Hind milk fails to be released as a result of not having letdowns. Which in turn, lowers supply. The result is less foremilk and less hind milk altogether. When this happens, even having letdowns will be insufficient because there’s simply not enough milk to satisfy baby. Letdowns are caused by oxytocin. Oxytocin is released when the nipple is stimulated.
When a baby first latches on they take shallow, pacifying sucks to stimulate a letdown. Milk begins to flow and they switch to deep long rhythmic sucks. To get additional letdowns for more milk it takes greater amount of stimulation. Thus the deep long rhythmic sucks produces additional letdowns. These letdowns often overlap each other where you cannot tell one stopped and another started.
If baby is tongue-tied, he or she cannot sustain deep, long, rhythmic sucking. The nipple lacks additional stimulation required for additional letdowns.
Foremilk gradually transitions to hindmilk while the milk is flowing out. This is because when milk is produced in the milk ducts, the fat is attached to the lining of the milk ducts while the watery part is contained inside. During letdowns, the milk ducts contract, causing the fat to break off into the watery part, creating a creamier milk. If only a first letdown is created and no additional letdowns follow, the fat is left on the lining of the milk ducts. Baby drinks the watery part and her thirst is quenched but she will remain hungry and unsatisfied.
Dehydration and malnutrition will cause low milk supply because your body has to work harder to make the milk. High blood sugar can also shut down the body’s ability to create milk.
High stress can lower supply because it hinders the release of oxytocin, which causes the letdowns. Because breastmilk is based on supply and demand, less milk demanded (aka milk leaving the breast) will result in less milk produced. The same result will also occur if baby cannot sustain long deep rhythmic sucks.