Why One Reading Does Not Prove a Food Is Harmful
Why a single glucose reading or app score cannot show that a food is harmful, with research on normal glucose swings and how differently people respond to the same meal.
Content last updated . This page has not been medically reviewed.
A Stanford study of people considered normal by standard tests found glucose reaching the prediabetic range 15% of the time and the diabetic range 2% of the time on CGM. [3] The authors read this as evidence that glucose handling is more varied than assumed. It does not mean those people had diabetes. Diabetes is diagnosed with laboratory tests, and a result is confirmed with a second measurement unless there are clear symptoms. [4]
The same meal, different people
Researchers who measured 46,898 meals in 800 people found high variability in the response to identical meals. [5] A UK study of 1,002 adults found the spread (population coefficient of variation) after identical meals was 68% for glucose. [6] In plain words: two people can eat the same food and see very different curves, and neither is necessarily wrong. [3]Personalized predictions can work in research. In one Israeli trial, a diet guided by an algorithm gave lower after-meal glucose responses. [5] That is a change in a biomarker (a measured signal), not proof of better health.
From a number to "harmful": the missing steps
Calling a food harmful needs several things a single reading cannot give:- A repeat pattern, not one spike, under comparable conditions.
- A health outcome, such as disease risk, not only a short-term glucose change. A 2025 systematic review of seven studies of CGM in people without diabetes concluded that evidence of a direct effect on hard cardiovascular outcomes remains limited. [7]
- Context: what else was eaten, how much, and when. The UK study found meal macronutrients explained about 15% of the variation in glucose responses, and gut bacteria about 6%, so most of the variation came from other things. [6]
See glucose "spikes" and oversimplified claims and glucose food responses and individual variation.
Frequently asked questions
Does a high number after a meal mean the food is bad for me?
Not by itself. Healthy people without diabetes spend a small part of the day above 140 mg/dL on CGM. [2] Whether a food is a problem depends on your overall pattern, health, and a clinician's view.
Why do my friend and I react differently to the same food?
Large studies find big person-to-person differences after identical meals. [5][6] Many factors play a part, and meal contents explain only a modest share. [6]
Are over-the-counter glucose monitors approved for people without diabetes?
In March 2024 the FDA cleared one over-the-counter system for adults 18 and older who do not use insulin, including people without diabetes who want to see how diet and exercise affect glucose. [8] That is a statement about one system, not about every wellness claim or every product. Clearance for measuring glucose is not proof that using it improves health.
Should I check a surprising reading another way?
If a reading seems wrong, diabetes guidance says CGM readings may be compared with a finger-stick test. [1] For a health decision, ask a clinician.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: established, mixed, insufficient evidence, marketing claim.
What a reading actually is
A continuous glucose monitor (CGM) is a sensor worn on the skin. It does not measure blood directly. It estimates glucose in the fluid between your cells, which is very similar to the level in your blood. [1] That is why diabetes guidance sometimes says to compare a CGM with a finger-stick test when you doubt the reading. [1] A food app that turns the trace into a "score" adds one more layer: the score is a company's calculation, not a measurement. See what a CGM measures and interstitial vs blood glucose.Healthy glucose moves, and sometimes goes high
In a US multicenter study, 153 healthy people without diabetes (ages 7 to 80, nonpregnant, nonobese) wore a Dexcom G6 sensor for up to 10 days. [2] Their average glucose was 98 to 99 mg/dL, or 104 in those over 60. [2] They spent a median of 96% of the time between 70 and 140 mg/dL, about 2.1% (30 minutes a day) above 140, and about 1.1% (15 minutes a day) below 70. [2]So even in healthy people, brief excursions happen. These numbers describe one sensor in one group. They are not personal targets and not pass or fail lines.
Safety and when to ask a professional
References
- National Institute of Diabetes and Digestive and Kidney Diseases (US), Continuous Glucose Monitoring, last reviewed June 2023. Read 2026-10-22.
- Shah VN et al. Continuous glucose monitoring profiles in healthy nondiabetic participants: a multicenter prospective study. J Clin Endocrinol Metab 2019. Read 2026-10-22.
- Hall H et al. Glucotypes reveal new patterns of glucose dysregulation. PLoS Biol 2018. Read 2026-10-22.
- National Institute of Diabetes and Digestive and Kidney Diseases (US), The A1C Test & Diabetes, last reviewed April 2018. Read 2026-10-22.
- Zeevi D et al. Personalized nutrition by prediction of glycemic responses. Cell 2015. Read 2026-10-22.
- Berry SE et al. Human postprandial responses to food and potential for precision nutrition. Nat Med 2020. Read 2026-10-22.
- Use of continuous glucose monitoring in non-diabetic individuals for cardiovascular prevention: a systematic review of its impact on guiding lifestyle interventions. Cureus 2025. Read 2026-10-22.
- US Food and Drug Administration, FDA clears first over-the-counter continuous glucose monitor, press release of 5 March 2024. Read 2026-10-22.