What breaks a fast (and what doesn't)

It's the question everyone asks in their first week: can I have this? The honest answer is that it depends on why you fast. If your goal is simply to eat less, a few calories here and there barely matter. If you want your body to spend the fast running on stored fat, what matters most is keeping insulin low, because insulin is the signal that tells your cells to store fuel rather than release it. This article sorts the usual suspects by what studies show they do.

Why insulin is the line that matters

While you fast, insulin falls and your fat tissue starts releasing fatty acids for your muscles and liver to burn. Anything that pushes insulin back up slows that release, even if it has few calories. That's why a fast isn't only about calories: a spoonful of sugar and a spoonful of olive oil have very different effects, even though the oil has more energy.

Fine while fasting

• Water, still or sparkling. Add lemon slices for flavour if you like.
• Black coffee, hot or iced.
• Plain tea, green tea and herbal infusions, without sugar or honey.
• A pinch of salt or sugar-free electrolytes on longer fasts.
• Your usual medication. Never change it to fit a fast without asking your doctor.

Coffee does more than keep you company. 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 ⁷. The dose was large, roughly what you'd find in several strong cups, so a single coffee does less. But the direction is clear: black coffee works with your fast, not against it.

Breaks your fast

• Juice, soft drinks, smoothies and anything with sugar or honey.
• Milk, cream or plant milks in your coffee, beyond a small splash.
• Protein shakes, collagen and amino acid drinks.
• Bone broth: it has protein and some calories.
• Snacks of any size, including fruit and nuts.
• Alcohol.

Milk deserves a special mention. In a Swedish study, milk and whey produced far smaller rises in blood sugar than white bread, yet whey released 90% more insulin ². That's useful after a workout, when you want to rebuild muscle. During a fast, it's exactly what you're trying to avoid.

The grey zone: sweeteners

Sweeteners are where the science gets interesting, because the answer changes with time. On the day you drink it, a sweetened zero-calorie drink seems harmless. In a British trial, swallowing sucralose didn't raise insulin or gut hormones, and simply tasting it and spitting it out had no effect either ³. In Singapore, drinks sweetened with aspartame, stevia or monk fruit didn't cause the blood sugar and insulin spike that a sugary drink did ⁴.

Sources

1. 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
2. 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
3. Ford HE, Peters V, Martin NM, et al. Effects of oral ingestion of sucralose on gut hormone response and appetite in healthy normal-weight subjects. Eur J Clin Nutr. 2011;65(4):508-13. doi.org/10.1038/ejcn.2010.291
4. Tey SL, Salleh NB, Henry J, et al. Effects of aspartame-, monk fruit-, stevia- and sucrose-sweetened beverages on postprandial glucose, insulin and energy intake. Int J Obes (Lond). 2017;41(3):450-457. doi.org/10.1038/ijo.2016.225
5. Romo-Romo A, Aguilar-Salinas CA, Brito-Córdova GX, et al. Sucralose decreases insulin sensitivity in healthy subjects: a randomized controlled trial. Am J Clin Nutr. 2018;108(3):485-491. doi.org/10.1093/ajcn/nqy152
6. Lertrit A, Srimachai S, Saetung S, et al. Effects of sucralose on insulin and glucagon-like peptide-1 secretion in healthy subjects: a randomized, double-blind, placebo-controlled trial. Nutrition. 2018;55-56:125-130. doi.org/10.1016/j.nut.2018.04.001
7. 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