Does dairy make you fat?

The primary proteins in milk are unique to milk, not found in any other tissues. Broadly speaking, there are two categories of milk proteins, the main group is the casein proteins ( about 3 or 4 depending on the species ), with the remaining proteins in milk grouped together as the whey protein group. All of these proteins are highly insulinogenic, meaning they produce a pronounced insulin response, even in the absence of carbohydrate. Keep that last bit in mind, it will be pivotal later.

Several studies have been performed to determine the effects of milk proteins on insulin levels, and a really interesting one compared the effects of ingesting cream vs. caseins. Cream is basically all fat, the lactose and proteins in milk have been processed out via centrifuging. So, we’re essentially comparing the insulin effects of ingesting milk fats vs. ingesting milk proteins, no carbohydrates allowed.

Take a look at the below table ( cribbed from Peter @ Hyperlipid ) and see what happens to your insulin when you ingest casein. One and two hours after ingesting casein, insulin levels are threefold higher than baseline, and this is a statistically significant result. Three hours later and we’re still more than double the baseline insulin level. But remember, we have not been ingesting any carbohydrates, so, by rights, our hapless experimental subjects should all be in hypoglycemic comas. But, happily, they’re not, because glucose levels have not budged from baseline.

Insulinogenic Effects of Casein

The burning question then becomes, how do you keep glucose levels constant in the face of tripled insulin levels and no dietary intake of carbohydrate? The one word answer is … glucagon.

The chart below comes from a different study that had subjects eat four different isocaloric meals that were high in protein, fat, carbohydrate, or alcohol. So, we’re eating the same amount of calories in each meal, but were changing the macronutrient ratios. Take a look at the insulin and glucagon response panels at the bottom of the diagram, the bit we’re interested in is the delta-AUC, or, the change in the area under the curve, which we can interpret as the overall effect of the meal. Well, as I’ve highlighted, every meal, with the notable exception of the high carbohydrate meal, causes both insulin and glucagon levels to increase in approximately the same magnitude. The high carbohydrate meal spikes insulin while at the same time suppressing glucagon, in fact lowering it somewhat over baseline.

Insulin / Glucagon Response to Different Macronutrient Isocaloric Diets

Glucagon has the opposite effect of insulin on lipolysis, that is, while insulin inhibits lipolysis, glucagon promotes it. So, in general, net net it is pretty much a wash in terms of lipolysis … one step forward, one step back. Only in the case of high carbohydrate meals do we have a net suppression of lipolysis, but not due to insulin secretion, but rather to the suppression of glucagon.

Now, we can finally answer the question: does dairy intake make one fat via the action of dairy on insulin and that hormone’s peripheral effects on lipolysis? No, because there is a compensatory secretion of glucagon by the pancreas in order to maintain normoglycemia and the peripheral effect of glucagon on lipolysis is to increase it. I tend to think that the anti-dairy paleo faction is probably overstating their case.