In one sentence
How far your glucose climbed above its pre-meal level in the two hours after a meal you logged: the headline of a meal comparison, and the number most people mean by "a spike".
Why it matters
It is the clearest thing a sensor shows a person who has never worn one: the same breakfast on two people, or two breakfasts on one person, and a difference you can see. In the study that founded personalized nutrition, 800 adults wore sensors for a week and the response to identical meals varied so much between people that the authors concluded universal dietary advice has limited use (Zeevi 2015); in 1,002 British adults, including identical twins, the person-to-person variation in glucose response to the same meals was 68 percent (Berry 2020). Your own peaks are the beginning of your meal library. The founder's surprise was oats: cold rolled oats in kefir took him to 10.8 mmol/L.
What moves it
The amount and kind of carbohydrate in the meal, first; then what came with it (protein, fat and fiber slow the rise), the order you ate it in, how fast you ate, what you ate at the previous meal, the time of day (a late dinner raised overnight glucose in healthy volunteers; Gu 2020), last night's sleep (a single short night lowered next-day insulin sensitivity by about a quarter; Donga 2010), stress, the luteal phase, and movement after eating, which your own record will show better than any study. And the sensor's lag, which makes a sensor's peak later and often a little lower than blood's.
How to measure it well
Log the meal at the time you start it, with what was in it; the window starts at the log time, and a meal logged an hour late is a meal measured on the wrong two hours. Eat the meal you are testing on more than one day before you decide anything about it. Compare meals on the same sensor site.
How Vitality Tracker uses it
Each logged meal gets its response curve and three figures: the peak rise, the two-hour iAUC and the dip after. The meal library ranks on the iAUC and shows the peak beside it; a meal eaten more than once shows its repeats together. Not a Vitality Score input.
How far to trust the number
Calculated from measured readings around one meal; it is the least stable of the three meal figures, because one high reading sets it and because the same meal varies within a person from day to day. Trust a meal's peak once it has repeated.
Sources
- Zeevi D, et al. Personalized nutrition by prediction of glycemic responses. Cell 2015;163:1079–94. doi:10.1016/j.cell.2015.11.001
- Berry SE, et al. Human postprandial responses to food and potential for precision nutrition. Nat Med 2020;26:964–73. doi:10.1038/s41591-020-0934-0
- Gu C, et al. J Clin Endocrinol Metab 2020;105:2789–802. doi:10.1210/clinem/dgaa354
- Donga E, et al. J Clin Endocrinol Metab 2010;95:2963–8. doi:10.1210/jc.2009-2430.