WHO’S AFRAID OF DAIRY FAT?

For the whole of my working life (about 50 years!), dairy fat has been considered harmful. That’s because it’s a source of saturated fats that are usually linked (associated) to an increased risk of atherosclerosis, or hardening of the arteries, particularly in the heart and brain. So dietary guidelines for the past 50 years have recommended reduced fat, low fat or skimmed dairy products. You still get lots of calcium, protein and B12 from these nutrient-rich foods. But a few months ago, I made the decision to switch to whole milk and full fat yogurt. I’ve always eaten full fat cheese such as feta and parmesan but in fairly small amounts. Why did I make that decision? Was it the right one?

Well, I was swayed by the mounting evidence in nutrition science that dairy fat was unique and behaved differently, especially when consumed in its original food matrix, as in the case of yogurt and cheese, but not butter. In fact, some of the evidence suggests that it reduces the risk of chronic diseases like type 2 diabetes. With a grandparent and parent who developed type 2 diabetes, I took note. But like everything in nutrition, it’s complicated!

It depends on what you would otherwise eat

Some of the most convincing evidence comes from the USA, where three very large cohorts of health professionals, have been followed for as long as 20-30 years. Their food consumption and health are studied at regular intervals and the data carefully validated. What did they find?

In a 2019 study, dairy fat consumption was not associated with risk of type 2 diabetes in these cohort studies of US men and women when compared with energy (calories/kilojoules) from carbohydrate. However, replacement of dairy with other animal fats or refined carbohydrates (i.e., starches and sugars) was linked to higher risk, and replacing dairy fat with carbohydrates from whole grains was associated with lower risk of type 2 diabetes (1).

In another Harvard study published in 2016, if you ate more dairy fat and less carbohydrate (e.g. grains), the relative risks of coronary heart disease and stroke were similar. But if you ate less dairy fat and more plant fat (polyunsaturated (PUFA) or otherwise), then you reduced your risk by 10-24%. Thus, replacing dairy fat with vegetable sources of fats and PUFAs may reduce risk. However, the authors noted that the food matrix may modify the effect of dairy fat on health outcomes, warranting further investigation (2).

Both studies described above are called observational studies which find associations. They cannot prove that dairy fat is beneficial or harmful because they cannot rule out “confounding” factors, even with the best methods of statistical adjustment. Only randomised controlled trials can be seen as evidence of proof of causation.

Here’s an example. In a study published in 2021, 72 participants with the metabolic syndrome were assigned to 1 of 3 diets with either limited intake of dairy foods, OR 3.3 servings of low-fat dairy each day, OR 3.3 servings of full fat dairy as milk, yogurt and cheese each day, for 12 weeks (3). What did they find?

Body weight increased on the full-fat diary diet (by 1.0 kg) compared with the limited dairy (0.4 kg) and low-fat dairy (0.3 kg). Importantly, insulin sensitivity appeared to decrease on both the full-fat and limited-fat dairy, even after adjusting for differences in weight. Reassuringly, there was no effect on liver fat, inflammatory markers, total, LDL or HDL cholesterol, triglycerides or diastolic blood pressure. However, there was a significant lowering of systolic blood pressure in the full fat dairy group and a similar trend in the low-fat group. The authors concluded that dairy fat when consumed in a complex whole-food matrix, does not adversely impact classic markers of cardiovascular disease.

What do we mean by food matrix? We mean the physical framework of the food as eaten – just as we know that the glycemic index (i.e., dietary carbohydrate) is affected by processing, so is dietary fat. In milk, yogurt and cheese, the fat exists as globules surrounded by a membrane (a thin layer of lipids and protein) that appears to be important. When we make butter, this membrane is disrupted.

To explore this, researchers at University College London undertook a cleverly designed 6-week randomized controlled trial to see if the cheese matrix mattered. In total, 164 volunteers were assigned to ∼40 g of dairy fat per day as 1 of 4 treatments: full-fat cheddar cheese, OR reduced-fat cheddar cheese + butter, OR butter + casein (milk protein) + Ca supplement, OR full-fat cheddar preceded by a 6-wk period where all dietary cheese was excluded (4).

What did they find? When all the fat was contained in the cheese matrix, participants had significantly lower total and LDL cholesterol. There was no difference in body weight, fasting glucose, or insulin between the groups, suggesting no effect on type 2 diabetes risk. Thus, dairy fat, eaten in the form of cheese, appears to have beneficial effects compared with the same nutrients eaten in different form (4).

We know that infants and young children must be offered full fat milk and dairy, but what about kindergarten children? Should they drink low fat milk? In the “Milky Way” Study, healthy 4–6-year-old children changed from whole-fat to reduced-fat dairy products for 3 months (5). This lowered dairy fat intake markedly (by about 13 grams per day), yet there were no changes to markers of body fatness or cardiometabolic disease risk. However, 3 months is not long, and further research is needed.

So… was my decision to switch to full fat yogurt and milk a good one? Considering that the rest of my diet is pretty good (not perfect) and I’m within the healthy weight range, I’m happy with my decision… so far. But I’ll be watching my blood chemistry (i.e., cholesterol) closely!

Read more:

1. Ardisson Korat and colleagues. Dairy fat intake and risk of type 2 diabetes in 3 cohorts of US men and women. Am J Clin Nutr. 2019
2. Chen and colleagues. Dairy fat and risk of cardiovascular disease in 3 cohorts of US adults. Am J Clin Nutr. 2016
3. Schmidt and colleagues. Impact of low-fat and full-fat dairy foods on fasting lipid profile and blood pressure: exploratory endpoints of a randomized controlled trial. Am J Clin Nutr. 2021
4. Feeney and colleagues. Dairy matrix effects: response to consumption of dairy fat differs when eaten within the cheese matrix-a randomized controlled trial. Am J Clin Nutr. 2018
5. Nicholl and colleagues. Whole-fat dairy products do not adversely affect adiposity or cardiometabolic risk factors in children in the Milky Way Study: a double-blind randomized controlled pilot study. Am J Clin Nutr. 2021

Emeritus professor Jennie Brand-Miller held a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney until she retired in December 2022. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.