor intermittently. 

Most CGMs have the option for alerts and alarms that respond to certain high and low BG thresholds that you can set based on your preferences. Studies show that this additional information along with alerts, helps to prevent low BG (hypoglycemia), increase time in range, and less BG variability compared to fingersticks with a traditional blood glucose meter (self-monitoring blood glucose or SMBG) (1). This is significant to delay the onset and slow the progression of diabetes-related complications such as kidney disease. 


There are currently four CGMs available on the market:

  1. Abbott – FreeStyle Libre & FreeStyle Libre 2 – 14 day wear
  2. Dexcom – G6 (G4 & G5 were previously available) – 10 day wear
  3. Medtronic – Guardian Connect & Guardian Sensor 3 – 7 day wear
  4. Senseonics – Eversense – 90 day wear (requires implantation)


Benefits of a CGM:


  • Almost eliminates the need for fingersticks (with the exception of calibration and/or confirming high and low blood sugars) – convenience
  • Can improve A1c and time in range with less variability
  • Reduce time spent in hypoglycemia and prevent hypoglycemic episodes (especially with Dexcom and Libre 2 alarms)
  • Prevents lows for people who have hypoglycemia unawareness or overnight lows (peace of mind for PWD, parents and caregivers)
  • Prevents diabetic acidosis if you forgot to take your insulin, are ill, have an insulin pump failure, injected expired/damaged insulin, or have injection site issues
  • Easy self-insertion (except for Eversense, which must be implanted)
  • Detailed reports allow you and healthcare team to see trends and make better adjustments to your insulin regimen
  • Helps you make more informed treatment decisions leading to less frustration, burnout, and time spent having to treat a high or low blood sugar – improved quality of life
  • Remote access to data available for monitoring by parents, caregivers, and healthcare providers


Blood glucose (BG) is constantly changing. There are literally, up to 100 different variables and it can be incredibly hard to guess how your body will react to each day. Diet, exercise, stress, sleep, hormones, insulin sensitivity, timing of insulin, damaged/heated insulin, pump site failures, illness, medications, extreme temperatures, altitude, damaged skin/tissue from repeated injections, and some days…which way the wind blows, can all affect blood glucose. Even with the same routine and the same exact food intake and physical activity 2 days in a row will likely have varied results in BG. This can be frustrating, time-consuming, and can lead to diabetes burnout. 

You can certainly manage these factors well and predict what will happen with your BG levels most of the time if you’re experienced, have a routine, and know your body well. But it’s not exactly accurate to rely solely on how your body’s feeling when making treatment decisions, especially if you have a hard time feeling your lows or you’re used to the feeling of having high BG all the time. 

Checking your BGs with a traditional blood glucose meter (not a CGM) is known as self-monitoring blood glucose (SMBG). It’s amazing that we can even check our blood sugars with a BG meter as fast and as accurate as we can these days compared to 30 years ago. But as technology advances, we see just how little information we have when we’re limited to checking our BG an average of 3-7 times per day versus an unlimited amount with a CGM. With SMBG, it’s recommended to check your BG level while fasting, before a meal and 2 hours after a meal, lows/highs, and at bedtime (if you have enough test strips). 

Even if you check your BG level 10 or more times each day, you still wouldn’t be able to obtain the data that you can get from a CGM because you only get to see a snapshot of what your blood sugar is at that moment. A CGM is more like a video that captures the full event, including what your blood glucose currently is, what it was earlier in the day, which direction it’s headed, and how fast it’s rising or falling. With all of this extra information, it’s much easier to make treatment decisions to avoid unnecessary highs and lows. You’re able to see how certain foods, exercise, and other factors affect your BG levels and make more informed treatment decisions. Less highs and lows leads to better glucose management, improved A1C, and lower risk of diabetes related complications.


Example of CGM data versus SMBG (fingerstick checks).


For example, a blood glucose level of 120 mg/dL (6.6 mmol/L) 2 hours after a meal might seem stellar for someone checking with a fingerstick, whereas the person with the CGM can see that they’re 120 mg/dL but declining quickly indicated by 2 arrows down on their CGM receiver or smartphone. The person with the CGM will be able to prevent a low BG from happening by consuming carbohydrates or have peace of mind by continuing to monitor by checking their BG every few minutes until it levels off. 


