What are the effects of dairy on health?

In The Carbohydrate Appropriate Diet, I wrote about the health benefits of dairy…and why some people should still avoid it. Since that book was published there has been a huge volume of research published and many reviews of that research summarising the most important results. There is also a lot of uncertainty around the topic of dairy. Some people promote it as a health panacea, others deride it as a pro-inflammatory dietary nightmare.

So, what does the evidence tell us about the impact of dairy overall, and for whom it is (and isn’t) appropriate?

At the time of writing The Carbohydrate Appropriate Diet, despite dairy avoidance being a common theme in alternative and complementary health circles, the evidence for the absolute avoidance of dairy by most people was very thin on the ground. In fact, the research had demonstrated that in an adequate protein diet, a moderate dairy intake helped to retain lean mass during dieting more effectively than the same protein intake but with lower dairy intake, and that high protein, high dairy intake increased fat-loss.¹ Typically, people lost more fat when they supplemented with calcium, and more again when the same amount of calcium was provided from a diary containing diet,^{2, 3} and it’s is likely that dairy promotes a synergistic effect because of its relatively high content of protein (especially the BCAAs found in dairy in abundance) and angiotensin-converting enzyme inhibitors, to mitigate fat-gain.⁴

Systematic reviews and meta-analyses of the published studies have concluded that the inclusion of dairy in energy-restricted weight-loss diets significantly reduces weight and body-fat and improves lean-mass.^{5, 6} There is also an association between the intake of dairy products and reduced rates of type-2 diabetes.^{7, 8} Furthermore, high-dairy consumption might reduce insulin resistance without negatively impacting bodyweight or cardiometabolic markers.⁹

What’s New?

You might think that not a lot has changed since 2014-2016 when I wrote and released The Carbohydrate Appropriate Diet. But since 2016 there have been nearly 60 systematic reviews or meta-analyses published on the effects of diary on various parameters of health! These include the studies I originally referenced but also many more.

So, let’s do a deep dive into what the research tells us about dairy and its effects on health, whether good or bad.

Cardiometabolic Health

Overall, the evidence points towards favourable associations, albeit moderate, between total dairy intake and cardiometabolic risk, including for hypertension (high blood pressure), diabetes, and stroke.¹⁰ A meta-analysis of 16 studies found that reductions in both type 2 diabetes and cardiovascular disease are associated with dairy intake but that this effect was only significant for women and not men (RR for T2D = 0.868; 95%CI, 0.82–0.92; P < 0.001; RR for CVD = 0.837; 95%CI, 0.75–0.93; P < 0.001).¹¹

The evidence points towards favourable associations between dairy intake and cardiometabolic risk

Heart Disease

Overall, dairy has little if any association with heart disease (RR~1.0),¹² while it’s possible that certain types of dairy (cheese in particular [SRRE=0·82; 95 % CI 0·72, 0·93]) might have a moderate, beneficial impact on the incidence of heart disease.¹³ Additionally, a weak, yet statistically significant association between total dairy and cheese (but not milk), and ‘pulse wave velocity’, a measure of arterial stiffness (an important marker for cardiovascular health) has been observed (ES = −0.03; 95% CI [−0.04, −0.01]).¹⁴

Because observational research focussed on questionnaires of food frequency and other methods of inferring food intake have serious limitations, a review was conducted of 13 studies (> 7000 participants) that had looked instead at circulating fatty acids derived from dairy, as this would provide a quantitative measure of dairy intake and cardiovascular risk. Fatty acids found almost exclusively in dairy (pentadecanoic acid (15:0), heptadecanoic acid (17:0) and trans-palmitoleic acid (trans-16:1n-7)) were compared to the incidence of heart disease, stroke, heart failure, and cardiovascular disease mortality and the risk of CVD. There were no associations between the concentration of pentadecanoic acid with heart disease and stroke, but a negative relationship with heart failure (RR = 0.72, 95% CI: 0.55–0.95). Other fatty acids from dairy were not associated with any cardiovascular disorder and the authors concluded that “Higher dairy fat exposure is not associated with an increased risk of cardiovascular disease”.¹⁵

Diabetes and metabolic syndrome

Total dairy product consumption is associated with reduced risk of metabolic syndrome,^16-18 along with a lower risk of key markers of metabolic syndrome such as high triglyceride concentrations in blood, abdominal obesity, and yoghurt consumption has been associated with a 16% lower risk of high blood glucose concentrations.¹⁷ It has been estimated that the overall protective effect of consuming dairy on the incidence of diabetes is ~10% (relative risk = 0.88 (95%CI: 0.80, 0.96), rate ratio = 0.92 (95% CI: 0.88, 0.97)).^{17, 19} Dose-response analysis has suggested that there is a benefit from increasing intakes of both total and low-fat dairy,¹⁸ and for every 200 g of dairy per day, there is ~4% reduced risk of developing diabetes.¹²

