In January 2024, Bulletproof Diet author Dave Asprey posted a video claiming oat milk "raises your blood sugar as much as drinking a Coke." The line travelled, and oat milk — a decade into being the default flat-white milk — acquired a reputation as a metabolic wolf in sustainable clothing.
There is a real mechanism underneath this. There is also a conclusion that the mechanism does not support, and — more interestingly — a measurement nobody appears to have made.
The mechanism is real, and it is the entire point of the product
Oat milk is sweet without added sugar because manufacturers use enzymes to break the oats' starch into shorter sugars, chiefly maltose. That is not a contaminant or a scandal; it is how you get a pourable, mildly sweet liquid out of a grain. It also means "no added sugar" and "not much sugar" are different statements, and the label only promises the first.
Where the numbers come from, and how far they stretch
Maltose does have a glycemic index of 105 — higher than glucose. That figure comes from the International table of glycemic index and glycemic load values, and it deserves its footnote: it is a single study, in six people, using 50 g of pure maltose.
Pure maltose in a lab is not a carton of oat milk. When the actual product is measured, it lands considerably lower. The Glycemic Index Foundation lists oat milk at 69, against roughly 30 for dairy milk. A separate laboratory analysis of 17 commercial plant milks found an oat drink that was "mainly maltose" nonetheless produced a moderate predicted GI of 59 — and noted, in passing, that the genuinely high-GI plant milk is rice milk, at 97 to 100. Rice milk never trends.
So oat milk does sit at the higher end of the plant-milk range. It is roughly twice dairy milk. It is not maltose, and the 105 figure does not belong in this conversation.
The comparison to Coke is arithmetically true and practically empty
Coca-Cola's GI is about 60. Oat milk's is about 69. On that basis the viral claim is, narrowly, correct.
Glycemic index compares equal quantities of available carbohydrate, not equal servings. An 8 oz serving of oat milk contains around 7 g of sugar, alongside fat, protein and a little fibre. A 12 oz Coke contains 39 g of sugar and nothing else. Per glass, Coke delivers over five times the sugar load.
"Higher GI" and "worse for your blood sugar in practice" are separate claims. The viral version depends entirely on readers not noticing the difference. And a 30 ml splash in a coffee — the way most oat milk is actually consumed — carries about 2 g of carbohydrate.
The measurement nobody has taken
Here is the part that surprised us, and the reason this piece is graded C rather than D.
We could not find a single human randomised trial measuring the postprandial glucose response to commercial oat milk against dairy milk. Not one. There is a large literature on oats and oat beta-glucan in solid form, studies of beta-glucan beverages, and the in vitro work above — but the retail product in a normal serving appears not to have been directly tested in people.
Which means the confident claim and the confident rebuttal are both running ahead of the evidence. Anyone telling you precisely what oat milk does to your blood sugar relative to dairy is extrapolating. We are not going to pretend otherwise in order to deliver a cleaner verdict.
The criticism that would have worked
If you wanted to make a serious nutritional case against oat milk, the sugar is not it.
Protein. An audit of 129 plant-based milk products found oat milk at about 1.0 g protein per 100 ml, against 3.6 g for cow's milk and 3.2 g for soy. If you are drinking plant milk for nutrition rather than ethics or taste, soy is the better-evidenced swap.
The health halo. Oat milk borrows oats' cholesterol-lowering reputation, which rests on beta-glucan. The British Heart Foundation's dietitian notes most oat milks contain roughly 0.5–1 g per 250 ml — you would need something like 750 ml to 1.5 litres a day to reach the 3 g therapeutic dose. The EU's authorised claim for blunting a meal's glucose rise requires 4 g of beta-glucan per 30 g of available carbohydrate; oat milk misses that by roughly an order of magnitude. The fibre in oat milk is not present at a level that meaningfully blunts its own glycemic response.
That is a real, sourced critique. It is just less exciting than "worse than Coke," so it isn't the one that travelled.