Example of Dexcom trend arrows from


Adjustments in insulin dosing and timing are more accurate when there is sufficient data available to your healthcare team.

Data from CGMs has been a huge game changer when it comes to adjusting insulin and making more informed insulin and meal choices.  


Blood Glucose and Diet

It’s very difficult to predict how food will affect each individual. The process of digestion and glucose absorption into the bloodstream is complex and varies based on composition of the meal (amount carb, protein, fat), rate of digestion, stomach & intestinal health, body size, insulin sensitivity, and physical activity. What we know from several research studies is that complex carbohydrates like whole grains and other high fiber foods like fruits, vegetables, legumes, lentils, nuts, and seeds can help to slow digestion and promote healthy bacteria in the gut, further promoting blood sugar control. Studies show that a high fiber diet promotes a healthy gut microbiome, which has a positive effect on BG levels. They also show that combining carbohydrate with protein and healthy fat helps to slow down the digestion of a meal, therefore reducing the glycemic response and reducing the severity of glucose excursions. 

Unless you’re treating a low blood glucose level, try not to eat carbohydrates by themselves. In most situations, it’s beneficial to pair carbohydrates with lean protein (preferably a minimally processed plant-based protein), healthy fat, & low carb vegetables to delay digestion and prevent a spike in blood sugar. This way of eating also helps to keep you full so you’re less likely to snack between meals, crave sugar, reduce inflammation, and manage weight. 

Blood glucose (BG) normally peaks around one hour after eating. This spike is temporary and is normal even for people without diabetes. However, the higher those peaks are and the longer they stay elevated, can have a big impact on vascular damage that leads to diabetes related complications. 


Here’s the healthy plate I created for my clients to help them make healthy balanced meals: 


Meal & Bolus Timing

It’s very important to make sure that the timing of insulin matches the rate of digestion. It takes about 15 minutes for rapid acting insulin to start working and it peaks at about 60-90 minutes. Ideally you should bolus for your meal 15 minutes before you eat if your BG is in your target range. If your premal BG is high, it’s typically recommended to wait longer to eat after injecting (around 30-45 minutes) to allow the insulin to start working before you consume more glucose. If your premeal BG is low or normal and trending down, you can bolus right at the start of the meal or treat the low first and wait until it’s normal again before you give insulin for the meal you’re about to eat. 

It’s important to wait at least 2 hours after taking a bolus to inject more insulin to avoid insulin stacking, which happens when too much insulin is in the bloodstream, causing low blood sugar. It’s much easier to monitor insulin dosing with an insulin pump or smart pen when you can see how much active insulin (insulin on board) you still have left in your body by looking at the information stored in the pump/pen. 

Without a CGM, you might take your BG and see that it’s 250 mg/dL (13.8 mmol) 90 minutes after your meal and then give a correction dose of insulin, only to plummet an hour later to less than 70 mg/dL (3.8 mmol). With a CGM, you would have seen that you were 250 but with 2 arrows down and knew that it’s best to monitor it closely over the next hour and it would likely level off. 


Carbs are Not the Enemy

Many people with both type 1 and type 2 diabetes turn to lower carbohydrate or keto diets as a way to manage their blood sugars and reduce the amount of insulin needed to cover the total amount of carbs they consume. However, research shows that high fiber plant-based diets, even those high in carbohydrates, help to reduce inflammation, reduce cardiovascular disease risk, improve glycemic control, and increase insulin sensitivity, which reduces the amount of insulin needed to lower blood glucose. Animal protein, especially red meat and meats cooked at high temperatures, are subject to oxidation, which leads to inflammation causing insulin resistance. 

If you’d like to start adding in some plant-based meals in your diet, try adding one new vegan or vegetarian recipe each week until you have a few different options you enjoy, and then build from there. Use beans or tofu in place of meat and add nuts, nut butters, and seeds to salads and side dishes for protein. To find healthy plant-based recipes, you can try some of the recipes on my blog (,, or Research also shows that plant-based diets help to slow the progression of kidney disease, another big reason to start eating more plant-based meals!


