Coffee and fasting

For many people, coffee is what makes the morning of a fast possible. The good news is that black coffee has almost no calories and fits comfortably into a fast. But coffee is more than a warm drink: caffeine changes how your body releases fat, handles sugar and feels hunger. Knowing how helps you use it well.

What coffee does during a fast

Caffeine works partly by triggering the release of adrenaline and related hormones, which tell your fat tissue to release fuel. In a laboratory study, a large dose of caffeine doubled the rate at which fat moved into the blood and raised energy use by 13% over the next few hours ¹. Much of that fat was recycled rather than burned, and the dose was high, so don't expect coffee to melt fat on its own. But it nudges your body in the same direction as the fast.

The blood sugar paradox

Here's where it gets interesting. In the short term, caffeine makes your body slightly less sensitive to insulin. When healthy volunteers were given caffeine through a drip, their insulin sensitivity fell by 15%, likely because adrenaline rose fivefold ².

How to drink it

• Drink it black, or with a splash of milk if that's what keeps you on track. Sugar, syrups and cream break your fast.
• Stay at or under about four 240 ml (8 oz) cups a day, roughly 400 mg of caffeine ⁵.
• Drink water as well: coffee counts towards your fluids, but it shouldn't be your only drink ⁶.
• Have your last cup at least 6 hours before bed ⁷.
• If you're sensitive to blood sugar swings, have your first meal before your strongest coffee rather than right after it.

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.

Sources

1. 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
2. Keijzers GB, De Galan BE, Tack CJ, et al. Caffeine can decrease insulin sensitivity in humans. Diabetes Care. 2002;25(2):364-9. doi.org/10.2337/diacare.25.2.364
3. Smith HA, Hengist A, Thomas J, et al. Glucose control upon waking is unaffected by hourly sleep fragmentation during the night, but is impaired by morning caffeinated coffee. Br J Nutr. 2020;124(10):1114-1120. doi.org/10.1017/S0007114520001865
4. Ding M, Bhupathiraju SN, Chen M, et al. Caffeinated and decaffeinated coffee consumption and risk of type 2 diabetes: a systematic review and a dose-response meta-analysis. Diabetes Care. 2014;37(2):569-86. doi.org/10.2337/dc13-1203
5. 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
6. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154. doi.org/10.1371/journal.pone.0084154
7. Drake C, Roehrs T, Shambroom J, et al. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med. 2013;9(11):1195-200. doi.org/10.5664/jcsm.3170