Dairy product consumption is associated with reduced risk of metabolic syndrome

Effects on Insulin, Glucose, and HbA1c

Dairy intake is associated with a reduced risk of type 2 diabetes but the reasons for this had not been clearly demonstrated.  In this review,  markers of metabolic syndrome and diabetes; blood glucose, fasting insulin, homeostasis model assessment of insulin resistance (HOMA-IR), and HbA1c (glycated haemoglobin – a measure of average glucose levels) were analysed in relation to dairy intake in 44 studies featuring over 3000 participants.²⁰

Key findings were:

• Fasting glucose was positively associated with increased dairy intake [34 studies, n = 2678; mean difference (MD): 0.07 mmol/L; 95% CI: 0.01, 0.12 mmol/L; P = 0.01, I² = 23%]
• However, HbA1c was negatively associated with elevated dairy product intake in 4 studies (n = 512; MD: −0.09%; 95% CI: −0.09%, −0.03%; P = 0.005, I² = 0%)
• Fasting insulin (29 studies, n = 1902; MD: −2.97 pmol/L; 95% CI: −7.05, 1.10 pmol/L; P = 0.15, I² = 21%) and HOMA-IR (13 studies, n = 840; standardized MD: −0.07; 95% CI: −0.26, 0.12; P = 0.49, I² = 38%) were not associated with elevated dairy consumption

While the risk of bias and some concern over the quality of some included studies, these results suggest that while fasting glucose may be elevated in response to increased dairy intake, average glucose levels do not appear to be elevated and there is little effect on insulin levels or insulin and glucose control.

Stroke and hypertension

Both total dairy intake and cheese show small yet statistically significant associations (SRRE=0·91; 95 % CI 0·83, 0·99; SRRE=0·87; 95 % CI 0·77, 0·99, respectively)[1] and milk a possible effect, between intake and risk of developing a stroke.^{13, 21} Each 200 g per day increment of milk intake is associated with a 7-8% lower risk of stroke.^{12, 22} The maximal likely benefit has been estimated though at a relatively limited intake of just 125 g per day of milk or 25 g per day of cheese.²²

Children in the highest intake group for dairy were 38% less likely to have childhood obesity

Obesity

Future obesity risk has been studied and reviewed in an analysis of 10 studies of > 46000 children and adolescents. Children in the highest intake group for dairy consumption were 38% less likely to have childhood overweight/obesity (pooled odds ratio (OR)=0.62; 95% confidence interval (CI): 0.49, 0.80). Each 1 serving per day increase in dairy consumption was associated with a reduction in body fat of 0.65% (β=0.65; 95% CI: −1.35, 0.06; P=0.07), and the risk of overweight/obesity was 13% lower (OR=0.87; 95% CI: 0.74, 0.98).²³

Dairy is a healthy food that is associated with improvements in markers of metabolic syndrome and diabetes, reduced risk of cardiovascular disease, and improved body composition.

Bone Health

Nutrients abundant in milk and other dairy foods, especially protein, calcium, vitamin D (in full fat and fortified dairy) and phosphorous, are important for the growth, repair, and overall health of bones. Despite the plausible benefits to bones from dairy, many claims have been made that dairy is ‘bad’ for bones.

Supplementing the diet with dairy products significantly increases bone mineral content

A recent systematic review summarised the available evidence from bone health in children and adolescents in relation to dairy consumption finding that “supplementing the usual diet with dairy products significantly increases bone mineral content during childhood”.²⁴ Another review (11 randomised controlled trials) found significant benefits to bone mineral content and bone mineral density were found with an average 8% increase in bone density following ~16 months of increased dairy consumption.²⁵

Osteoporosis, fracture, and bone mineral density

Total dairy intake (but not milk intake) was associated with a reduced risk of developing osteoarthritis (RR for total dairy 0.63; 95% CI: 0.55–0.73). However both total dairy intake and milk intake were associated with reduced risk of osteoporosis (22% and 37% reduced risk respectively) and milk also associated with a 25% reduced risk of hip-fracture in cross-sectional and case-control studies, while linear regression suggested a 9% greater risk of hip fracture for every 200 g increase in milk consumption per day.²⁶ A 2018 review of dairy intake in healthy adults (10 cohort studies) showed no strong associations between milk or total dairy intake and fracture but a < 3% lower average bone mineral density in those with lower dairy intakes and equivocal results for later milk consumption.²⁷

Overall, bone mineral density and bone health overall are likely to be improved by dairy consumption.

Kidney Function

Studies suggest a favourable effect of dairy intake on kidney disease but the data is limited and further research needs to be conducted.²⁸

Cancer

Liver

A total of eight cohort and case-control studies featuring over one million participants suggested an association between dairy intake and liver cancer (RR 1.38, 95% CI: 1.00-1.91, p < 0.05) but this association was not shown for milk or yoghurt and was not significant for cheese.²⁹

This suggests that other dairy or diets featuring higher amounts of other dairy foods, such as butter, might have some association with liver cancer, but it is extremely difficult to find causality.

