Can you drink coffee with milk while fasting?

Milk in your coffee while fasting breaks the fast, at least in energetic terms. Milk contains carbohydrates (lactose), proteins and fats, and through these it stimulates insulin secretion. One study compared a meal containing dairy with a meal containing lean meat: the insulin response was comparable. This means that milk does prompt the pancreas to produce insulin, although the effect was no greater than that of a regular high-protein meal.

Whether that is a problem depends on what you want to achieve with fasting. In a time-restricted eating (TRE) protocol that resulted in reduced fat mass in dancers, coffee or tea was only permitted without sugar or milk. In that protocol, milk was explicitly considered a 'fast-breaker'. This was a small study with ten participants per group, so this does not represent a firm line of evidence, but it does illustrate how serious intervention protocols typically treat the matter.

On the other hand, there is no reason to fear milk or milk proteins when eating outside the fasting window. Supplementation with milk proteins (including whey) has even been linked to slightly improved blood sugar levels in adults: fasting blood glucose fell by an average of 1.8 mg/dL and insulin resistance decreased. Whey also slows the glucose peak after a meal and suppresses appetite. These are beneficial effects, but they occur during the eating period, not during the fast itself.

If you want to preserve the metabolic benefits of intermittent fasting, the safest option is to drink your coffee black. A small splash of milk may be negligible enough in practice to change little for most people, but this has not been tested in studies. If you are in doubt, drinking black coffee during the fasting window is the safest choice.