Feature Article for the British Association of Nutrition & Lifestyle Medicine (due to be published)

Atiya Khan

There are conflicting messages around dairy, with some recommendations to include daily, avoid full-fat or avoid completely1.  It appears as a section on the Eatwell Guide issued by the British Nutrition Foundation, suggesting dairy and alternatives such as soya with the advice ‘Choose lower fat and lower sugar options’2. Milk, cheese, yoghurt and fromage frais are recommended for their calcium content, alongside other foods, including fortified foods3. The BANT Wellness Solution plate, however, has dairy outside of the main plate, advising to ‘Limit dairy to a small matchbox of cheese, half a cup of live unsweetened yoghurt or a small glass of milk a day’4.  

Lactose intolerance, affecting approximately 5 in 100 people in the UK, is the most commonly diagnosed adverse reaction to cow’s milk in adults and adolescents, although people may self-diagnose so true numbers are unknown5,6,7. As for allergies, over 20% of the UK population has at least one or more allergic disorder, with 44% of British adults suffering one or more allergy. The prevalence of eczema has increased two to threefold in industrialised countries during the last 30 years, asthma is expected to increase from 300 million to 400 million worldwide by 2025 and there has been a 615% increase in hospital admissions for anaphylaxis in the UK between 1992 to 20128. One of the most common allergies is to dairy, especially for children, with 2 in every 200 children under 4 allergic to milk and more common in babies9

This article sets out a review of the research on dairy, its nutrients, bioavailability, effects on inflammation and some conditions it may affect. 

Nutritional value of dairy 

Dairy contains a variety of nutrients: vitamins A, B6, B12, D and K; minerals calcium, iodine, magnesium, potassium, phosphorus and zinc; fats; and proteins. Calcium can make up to 60% of the recommended daily allowance. There is now more interest in fermented dairy, as a source of vitamin K and its role in the intestinal microbiota. Accordingly, the type of dairy product consumed is relevant, with salted cheeses causing a high sodium intake, soft cheeses with less calcium and fat content dependent on different factors, including milk source, processing, fat removal and animal conditions. Desserts contain additional sugars and vegetable fats, while yoghurt is a high quality protein, helping to maintain body weight, support muscle and bone growth, with a research paper advising one yoghurt serving a day.1,10

The levels of nutrients in milk are proportionate to the growth of a calf, with the composition of milk changing according to stage of lactation, breed, age and health of the cow11. Calcium is often raised as a concern for those going on a dairy free diet, with a study on women between 19-50 years old following a dairy free diet showing they met 44% of the calcium requirement and 57% of magnesium and potassium requirements. While the study acknowledges that dietary supplements and fortified foods can be used to meet these requirements, other components of dairy such as amino acids are beneficial for health, suggesting that a lactovegetarian diet is more beneficial than vegan.12,13  

Bioavailability 

A study using ‘normal dietary conditions’ showed that 30-40% calcium in cheese in milk is absorbed in the gut, while 28-36% is absorbed from fortified cereals, soy and rice drinks. Absorption of calcium from green leafy vegetables is limited due to fibre, phytic and oxalic acid, amounting to around 5% calcium absorbed, so a higher quantity of vegetable intake is required to meet the same intake as milk13. However, this is in the context of a Western diet, with one study commenting that while it is possible to get adequate calcium from a plant based diet, it is more convenient to include dairy14. Milk fats are thought to form insoluble soaps with calcium, hence theoretically reducing absorption but the soaps disassociate with the low pH of the stomach, and re-associate in the ileum, by which time most calcium absorption has already occurred. Lactose, proteins and phosphopeptides in milk help with solubility of calcium for absorption in the distal intestine13, 15, 16.

Inflammation

Cow’s milk is a common food allergen for atopic conditions17. Plant based diets and activities are presented as anti-inflammatory, with the Western diet and inactivity as pro-inflammatory, with the latter having over 70% of caloric intake from refined sugars, vegetables oils, highly processed cereals and dairy products, with a higher ratio of omega-6 to omega-3 polyunsaturated fatty acids, contributing to chronic diseases such as obesity and osteoporosis18,19.  The Mediterranean diet is cited as a ‘health protective’ diet, being largely plant based. The China study explains that foods of animal origin, even in small quantity, can increase plasma cholesterol concentrations, correlating with death rates from chronic degenerative diseases20.

