This question was asked of me by my 6th grader. I had a hard time answering.
That depends on the rest of your diet, and the amount of milk. Healthy and unhealthy are often inappropriate adjectives to use for single foods, as few of them are all good in any amount, and few are all bad in all amounts.
Natia is correct, but in general restricting calories is the primary need for humans in the western world. The number of calories on skim milk is lower and so in that case it would be healthier.
As with most things in life it is not the substance that is unhealthy but the amount can make it unhealthy if in excess (or if you are lactose intolerant any amount).
Define “healthier.”
Whole milk, obviously, has more fat. However, skim milk has had the Vitamin D stripped out of it (it’s fat-soluble, so taking out the fat takes out the Vitamin D). In most cases they put it back, but studies suggest that absorption of Vitamin D does not occur without fat.
This study from 2016 found that children who drank whole milk vs skim had higher Vitamin D levels and lower BMI: Relation between milk-fat percentage, vitamin D, and BMI z score in early childhood | The American Journal of Clinical Nutrition | Oxford Academic
For what it’s worth I told her that the correct answer was whatever her life skills teacher wanted to hear, and she said that meant skim was healthier. We only buy whole milk though, my kids need the calories.
I know a pediatrician recommended whole milk over skim for our daughter, but I can’t remember why. She doesn’t drink a lot of milk, but when she’s going through a “cereal for breakfast” phase we get whole based on that long-ago advice.
As this is GQ I need to point out that free and total vitamin D are very different and some cultural bias and specifically Eurocentric studies have been found to be problematic over the past couple of years.
It will take some time before bioavailability vitamin D is understood and for useful studies to be produced but in general the reproducibility and validity of studies like you posted should be considered as invalid or as less conclusive than generally considered.
https://onlinelibrary.wiley.com/doi/abs/10.1111/cen.13825
Note that this study is being cited as it is accessible, but the “Ethnicity” considerations it references have also been found to be unreliable due to significant diversity within even isolated populations. The point being that almost all of these studies used total vitamin D blood levels and there are serious implications on needs and availability based on multiple complex gene interactions.
As an example in Black Americans, the incidence of falls, fractures, or osteopenia are significantly lower compared to White American counterparts with similar vitamin D levels. While the implications are currently unknown, it is quite possible that the mutations that produced light skin, which have been assumed to help with vitamin D synthesis in higher latitudes may actually result in a greater need but further research with more data that is not as biased will be required to know for sure.
In general most Americans get plenty of vitamins even with poor quality heavily processed food diets as our intake is fairly high. Without a known deficiency there is very little need to consider small intake differences like stated above for the average consumer today.
It has become clear that the extreme push in decades past toward low-fat foods (motivated by calorie reduction or general health) was misguided, at least to the extent that the replacements were high in sugar and other processed high-GI carbs. So certainly don’t replace whole milk with low-fat chocolate milk! But whole milk does have a lot of calories, so if you drink a lot of it and you’re overweight, replacing some with low-fat milk is sensible; conversely if you need the calories, whole milk is fine.
The overarching rule with diet is that it’s difficult to do rigorous science, since we can’t do the controlled decades-long experiments with human populations that would be ideal. So treat any recommendation for any kind of extreme diet with skepticism. Don’t seek to eliminate things from your diet, don’t believe evidence-free recommendations to eat excessive amounts of “superfoods”, don’t buy in any extreme way into fads like the paleo just-so fantasy tale except the limited aspects that happen to coincide with actual evidence.
I should also call out a common issue with many studies which indicate specific food types are correlated with BMI, which is almost always a false correlation.
People with weight problems are more likely to buy “low calorie” foods, because they are trying to reduce their weights. While the text of the studies may call this out it is rarely called out by the abstract or pop-science articles.
People with high BMIs are far more likely to drink skim milk or diet-soda etc… because they are trying to restrict calories while people with “normal” or “low” BMIs have a far lower incentive to buy these products. People respond differently to over and underfeeding, and it is a complex subject. The scientific community is working hard to improve the quality of studies and is addressing some limitations but note that epidemiology has serious challenges in performing studies and there is also a social and economic aspect that tends to favor easy solutions to complex problems.
The reason the OP’s discussion was complex is this is a far more complicated discussion than most people realize. While out of date this chart will help demonstrate how complicated it is.
http://www.shiftn.com/obesity/Full-Map.html
For the most part it is still a problem of decreasing activity levels mixed with increased intake levels, but reversing that trend is rarely as simple as buying low calorie foods or simple diets, but both the USDA recommendations and the products we are encouraged to buy try to simplify this complex subject to simple solutions, which rarely results in the intended outcomes.
Sterons: Consider that you had a hard time answering means you are probably better informed than most of us. The fact you realized this should be considered a positive thing.
Huh? Skim milk doesn’t have the fat added back into it (whole milk was usually once skim milk with the fat added back into it so that fat content can be identical from batch to batch). That is not a “stripping” of Vitamin D.
In most cases they put what back to skim milk? Vitamin D, right? They aren’t “putting it back” as their was virtually none in the fat to begin with. They fortify both skim and whole milk with Vitamin D. It’s not a waste by putting it into the skim milk, either. We will use fat stored to process Vitamin D. Having fat in the same meal as foods containing Vitamin D can help or hinder absorption. The following study showed that higher fat content in the same meal resulted in lower Vitamin D absorption than a specific lesser amount.
How were those studies performed? I’d like to see one.
Raw milk (which is high risk and unsafe in a post-industrial world) contains a small amount of vitamin D, pasteurized milk has *slightly *less content before it is fortified.
Raw milk (which is high risk and unsafe in a post-industrial world) has about 0.04 mg per quart of vitamin D, where commercial fortified whole milk seems to have 0.41 mg per quart. Note that this is 10 times as much.
I doubt consumers would be OK with differing fat levels or cream rising to the top and want their milk consistent and homiginized thus what
x-ray vision mentioned.
One quart of fortified whole milk would give you 98% of the current daily value for vitamin D, when you would have to drink two and a half gallons of raw milk to get the same amount.
It is usually claimed that a quart of milk contains about 400 IU of Vitamin D. The conversion from D in IUs to mcg is done by dividing by 40.
https://dietarysupplementdatabase.usda.nih.gov/ingredient_calculator/help.php#q10
Mcg to mg is done by multiplying by .001. So…
400/40=10
10x.001=.01mg
.01 mg of Vitamin D in a quart of fortified milk.
Unfortified milk can have Vitamin D content ranging from 0.3–1.0 μg/kg and fortified milk in the US contains about 7.05 μg/kg.
So anywhere from 7 to 24 times as much.
Oops, yes I pulled my IU conversion numbers from a bad source, thanks for correcting my math error.
No problemo. 
Since the discussion turned into one about vitamin D and milk, it should be noted that 20 to 30 minutes of sunshine on bare skin threes times a week reportedly gives people amble vitamin D without drinking a drop of milk.
Not that simple. We’re still not sure what an adequate amount entails. There are many variables such as age, location, time of year and one of the biggest is skin color. A very dark-skinned person needs much longer exposure than a fair-skinned person.
Another data point: consuming low-fat dairy products can affect ovulation and fertility. Reduced Fertility In Women Linked To Low Fat Dairy Food
My kids’ pediatrician told me to give them 2% milk. He said that they needed the fat, that skim wasn’t a good choice for them.
How much bare skin? It is winter ya know! Will 3 square inches suffice?