Question

1. When a baby drinks milk, that milk seems to simply “disappear” as the baby grows larger and gains weight. Milk is full of fats, proteins, and carbohydrates, but so is a baby’s body. Human milk contains over 200 components, including lactose, casein, and triglycerides. Yet, a baby’s body has thousands of different types of proteins, carbohydrates, and fats. The milk travels through the baby’s mouth, stomach, and small intestine.

A new parent, insists that “the baby grows because the milk’s proteins just go straight into the baby’s muscles.”

Based on what you know, what best explains how the matter in milk becomes part of a baby’s growing body?

Responses

A. The proteins in milk directly attach to the baby’s muscle cells without changing form.
A. The proteins in milk directly attach to the baby’s muscle cells without changing form.

B. The proteins in milk are broken down into amino acids, which enter the bloodstream and are rebuilt into new body proteins.
B. The proteins in milk are broken down into amino acids, which enter the bloodstream and are rebuilt into new body proteins.

C. The milk molecules mix with blood in the stomach and are stored in the muscles as they are.
C. The milk molecules mix with blood in the stomach and are stored in the muscles as they are.

D. The body absorbs whole milk molecules through the skin and turns them into energy.
D. The body absorbs whole milk molecules through the skin and turns them into energy.

E. The lactose and fats in milk remain unchanged and become muscle proteins after several days.
E. The lactose and fats in milk remain unchanged and become muscle proteins after several days.

Answer 1

Correct answer: B.

Why: Digestion breaks milk proteins into amino acids (and fats into fatty acids/glycerol, carbs into sugars) in the stomach and small intestine. Those small molecules are absorbed into the bloodstream and taken up by cells, which rebuild them into the many specific proteins, lipids and other molecules the baby needs using the instructions in its DNA. The other options are wrong because whole milk molecules are not simply inserted intact into muscles, they are not absorbed through skin, and lactose or fats do not magically turn into muscle proteins unchanged. (Newborns can also get intact antibodies from breast milk for immune protection, but most dietary proteins are digested first.)