'Fast' and 'slow' carbs are an oversimplified, inaccurate model

A food's glycemic index depends heavily on what you eat it with

Splitting carbs into 'fast' and 'slow' is based on glycemic index — a measure of how quickly a food raises blood sugar compared to pure glucose. The idea is simple and intuitive, but it ignores an important factor: glycemic index is measured for a food eaten alone on an empty stomach, and in real meals carbs are almost never eaten in isolation.

Adding protein, fat, or fiber to a meal significantly slows absorption of even 'fast' carbs, blunting the resulting blood sugar spike — a plate of white rice with chicken and vegetables behaves very differently in the body than plain white rice eaten on its own. There's also a related metric, glycemic load, which accounts for both the speed and the actual amount of carbs in a real-world portion, and it's often a more useful guide than the isolated glycemic index.

Practical takeaway: don't sort foods into 'bad fast' and 'good slow' carbs in isolation from the rest of the meal — the overall composition of the plate and the portion size matter more than one ingredient's isolated glycemic index.