Does milk in coffee break a fast?

Why milk is a special case

Milk raises insulin more than its sugar would suggest, because its protein triggers insulin too. Researchers in Sydney fed people 38 everyday foods in portions with the same calories: protein foods released much more insulin than their effect on blood sugar would predict ². In a Swedish study, whey, the protein in milk, released 90% more insulin than white bread ¹.

That's why a fast that's mostly about keeping insulin low is cleanest with black coffee. But a splash of milk is a small amount. If it's what makes your morning coffee possible, many people accept the trade-off.

MYTH

Butter or MCT oil in coffee keeps you fasting.

FACT

A spoonful of butter or oil adds around 100 calories or more. It won't spike insulin like sugar, but it ends a fast in the calorie sense, and it replaces fat your body would otherwise burn from its own stores.

What about plant milks and creamers?

Fasting Tracker's guide puts milk, cream and plant milks in the same group: fine as a small splash, a fast breaker beyond it. Many creamers and sweetened plant milks also contain sugar, which is the clearest fast breaker of all.

Black coffee works with your fast

In a laboratory study, caffeine doubled the rate at which fat was released into the blood and raised energy use by 13% over the following hours ³. The dose was large, so a single cup does less, but the direction is clear. Stay at or under about four 240 ml (8 oz) cups a day ⁴.

Sources

1. Nilsson M, Stenberg M, Frid AH, et al. Glycemia and insulinemia in healthy subjects after lactose-equivalent meals of milk and other food proteins: the role of plasma amino acids and incretins. Am J Clin Nutr. 2004;80(5):1246-53. doi.org/10.1093/ajcn/80.5.1246
2. Holt SH, Miller JC, Petocz P. An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods. Am J Clin Nutr. 1997;66(5):1264-76. doi.org/10.1093/ajcn/66.5.1264
3. Acheson KJ, Gremaud G, Meirim I, et al. Metabolic effects of caffeine in humans: lipid oxidation or futile cycling? Am J Clin Nutr. 2004;79(1):40-6. doi.org/10.1093/ajcn/79.1.40
4. Wikoff D, Welsh BT, Henderson R, et al. Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and children. Food Chem Toxicol. 2017;109(Pt 1):585-648. doi.org/10.1016/j.fct.2017.04.002