Testicular

There is no strong evidence that dairy consumption is associated with risk of testicular cancer.³⁰

Bladder

Some studies have reported that milk and dairy product consumption reduces bladder cancer incidence, whereas others have reported null or opposite findings. A meta-analysis of 26 cohort and case-control studies found that medium compared with low consumption of dairy was associated with lower risk of bladder cancer for total dairy (RR = 0.90; 95% CI: 0.81, 0.98), milk (RR = 0.90; 95% CI: 0.82, 0.98), and fermented dairy products (RR = 0.87; 95% CI: 0.79, 0.96). However, high compared with low consumption of whole milk was significantly associated with a higher risk (RR = 1.21; 95% CI: 1.04, 1.38). These findings suggest a decreased risk of bladder cancer associated with medium consumption of total dairy products and with medium and high consumption of milk and fermented dairy products.³¹

These results are unclear but suggest a likely reduction in bladder cancer risk from dairy, however, high consumption of full-fat milk provides an anomalous finding.

Colorectal

A review of 29 studies featuring greater than 22000 participants showed a consistent and significant decrease in colorectal cancer risk associated with higher consumption of dairy products (RR: 0.80; 95% CI: 0.70, 0.91) and milk (RR: 0.82; 95% CI: 0.76, 0.88). Cheese consumption was also inversely associated with the risk of colorectal (RR: 0.85; 95% CI: 0.76, 0.96) and proximal colon cancer (RR: 0.74; 95% CI: 0.60, 0.91). No significant associations with colorectal cancer were found for the consumption of low-fat dairy products, whole milk, fermented dairy products, or cultured milk. The study concluded that consumption of total dairy products and total milk was associated with a lower risk of developing colorectal cancer, while low-fat milk consumption was associated with a lower risk of cancer restricted to the colon. Cheese consumption was associated with reduced colorectal cancer risk, specifically proximal colon cancer.³²

Dairy appears to be associated with a lower risk of colon cancers.

Prostate

In a review of high-quality meta-analyses (2 to 32 cohorts with > 800,000 subjects) high vs low consumption of dairy was associated with a possible increase in risk for prostate cancer (RR 1.06 to 1.68) and a suspected dose-response of 1.07 per 400 g consumed per day. However, the inconsistency in results and relatively small effect size cast doubt on this finding. All sub-group analyses had risk ratios that crossed 1.0 suggesting no likely increase in risk, and interestingly more recent studies have reduced risks compared to earlier ones.³³

So, while some data suggest an increase in prostate cancer risk from increased total dairy, overall this effect is unlikely.

Cognitive Function

While dairy is likely to be associated with improved health outcomes for ageing such as reduced frailty and reduced rates of muscle loss, effects on mental health have been contradictory.³⁴ The existing, mostly observational, evidence does not show any strong associations between dairy intake and cognitive function and the quality of evidence and potential for risk of bias makes it difficult to make any conclusions regarding the effect of either milk or total dairy intake on the risk of cognitive decline or neurological disorders in adults.³⁵

Dairy does not appear to have an effect, either positive or negative, on cognitive function but is associated with other health benefits.

Milk, Dairy and Pregnancy

A 2018 systematic review of the literature looked at the effect of milk and dairy consumption during pregnancy on pregnancy and lactation outcomes. A final inclusion of 17 studies (six prospective cohort studies, three intervention studies, three retrospective cohort studies, three cross-sectional studies, and two case-control studies showed a trend (not definitive) towards improved infant birth weight and length but a lack of quality data prevented any conclusions being drawn about the effects of dairy intake on preterm deliveries, spontaneous abortion, or lactation.³⁶

The effects of dairy on pregnancy and birth outcomes is unclear. There is likely to be a beneficial effect IF dairy intake aids overall nutrient status and energy sufficiency.

Dairy and acne

Another common belief, that has been considered by many in scientific circles to be folklore, is that dairy is associated with acne. In a recent systematic review of the literature (to the end of 2017), 14 studies with over 78000 participants there was an approximate increase in acne of 25% related to all dairy intake (OR 1.25; 95% CI: 1.15–1.36; p = 6.13 × 10⁻⁸) and this effect was dose-dependent: 1.24 (0.95–1.62) 2–6 glasses per week, 1.41 (1.05–1.90) 1 glass per day, and 1.43 (1.09–1.88) ≥2 glasses per day compared to intake less than weekly. While these results were both meaningful and significant, lack of consistency in the study results suggests caution in their interpretation but based on the accumulated evidence, dairy is likely to increase the risk of acne.³⁷

In 14 studies with over 78000 participants, there was an approximate increase in acne of 25% related to all dairy intake

Increasing dairy intake may exacerbate acne. Those suffering from acne could reduce or eliminate dairy to see if they benefit.

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[1] SRRE = standardised relative risk estimate