Feedback Inhibitor of Lactation: Breaking it Down

It’s easy to get tangled up in something that seems mysterious or complex. I’ve been there! One of those things came up recently during a webinar session — feedback inhibitor of lactation (FIL). Give me 2 minutes to break this down into simple terms in case you weren’t in attendance that evening.

Types of chemical signaling (messaging)

The body has different types of messaging including endocrine, autocrine, paracrine, juxtracrine, and more. Here, I’ll describe endocrine and autocrine. (See illustration.)

Endocrine control

The prefix “endo” means within, inside, or containing. Hence, endocrine signaling occurs when one particular cell targets a distant cell by moving within the blood stream. I call this “remote control.”

In the first several days after birth, milk production is governed primarily by endocrine (hormonal) control.

An easy way to remember this is by reflecting on how new mothers feel and behave during the first few days after birth. They may describe themselves as feeling “hormonal” when they exhibit high sensitivity and/or tearfulness.

Even in mothers who choose to formula feed, the milk “comes in.”  It’s the hormones saying, “OH! It’s time to make the milk!”

Autocrine control

The prefix “auto” means “self.” Therefore, as you might expect, autocrine control means that a cell targets itself. I call this “local control.”

Sources vary on the precise time at which the switch happens, but several days after birth, milk production is governed primarily by autocrine control.

Here, the key to milk-making is frequency of milk removal. The more the baby suckles, the more the milk is produced.

This is also easy to remember. Consider the woman who is formula-feeding. Her milk dries up within about 10-14 days.  

Understand, however, that milk is always produced by both endocrine and autocrine signaling. It’s just a matter of which influence is dominant.

Before I explain feedback inhibitor of lactation, I need to explain feedback loops.

Positive and negative feedback loops

Metabolic functions are regulated by positive and negative feedback loops.

Consider the furnace in your house. It’s chilly; you set the temperature for 70 degrees. The furnace kicks on when the temperature dips below 70 degrees. When the room temperature reaches 70 degrees, the furnace stops emitting heat. That’s an example of a negative feedback loop.

Negative feedback reduces change. It moves the system closer to the target of equilibrium. Otherwise stated, it stops when it gets to where it’s supposed to be.

Conversely, consider what happens when a piece of fruit ripens. When one peach becomes ripe, it gives off ethylene (C2H4) — a gas. Nearby peaches, exposed to the ethylene, quickly become ripe. That’s an example of a positive feedback loop.

Positive feedback amplifies change. A positive feedback loop moves a system further away from the target of equilibrium. Otherwise stated, it keeps going and going. It does not stop until the event has stopped.

In the human body, the best example is oxytocin and the contractions of labor. The contractions come faster and faster, harder and harder. The contractions do not stop until the baby is born.

Feedback inhibitor of lactation and milk-making

When milk is regulated by autocrine messaging, more milk removal means that more milk is produced. That’s a positive feedback loop.

Conversely, when milk remains in the breast (i.e., there is not adequate removal), milk production slows. (Animal studies showed this in 2019 and 2016). Specific proteins signal the brain to reduce the rate of milk production.

That’s a very basic explanation of the feedback inhibitor of lactation (FIL).

Thank you to the aspiring lactation consultant who raised this question.