Any drink consumed during the fasting window affects two metabolic parameters: the insulin response and the state of autophagy. The question is not just “is it caloric?”, but “does it trigger a sufficient insulin secretion to interrupt lipolysis?”. We use this criterion to sort drinks compatible with intermittent fasting from those that compromise it.
Insulin Response and Fasting Break Threshold
A caloric intake, even minimal, does not necessarily break the fast in a metabolic sense. What matters is the magnitude of the insulin spike it causes. Plain water, unflavored sparkling water, and herbal teas without additives do not trigger any measurable insulin response.
Black coffee raises a more nuanced question. Caffeine alone does not cause significant insulin secretion. However, drinking coffee on an empty stomach increases gastric acidity, which can cause heartburn and reflux in sensitive individuals. We recommend having it mid-morning rather than upon waking, and opting for a less concentrated filter coffee with lower chlorogenic acid than an espresso.
Plain green tea and black tea are compatible with fasting. The L-theanine in green tea actually mitigates the harsh stimulating effect of caffeine, making it a better choice than coffee for those prone to anxiety during the food restriction period.
The question of what to drink without breaking the fast therefore depends less on the number of calories than on the type of macronutrient involved: an addition of sugar, even a spoonful, is enough to restart the insulin cycle.
Milk Coffee, Broth, and “Zero” Drinks: What Really Breaks the Fast
Adding cow’s milk to coffee, even in small amounts, introduces lactose (a sugar) and casein (a protein). Both stimulate insulin. Any dose of milk, whether plant-based or animal, interrupts the fasting phase.
Bone broth, often recommended in extended fasting circles, contains amino acids that activate mTOR, a cellular signaling pathway opposed to autophagy. It is useful for refeeding, not during the restriction window.
“Zero calorie” drinks sweetened with artificial sweeteners (aspartame, sucralose, acesulfame K) deserve caution. Some data suggest that these sweeteners provoke a cephalic insulin response: the sweet taste, even without calories, triggers a metabolic preparation for glucose absorption. The magnitude of this effect varies among individuals, but artificial sweeteners potentially compromise the metabolic fasting state.
Drinks to Avoid During the Fasting Period
- Fruit juices, even freshly squeezed: their glycemic load is comparable to that of a soda, without the fiber of the whole fruit to slow absorption
- Diet sodas and flavored waters sweetened with sweeteners: risk of cephalic insulin response and maintenance of a preference for sweetness
- Cow’s milk, oat milk, soy milk: all provide carbohydrates or proteins in sufficient quantities to break the fast
- Energy drinks (even sugar-free): the combination of concentrated caffeine, taurine, and sweeteners on an empty stomach frequently causes digestive issues
Hydration and Electrolytes During Intermittent Fasting
Dehydration during intermittent fasting is often underestimated. About one-third of our daily water intake comes from food. Skipping one or two meals mechanically reduces this intake.
Regularly drinking water throughout the fasting window is the primary recommendation. Plain water remains the standard. Natural sparkling water (without flavor, without sugar) is an alternative, provided it does not cause bloating that could be mistaken for hunger.
Drinks containing electrolytes (sodium, potassium, magnesium) without sugar or sweeteners do exist. They can be useful in cases of prolonged fasting or physical activity. Their use is not trivial: electrolyte intake warrants medical advice for individuals on diuretics or treated for hypertension, or in cases of kidney disease.
Warning Signs to Watch For
Persistent headaches, muscle cramps, or dizziness during fasting often signal a hydration or electrolyte deficit. These symptoms should not be “endured” in the name of discipline: they indicate that the body is lacking resources.
Drug Interactions and Fasting: A Neglected Angle
The majority of articles on intermittent fasting address drinks without discussing interactions with medical treatments. This is a significant gap.
Individuals on insulin or hypoglycemic sulfonamides risk hypoglycemia when a meal is omitted without adjusting their treatment. Timing of medication should not be altered without medical supervision.
The case of SGLT2 inhibitors, such as empagliflozin, is more specific. Fasting combined with very low food intake, vomiting, or dehydration can increase the risk of euglycemic diabetic ketoacidosis, a complication that sometimes occurs without very high blood sugar. Recommendations suggest contacting a healthcare professional rather than continuing fasting alone.
GLP-1 agonists, such as semaglutide, add another parameter: their appetite-suppressing effect combined with fasting can lead to excessive caloric restriction, with a risk of muscle loss and deficiencies. Any treatment that alters carbohydrate metabolism requires medical advice before practicing intermittent fasting.
The choice of drinks during intermittent fasting is based on a simple principle: water, black coffee, plain tea, and herbal teas remain the only options that do not trigger an insulin response. Anything with a sweet taste, a dairy base, or a protein profile interrupts the restriction phase. And for individuals on treatment, the question is not “what to drink,” but “should I fast” – a decision that rests with the treating physician.
