Smoking cigarettes reduces Omega-3 fatty acid uptake, especially DHA, into breastmilk. Omega-3 fatty acids play key roles in infant brain development. Intake of DHA, in particular, is associated with improved mental and psychomotor development. DHA is the most abundant Omega-3 fatty acid in a newborn's brain. Maternal intake of Omega-3 fatty acids is associated with larger brain volumes in breastfed or mostly breastfed infants.

And in more bad news about smoking, maternal cigarette smoking impairs the uptake of some nutrients into breast milk. For example, iodine concentrations in the breast milk of women who smoke are roughly half of those of non-smoking women's breast milk. Iodine is critical for brain development and is also important for thyroid function. Rodent studies have found that maternal nicotine exposure impairs thyroid function in offspring and promotes resistance to the hormone leptin, which is associated with obesity.

Smoker's breast milk has lower levels of antioxidant vitamins, vitamin C and E, which could drive a pro-oxidative state in the infant. This was demonstrated in a really interesting study where researchers measured levels of ethane, a marker of oxidative stress in the exhaled air of infants. The breastfed infants of women who smoked exhaled seven times more ethane than the infants of women who did not smoke.

Research indicates that exposure to cigarette smoke influences breastfeeding duration, a study of more than 1,200 mother-infant pairs found that women who were exposed to household secondhand smoke were 30% more likely to stop breastfeeding early compared to women who were in non-smoking households.

Other things can be transferred from the mother's breast milk as well. A whole slew of harmful substances can transfer from the mother's blood into her breast milk. For example, heavy metal concentrations like cadmium are increased in breast milk.

Cadmium impairs the metabolism of nutrients essential to infant development, including selenium, zinc, copper, and magnesium. If that's not bad enough, cadmium is also a carcinogen. Cadmium levels in the transitional milk of women who smoke are approximately four times higher than the milk of women who don't smoke. Most drugs can be taken up into breast milk via passive diffusion, but how much ends up in the breast milk depends on the drug size and chemical characteristics, but also factors that alter the mom's drug metabolism.

An infant's capacity to metabolize drugs is much lower than the adult, especially in the early weeks of life. So most of the adverse events related to maternal drug use occur in infants less than two months of age. Drugs that should absolutely not be taken during breastfeeding include anticancer drugs, lithium, oral retinoids, high dose iodine, Amiodarone, and gold salts. But even drugs considered social drugs, including alcohol, nicotine, caffeine, and cannabis can carry some risks to the infant. Alcohol also passes into breast milk, but the amount that an infant is exposed to is low due to the infant's high body water content. Unfortunately, infants detoxify alcohol less efficiently than adults do. So high maternal alcohol intake could have negative effects on breastfed infants, including altered sleep patterns, decreased milk intake, weight gain, alcohol induced hypoglycemia, and impaired motor development.

Alcohol metabolism occurs according to zero-order kinetics. That means that it is broken down at a constant rate. So drinking water, exercising, or pumping and dumping won't speed up the process. Nicotine transfers into breast milk too and can have harmful effects on infant health. Nicotine concentrations in breast milk of women who smoke are three times higher than the mom's plasma levels. But an infant's capacity to eliminate nicotine is three to four times less than that of the adult.

A study involving 15 mother-infant pairs found that infants slept approximately 30 minutes less when they were fed immediately after the mother smoked, compared to when they were fed after the mother abstained from smoking. And the babies of women who smoke cigarettes are more susceptible to respiratory infections and colic, and typically exhibit poor respiratory function after breastfeeding. Not only does nicotine affect the infant, but it also impairs milk production by altering levels of maternal prolactin, the principal hormone involved in the production of milk.

Quitting smoking can be very difficult though. Nicotine patches might be a good option for women who are having problems quitting. Breast milk concentrations of nicotine obtained from nicotine patches or approximately 70% lower than those obtained from smoking. On the other hand, findings from rodent studies show that even low levels of nicotine exposure during pregnancy or early in life can increase the risk of sudden infant death syndrome, or SIDS, by inhibiting the neonate's ability to auto resuscitate.

Of course, many women's favorite drug is caffeine. It's found in a wide variety of foods and beverages. My favorite is coffee. And it's found in other products as well, such as gum. The amount of caffeine present in breast milk varies according to differences in the mother's caffeine metabolism. And it's typically low, approximately 1% of the maternal blood concentration, which usually peak one to two hours after ingestion. But caffeine metabolism is very poor in infants. So infants of women who consume extremely high quantities of caffeine, 750 milligrams per day or more, which is about six to eight cups of coffee, could potentially achieve toxic concentrations of caffeine from breast milk.

Cannabis is a broad term for the psychoactive components of the marijuana plant. Breast milk samples from 50 lactating women who reported using marijuana showed that 63% of the samples contained detectable levels of THC, the primary psychoactive ingredient in marijuana. These levels were present up to six days after the last reported use. Marijuana exposure via breast milk during the first month of an infant's life is associated with decreased infant motor development at the age of 12 months.