How High Fat Meals Affect BG

We all know that too many carbs can spike your blood sugar, but what about too much fat? Have you ever been confused as to why your blood sugar is within range 2 hours after a meal but then slowly starts to spike up? This can be caused by a high fat meal such as pizza, especially if you have a few slices with high fat toppings like cheese, pepperoni, or sausage. If you want to prevent this effect, you can limit the pizza to vegetable toppings, have just a slice or 2 (depending on size) and pair it with high fiber, low carb vegetables like a salad, roasted broccoli, carrots, cauliflower, asparagus, peppers, mushrooms, etc. 

*Note: Using the extended bolus, dual-wave bolus, or combination bolus features of an insulin pump can be really helpful to manage these situations by allowing a slow release of insulin versus giving all the insulin at once, which doesn’t quite match up with the timing of peak insulin action and the spike in BG. Even if you count carbs correctly and bolus the right amount of insulin, the high fat content in the meal slows the BG release so much that the insulin peaks before the blood sugar actually gets released into the bloodstream. This can sometimes cause a lower blood sugar reading before the carbs from the pizza are fully digested and released into the bloodstream. 

CGMs allow you to see how food affects your blood sugar in real time either automatically or manually scanned (intermittently) anytime you need to obtain BG data. People with diabetes are encouraged to check their BG levels before meals (preprandial) and again 2 hours after the start of a meal (postprandial) to determine their glycemic response to a meal. This is great to determine whether or not you need to give more insulin to correct but doesn’t tell you how high your BG spiked and for how long. CGMs also see patterns that you can’t see with SMBG, such as the need for basal insulin adjustments at certain times of day. It’s important to note that it’s normal for blood sugar to rise after ingesting food even for people without diabetes so it’s most important to pay attention to how high it spiked and how long it’s elevated rather than trying to avoid any increase at all. The higher and longer your BG spikes, the greater risk for damage to vessels and diabetes related complications. 


How CGMs empower you to make more informed treatment decisions:

I notice that a lot of my clients are more inspired to eat healthier and take the right amount of insulin when they start wearing a CGM. Just knowing their BG is spiking too high after a meal encourages them to make healthier meal choices and want to learn more about how to have better control. CGMs are a powerful learning tool to allow you to discover how different meals affect your glucose levels by allowing you to check your BG as much as you need to to get the data you need. Some people are amazed at how certain foods actually impact their BG levels and can improve their A1c much faster than without a CGM. 

You can test your glycemic response to certain meals or foods by monitoring your BG with CGM data one hour after the start of a meal. This will give you the peak glycemic response and can tell you how your body reacts to a certain meal. There are several variables and many things to consider when doing these tests. 


The following variables must be constant each time you evaluate your post-meal glycemic response:

  • Carbohydrate – must be accurate and contain the same number of grams each time (can be a different type) 
  • Premeal BG – must be as close to the same as possible each time to accurately measure the difference in BG from pre and post meal. (Ex. Starting with a BG of 100 mg/dL (5.5 mmol/L) and spiking to 200 mg/dL (11.1 mmol/L) is a 100 point increase versus starting at 150 mg/dL (8.3 mmol/L) and spiking to 200 mg/dL (11.1 mmol/L), which is only a 50 point increase)
  • Physical activity – same intensity and duration or none at all
  • Insulin dose and timing – must bolus the same amount of insulin at the same time (the basal insulin should also be the same on the days you’re testing
  • Time of day – don’t compare your response to breakfast to your response to carbs at dinner since insulin sensitivity varies at different times of the day
  • Other factors – keep medication timing and dosing the same (especially steroids) & avoid testing on days when you’re ill, stressed, or menstruating unless you’re testing specifically to measure your glucose response to these factors


Types of meals you can evaluate:

  • Different types of carbohydrates on their own (ie. white bread versus whole wheat bread) 
  • Carbs + protein (Ex. White or brown rice and chicken or rice and beans)
  • Carbs + protein + vegetables (Ex. Rice, chicken, broccoli or rice, beans, and peppers) 
  • Carbs plus protein and fat (Ex. Rice, chicken, broccoli, avocado or rice, beans, peppers, & avocado)
  • High fat meal (Ex. pizza, cheeseburger, pasta with bolognese or alfredo sauce)
  • **Try to test foods you normally pair together, rather than single food items, which you wouldn’t normally eat on their own


