Extreme Constipation with Milk

[SeattleRain]

My son is almost a year (2 weeks away) and has been thus far, formula-fed with milk-based formula, regular ol' Enfamil. In the interests of saving a little bit of money, I ran out of formula this week and just gave him whole, organic milk. He's so close to his 1st birthday, and I was planning on switching him then, that I didn't think it would be a big deal. He's been on the milk for over a week and he's had EXTREME constipation that has not resolved despite making some food changes. He drinks approximately 18oz a day (6 bottles). Here's what we've tried so far to resolve the constipation:

* 6 oz of mango-pear juice (cut with 6 oz of water) over the course of the day in his sippies

* No other dairy other than his milk bottles

* Only whole grain crackers and breads

* Seed butter blend including flax seeds and oil (4g fiber per 2T!)

* Upping the fruit and veg that we offer

* Free access to water throughout the day

* Hummus and other beans (he really only likes hummus)

I'm sort of at a loss. I'm starting to think that this might have to do with the milk, and not have to do with the diet. I don't think he's allergic to milk, since he's on a milk based formula and has done fine. He's so close to that magic age of 1 that it seems like he should be ready to drink milk. So what could be going on here? What should I do? If he's not ready to drink milk, how long should I keep him on formula? Should I switch him to toddler formula? Should I reduce the amount of milk he gets? He's somewhat underweight, so I like the amount of fat that milk gives him, as opposed to simply offering him water.

[Greenlea]

You may need to gradually switch to milk.  Half and half or even less.  Just because he is one doesn't mean he is automatically ready for milk - and it also doesn't mean kids under one aren't ready.  Also, I thought milk enzymes in formula are different and easier for babies tummies than real whole milk, so he may just not be stomaching it right.

Also, just because he is one doesn't mean he HAS to have milk.  Sure he may still be needing or wanting his formula, but once my son got off the formula bottle, I didn't not give him cow's milk.  

[SeattleRain]

I switched him to Enfagrow, their toddler formula. I thought maybe he just wasn't ready.

He still seems to want the milk though, so I don't think I can totally cut it out. Either I go to whole milk or stick to formula, but he seems to want it and I don't mind giving it to him. I know some people don't do the milk thing, but I don't think there's anything wrong with it if a baby isn't breastfeeding and getting calories/milk other ways. Expecting a 1 year old to eat all their calories seems like a challenge.

Is the constipation indicative that he's not ready? Has anyone else had a situation like this? What helped it? Did the child ever get past it?