My maternal grandfather was a proud dairy farmer. In his honor, let me devote a post to the virtues of milk.
Some people have lactose intolerance. But as the descendant of a long line of milk drinkers who evolved the ability to digest milk well in adulthood, I have no such problem.
The dietary effects of milk are complex. Drinking milk causes a rise in insulin, which among other things is a hormonal instruction for the body to take sugar out of the blood and turn it into stored fat. On the other hand, through the hormone incretin, milk slows down stomach emptying and makes people feel full or satiated for longer, so they eat less of other things. On balance whole milk is good for weight loss, skim milk is less helpful. Jason Fung carefully reviews the academic literature on milk in his excellent book The Obesity Code (in chapter 17, "Protein"). All the quotations below are fromThe Obesity Code
Whey Stimulates Insulin ...
The hormonally powerful part of milk is not the fat or the milk sugar (lactose), but the whey:
Dairy, meat and the insulin index proteins differ greatly in their capacity to stimulate insulin, with dairy products in particular being potent stimuli. Dairy also shows the largest discrepancy between the blood glucose and insulin effect. It scores extremely low on the glycemic index (15 to 30), but very high on the insulin index (90 to 98). Milk does contain sugars, predominantly in the form of lactose. However, when tested, pure lactose has minimal effect on either the glycemic or insulin indexes. Milk contains two main types of dairy protein: casein (80 percent) and whey (20 percent). Cheese contains mostly casein. Whey is the byproduct left over from the curds in cheese making. Bodybuilders frequently use whey protein supplements because it is high in branched-chain amino acids, felt to be important in muscle formation. Dairy protein, particularly whey, is responsible for raising insulin levels even higher than whole-wheat bread, due largely to the incretin effect.
... But Whey's Stimulation of Incretin Is Also Very Satiating
Like many people, I find whole milk very satisfying, and crave other food a lot less after a glass of milk. There is a hormonal reason for this. Jason Fung explains:
But are dairy and meat are fattening? That question is complicated. The incretin hormones have multiple effects, only one of which is to stimulate insulin. Incretins also have a major effect on satiety. ... Incretin hormones play an important role in the control of gastric emptying. The stomach normally holds food and mixes it with stomach acid before slowly discharging the contents. GLP-1 causes stomach emptying to significantly slow. Absorption of nutrients also slows, resulting in lower blood glucose and insulin levels. Furthermore, this effect creates a sensation of satiety that we experience as “being full.” A 2010 study compared the effect of four different proteins: eggs, turkey, tuna and whey protein—on participants’ insulin levels. As expected, whey resulted in the highest insulin levels. Four hours afterward, participants were treated to a buffet lunch. The whey group ate substantially less than the other groups. The whey protein suppressed their appetites and increased their satiety. In other words, those subjects were “full.” ...
So the incretin hormones produce two opposing effects. Increased insulin promotes weight gain, but increased satiety suppresses it—which is consistent with personal experience.
On Net, Whole Milk Aids Weight Control. Low-Fat Milk is Neutral
Airtight experimental evidence on the net effect of milk is not available, but there is some associational evidence suggesting that whole milk aids in weight control:
These two opposing effects—insulin promotes weight gain, but satiety promotes weight loss—cause a maddening debate about meat and dairy. The important question is this: Which effect is more powerful? ...
The story with dairy is entirely different. Despite the fact that its consumption causes big increases in insulin levels, large observation studies do not link dairy to weight gain. If anything, dairy protects against weight gain, as found in the Swedish Mammography Cohort. In particular, whole milk, sour milk, cheese and butter were associated with less weight gain, but not low-fat milk. The ten-year prospective CARDIA Study found that the highest intake of dairy is associated with the lowest incidence of obesity and type 2 diabetes. Other large population studies confirmed this association. The data from the Nurses’ Health Studies and the Health Professionals Follow-up Study shows that overall, average weight gain over any four-year period was 3.35 pounds (1.5 kilograms)—pretty close to 1 pound per year. Milk and cheese were essentially weight neutral.
Yogurt is an important case to talk about because so many people think that yogurt is a health food. Plain full-fat yogurt should be just as good for weight loss as whole milk if not a bit better, but the low-fat, high-sugar yogurt many people eat is bad stuff. It is unlikely that the good effects of the yogurt content are able to overcome the bad effects of the added sugar.
Reigning Dietary Theories as Reflected in the Supermarket May Be Off Target
Going down the aisles of grocery stores, I notice first all the aisles that are temples to refined carbohydrates. Then when I look closer at things that claim to be especially healthy foods, I notice many low-fat products (see "Jason Fung: Dietary Fat is Innocent of the Charges Leveled Against It") and dairy-free products. These may not be the most important directions to go (except for those with lactose intolerance or another specific milk intolerance).
Zero added sugar of any type and no artificial sweeteners is likely to be a more helpful direction to go for health. (See Sugar as a Slow Poison.) Of course, ruling out added sugar and artificial sweeteners rules out a large share of all processed foods, which may be helpful in its own right given the incentives manufactures have to maximize taste and shelf-life with little regard for health other than doing the minimum to be able to pretend something is healthy.
Milk may be only a few thousand years old as a common food for adult humans, and then only in some ancestries. But a few thousand years in some ancestries is a lot longer and more broadly tested than many of the processed foods advertised today have been tested. I recommend worrying more about new types of processed foods that are less than 200 years old in widespread consumption than about whole milk.
Don't miss these other posts on diet and health and on fighting obesity:
Julia Belluz and Javier Zarracina: Why You'll Be Disappointed If You Are Exercising to Lose Weight, Explained with 60+ Studies (my retitling of the article this links to)
Also see the last section of "Five Books That Have Changed My Life" and the podcast "Miles Kimball Explains to Tracy Alloway and Joe Weisenthal Why Losing Weight Is Like Defeating Inflation." If you want to know how I got interested in diet and health and fighting obesity and a little more about my own experience with weight gain and weight loss, see my post "A Barycentric Autobiography."