What this tells you:

  • How different foods affect your BG and which ones to choose most of the time
  • If your insulin to carb ratio is accurate 
  • What other factors are affecting your BG levels


Another example — You can check your glycemic response to certain breakfast foods with the exact same carbohydrate content by having 30g steel cut oats one day and 30g cold breakfast cereal with milk the next day (30g including the milk of choice). Another example would be 30g plain steel cut oats versus 30g steel cut oats with a handful of raw nuts added. You don’t need to use 30g specifically for your challenge, but make sure the 2 meals have the exact same carbohydrate content for an accurate test. 

The post meal information you get from your CGM shows you how high and how long your BG stays elevated. BG targets are set for each individual based on medical history, age, type of diabetes and individual goals. 


The general guidelines set by the American Diabetes Association (ADA) and the American Association of Clinical Endocrinologists (AACE) recommend the following blood glucose targets for most people with type 1 & type 2 diabetes:


Timing ADA recommendations AACE recommendations
fasting and before meals 80-130 mg/dL (4.4-7.2 mmol/L) for nonpregnant adults  <110 mg/dL (6.1 mmol/L)
2 hours after eating a meal <180 mg/dL (10 mmol/L) for nonpregnant adults <140 mg/dL (7.8 mmol/L)


Choose the meals that have the lowest peak after one hour and return to your target range 2 hours after the start of the meal. If you have low BG after a meal, assess which factors need to be adjusted accordingly (typically insulin, medication, physical activity, or alcohol). 


When using your CGM as a tool to discover how different meals affect your BG, first ask yourself these questions:

  • Have I made sure all variables are constant? (basal/background insulin is adequate, accurate carb counting, same time of day, pre-meal blood sugar in range, same bolus dose & timing, same physical activity, etc.)
  • Which meals have the highest glucose peak and why? 
  • How long does it take for the glucose to peak? 
  • Which meals have the longest post-meal BG and why? 


These are some potential causes for post meal BGs higher than expected:

  • Inaccurate carb counting – use household measuring utensils and read food labels
  • High fat meal 
  • Meal timing – insulin to carb ratios can be different at different times of the day 
  • Am I stressed, sick/feverish, sedentary, injecting into an overused injection site) poor absorption)


CGM data can help you determine if you need to

  • Adjust insulin to carb ratios
  • Give more/less insulin
  • Prevent insulin stacking (if not on a pump)

In summary, CGMs can be excellent tools to improve blood glucose management by providing more data and giving you more opportunities to prevent and treat high and low blood sugars. When using a CGM to test post meal glucose responses, make sure all other variables are constant and review the information with your healthcare team to make insulin dose changes as needed. 


Looking for some help?

I hope this article was helpful for you! If you’re looking for some additional guidance on blood glucose management, visit my website at to learn more about my 1:1 nutrition counseling services or click here to set up an introductory call with me. I provide virtual nutrition coaching for people all over the world for diabetes, kidney disease, digestive issues, thyroid health and auto-immune diseases. 


This article was written by:

Nicole DeMasi Malcher, MS, RDN, CDCES
Registered Dietitian Nutritionist
Certified Diabetes Care & Education Specialist
Owner, DeMasi Nutrition – Virtual Nutrition Coaching Practice based in Los Angeles, CA U.S.



  1. Use of Factory-Calibrated Real-time Continuous Glucose Monitoring Improves Time in Target and HbA1c in a Multiethnic Cohort of Adolescents and Young Adults With Type 1 Diabetes: The MILLENNIALS Study.
  2. The Use of Continuous Glucose Monitoring to Evaluate the Glycemic Response to Food. Janine Freeman, RD, LD, CDE and Lynne Lyons, MPH, RD, CDE. Diabetes Spectrum 2008 Apr; 21(2): 134-137.
  3. The Big Picture: Checking Your Blood Glucose. Accessed 10/25/2020.