A study reviewing the effects of dairy products on inflammatory markers in relation to cardiovascular diseases, obesity and metabolic syndrome in both healthy and diseased individuals showed varied results, including:

  • no correlation between dairy and inflammatory markers21;
  • insignificant reduction of CRP with dairy consumption in obese members of the group22;
  • some saturated fats C15:0 and 17:0 fatty acids may be responsible for health benefits in overweight adolescents23;
  • low fat milk had no significant effect on CRP24;
  • low fat dairy could increase s-TNFR25;
  • no conclusive increase in markers after a high fat dairy meal26;
  • 4 weeks of 3 daily servings of both low and high fat dairy in individuals with low-grade systemic inflammation, had no adverse effects on inflammation27;
  • increasing dairy reduces oxidative and inflammatory stress in metabolic syndrome28;
  • a protective effect (a reduction of CRP, IL-6 and TNF-α) in healthy individuals (men and women aged 18-89) in Greece, with a weekly intake of 11-14 servings compared to less than 8 servings29;
  • yoghurt could be protective against inflammation, while cheese could be pro-inflammatory.1,30

The effects of low fat dairy are often reviewed, making the findings limited, as another study suggests that it is the fats in dairy which offers anti-inflammatory and protective effects in pathology, albeit with an unknown mechanism, perhaps due to specific fatty acids or the food matrix itself,while saturated fat from meat was found to increase the frequency of cardiovascular disease1,13,31,32. Another review indicates however that low fat dairy may actually be protective for cardiovascular risks33. It is possible that milk may lower the risk for hypertension by inhibiting angiotensin-converting enzyme (ACE)11,34.

There was no clear correlation between CRP and total caloric intake, including total fats and saturated fats for a group of both men and women. For women however, higher total fat was observed with lower CRP levels35. However, IL6 increases with fats and carbohydrate meals13,36,37. Another study on 38 young men showed that a diet high in conjugated linoleic acid from butter showed higher lipid peroxidation but did not change the markers for inflammation, cardiovascular disease or diabetic risk markers38

A systematic review of 52 clinical trials used an Inflammatory Score for anti-inflammatory activity in humans, showing a stronger anti-inflammatory activity for those with metabolic disorders but a pro-inflammatory effect in people allergic to cow’s milk. Both low and high fat dairy, and fermented diary had an anti-inflammatory activity. The review acknowledged a lack of agreement on clinically relevant inflammatory markers, as over 50 markers were used39

The correction between milk and mucus production is one that has been reported to be ‘not statistically significant, and it appears to be more likely to be reported in those that hold the belief40,41. In the cases of cow’s milk allergy, however, there are cases of links with asthma symptoms41. In a study on rats, however, bovine alpha-lactalbumin was found to support the gastric mucosa by increasing mucin production42

Fermented dairy

Probiotic fermented milk, kefir, has shown promising effects in reducing HbA1c in a study of diabetic patients aged between 35 to 65 years, but did not significantly affect serum triglyceride, total cholesterol, LDL or HDL cholesterol43. Yoghurt has been shown to increase HDL in hypocholesterolaemic women, reducing the radio of LDL/HDL ratio44. Fermented dairy products are linked with lowered LDL cholesterol, reduced hypertension risk and cardiovascular disease risk1,33,45,46. A meta-analysis showed long term-cheese consumption did not increase the risk of all-cause mortality, although there is some association with increased risk of prostate cancer and Parkinson’s disease48,49,50. Fermented dairy may offer cardioprotective benefits by the bacterial metabolites arriving to the digestive tract alive. The vitamin K2 content of fermented dairy, in conjunction with vitamin D may support the positive cardiovascular effects51.

During fermentation, bacteria and yeasts degrade lactose into breakdown products according to the species present, which influence taste and health benefits. Bacteria bind with toll-like receptors on dendritic cells and other leukocytes in the small intestine and colon mucosa, upregulating the Th1 regulatory system. An imbalance in Th2 is related to atopic disease while Th1 imbalance related to Crohn’s and gastritis.11,52

Lactose intolerance

Variations in the gut microbiome can influence tolerance to lactose, especially to yoghurt, as it contains less lactose and may contain bacterial lactase11,53. Some lactose intolerant people may still be able to tolerate 12-14g of lactose at any one time (around 240ml milk), or have dairy containing less lactose, such as hard cheese and yoghurt54

Children

For infants up to 3 years old, a review showed a higher risk of iron-deficiency anaemia for those having cow’s milk rather than formula, no difference in risk for developing type 1 diabetes mellitus, and no other detrimental health consequences55.Cow’s milk is low in iron, with levels further lowered in 40% of infants by intestinal blood loss of iron, although this reduces with age, stopping after age 1. Calcium and casein also lower the absorption of non-heme iron. The amount of protein and minerals in cow’s milk is higher than infant requirements, which is then excreted in the urine, potentially putting infants at risk of dehydration if their fluid intake is low.56Allergy prevention by exclusive breastfeeding up to 6 months is preferred, or a reduced allergenic formula, with the major allergens of cow’s milk in casein and whey protein17.

During mid-pregnancy, higher total protein intake or animal protein from meat or dairy was linked to a higher risk of gestational diabetes mellitus, while vegetable protein either pre-or during pregnancy did not influence the risk for a group of Chinese women.57

Osteoporosis 

A 12 year study showed women with increased calcium from dairy had a higher risk of hip fracture, with a subsequent study showing vitamin D is linked to lower hip fractures in post-menopausal women 58,59. A cohort study of 20 years found a link between high milk intake and mortality for both men and women, again with a higher incidence of fracture in women60. A study on 96,000 post-menopausal women and men over age 50 showed that drinking more milk during adolescence did not lower the risk of hip fracture in adult women; whereas for men, there was an increased risk of hip fracture by 9% with each extra glass of milk consumed during adolescence, but this may have been because of increased height61

Finding a comparative dairy-free group which had adequate calcium and vitamin D levels was noted to be difficult, alongside the difficulty of managing dietary studies58.Restricting dairy did not lower bone mineral density or bone resorptive markers in children in Hong Kong, but they did have a lower intake of calcium, vitamin D and iron62. A study on a Korean population however showed that increased calcium lowered the risk of osteoporosis, and that one serving a day of milk or dairy can reduce risk63. The phosphorous in dairy may also support bone health13,15,16

The hypothesis of an ‘acidic’ diet being associated with osteoporosis was not supported by a systematic review, with no proof of a causal link between changes in urine calcium and bone resorption markers and the acid-ash theory.  It noted that fruit and vegetables are supportive of bone health but perhaps through another mechanism64

Cancer

Fermented dairy, in particular yoghurt, was associated with decreased cancer risk. Bladder, colorectal and oesophageal cancers were also reduced by fermented dairy65.  Other reviews show milk and dairy have a negative correlation with cancers including colorectal, bladder, gastric and breast and no link with pancreatic, ovarian or lung, with varying results with prostate cancer68. Higher intakes of dairy, i.e. more than 2.5 daily servings, may be linked to a higher risk of prostate cancer, while another study found a lowered risk of lung, breast and ovarian cancers in lactose intolerant individuals49,66,67

Acne, breast cancer and prostate cancer are associated with dairy intake, which may be due to an increase of Insulin-like Growth Factor-1, either from the milk directly or made from the steroid hormones in milk, or both. 5alpha-pregnanedione (5α-P)in milk, a precursor to dihydrotestosterone (DHT), may be a route for prostate cancer, but can also stimulate oestrogen receptors in breast cancer cells69,70.

However, while alcohol and processed meat are implicated as causal factors in cancer, identifying one protective agent is difficult, other than fibre for colorectal cancer. Instead a more system-wide approach is useful for cancer protection, whereby there is an environment for healthy cell division, by nutritionally adopting mostly plant-based foods with modest meat, dairy, fish, low in alcohol and salt preserved foods with an active lifestyle.71 

Other types of milk and processing

Goat’s milk is often considered to be a preferable milk to cow’s but it may not have better nutritional value or less allergenic potential72. Goat’s milk is high in taurine and may be involved in managing the immune response before any damage to tissues and the leukocytes themselves11,73. Ovine milk has higher levels of protein, fats, minerals and vitamins than cow’s or goat’s milk, being high in oleic acid helping to reduce LDL cholesterol. Ovine milk may also contain ACE inhibitor peptides1

Homogenisation, by breaking up the fat globules into smaller lipids, may have consequences for health. Pasteurisation, in heating the proteins causing them to react with carbohydrates, may create products detrimental to health, as well as reducing some vitamins and antioxidants. Storage may cause the reduction of glutathione. Using some non-thermal processing may be preferable11

Organic dairy products were found to have higher protein, omega-3 fatty acid, and a higher omega-3 to omega-6 ratio (0.42 to 0.23). This may be due the requirements of feeding for organic status74.

Conclusion

Dairy products can contribute to meeting nutrient requirements, and fermented foods can help with reducing inflammation, in the context of appropriate consumption in moderate amounts11,68,75,76.  There are inconsistent findings on the association of dairy with degenerative disease; a reductionist approach in identifying one nutrient group with a health effect overlooks a wider or top-down approach at the person’s overall food intake and lifestyle31,77,78.

References are available here.