Dairy: Separating the Curds from the Whey

Dairy products always seem to assume a position of prominence in food group charts and in dietary recommendations issued by certain groups. But are they really the nutritional powerhouses some claim them to be, the key to prevention of chronic disease, or has politics taken over?

Dairy products are frequently touted as a wonderful source of protein, but that fails to impress those who realize that all whole foods contain protein and as long as one consumes an adequate amount of whole foods to maintain one's weight, one should easily obtain enough protein. Dairy products are also a good source of vitamin B-2, also known as riboflavin, but, again, riboflavin is widely distributed among foods and should not be a problem for anyone. Finally, dairy products are a rich source of well-absorbed calcium. Dairy proponents claim that the large majority of calcium in the average American diet comes from dairy products. That is also true, but it does not mean that dairy products are a more important source of calcium than other calcium-rich foods. It is simply a reflection of how poor the average American diet is, devoid of the much more healthful sources of calcium such as the dark, leafy green vegetables.

Kale vs. Cow's Milk: You Choose

Let us compare milk with kale, for example. Both are rich in well-absorbed calcium. Milk, except for the non-fat version (which actually derives 5% of its calories from fat), contains much cholesterol and saturated fat, Kale contains no cholesterol and is low in fat, especially saturated fat. Kale is a better source of protein than most people realize: about one third of its calories come from protein. Kale is a source of iron; dairy products contain negligible amounts of iron, which means that replacing other foods with dairy products could promote iron deficiency in people who have low iron stores. Kale is a good low-fat source of vitamin E; cow's milk, as opposed to human milk, is low in vitamin E. Kale, like milk, is a good source of the B-vitamins; it only lacks vitamin B-12. Kale, like other dark, leafy greens, is a rich source of vitamin K, which is involved in blood clotting and probably in bone health as well. Kale is a good source of vitamin C; milk has almost none. Kale, although low in fat, contains omega-6 and omega-3 essential fatty acids; milk contains hardly any. Kale is a rich source of the mineral magnesium, which is part of the chlorophyll molecule. Milk contains much less. Magnesium, by the way, is a major component of bone and may be at least as important as calcium in preventing osteoporosis. Kale is a cruciferous vegetable, containing compounds that lead to the formation of phase II enzymes by the liver, which we think help prevent cancer and other chronic diseases as well. Kale, in fact, contains numerous phytochemicals (plant-derived compounds) that promote health - preventing cancer through a variety of mechanisms, reducing inflammation in the body, and acting as antioxidants. Kale not only contains beta-carotene, both the cis and the trans forms, but is also the richest dietary source of lutein, a carotenoid that we believe helps prevent age-related macular degeneration, cataract, and possibly other diseases as well. If you had the choice of dietary sources of calcium, which would you pick?

Cows vs. Humans

The differences between cow's milk, which is intended for cows, and human milk were highlighted by the recommendations issued by the American Academy of Pediatrics in 1992.¹ Its Committee on Nutrition found that "infants fed whole cow's milk have low intakes of iron, linoleic acid, and vitamin E, and excessive intake of sodium, potassium, and protein." Because of these facts and the association between cow's milk ingestion in young infants and gastrointestinal blood loss, the Committee recommended that infants should not receive cow's milk during the first year of life.

Does Dairy Lower Blood Pressure?

The benefits attributed to milk consumption by its proponents have not been limited to bone health, which will be discussed later. Claims have been made that milk consumption reduces blood pressure. While it is true that higher intakes of calcium, because of its interactions with sodium, are associated with lower blood pressures, a review of the various studies showed that this association disappeared in people who kept their sodium (salt) intake at the low level where it should be. The Dietary Approaches to Stop Hypertension (DASH) study, which showed a lowering of blood pressures in people who consumed a diet rich in fruits, vegetables, and low-fat dairy products, has been cited incorrectly by some as evidence that dairy products lower blood pressure. The authors of that study made it clear that "the trial was not designed, however, to identify the effective and ineffective components of the diet."² In fact, studies have shown that vegetarian diets by themselves, or simply increasing the consumption of fruits and vegetables,³ can have a significant blood pressure-lowering effect. In the EPIC-Oxford Study, a comparison of meat eaters, fish eaters, vegetarians, and vegans showed that vegans had the lowest blood pressures.⁴

Calcium and Colon Cancer

Then there was the claim that dairy product consumption could reduce the risk of pre-malignant polyps as well as cancer in the colon (large intestine). Indeed, low calcium intake does seem to be a risk factor, but it was very nicely shown that, so long as calcium intake was at least 700 mg a day (below the RDA), increasing the calcium intake above that level appeared to have minimal, if any, effect.⁵ There have been some suggestions that vitamin D may also have a protective effect with regard to colon cancer. But vitamin D is not really a vitamin at all; it is a steroid hormone that is preferably obtained by moderate exposure of the skin to sunlight. Dairy products actually contain small and variable amounts of vitamin D. That is why they are fortified, and other foods can be and are fortified as well.

The Latest Weight Loss Scam?

Most recently, claims have been made that dairy products have some intrinsic ability to promote weight loss. These tenuous claims were based on a few observational studies that could not demonstrate any cause and effect relationship. Obviously, many such studies receive support from the dairy industry, a fact that colors the results. More recent, well-performed studies refuted these claims. In one clinical trial, increased consumption of dairy products in healthy, young women over a one year period had no effect on weight or fat mass.⁶ The Health Professionals Follow-up Study, a large prospective study, found no relation between dairy product consumption and weight gain in men followed for 12 years.⁷ Another large prospective study actually found that children who drank the most milk (including skim and 1% milk) gained the most weight.⁸

Bone-Building Boondoggle

The medical literature regarding calcium intake and osteoporosis is huge and it is impossible to adequately cover the subject in this space. Suffice it to say that osteoporosis is a multifactorial disease. Calcium is only one small piece of the puzzle. Weight-bearing exercise is one of the most important determinants of bone health. Among the nutrients, calcium, magnesium, zinc, vitamin K, and, of course, "vitamin D" all appear to play a role. The inordinate attention given calcium is related to the strength of the dairy and supplement industries, not the scientific evidence. Calcium clearly plays a role, as shown by clinical trials in which large dosages of supplemental calcium in elderly women have been shown to promote bone mineralization and reduce the risk of osteoporotic bone fractures. But does a high dietary intake of calcium, as might be achieved by increasing dairy product consumption, reduce the risk of osteoporosis? The answer, which may be surprising to some, is that there is no good evidence that a high dietary intake of calcium, as opposed to a moderate intake, makes any difference (there is some evidence, however, that a very low intake does increase the risk).^9,10 That does not mean that it is not possible that under some circumstances it could make a difference. For example, we know that high salt intake causes a significant loss of calcium in the urine. It is conceivable that people who limit their sodium intake to 2 grams a day, which few do, might indeed benefit from higher calcium intakes. Further studies should shed light on this matter.

On the other side of the coin, you may occasionally hear zealots claim that milk consumption increases the risk of osteoporosis. This is also false. While some observational studies do indeed show an association between higher intakes of milk or calcium and osteoporotic fractures, the explanation lies in the fact that milk consumption is associated with a more affluent lifestyle, marked also by decreased exercise and other lifestyle factors that increase the risk of osteoporosis.

The Cancer Connection

Aside from the nutritional and medical aspects we have already discussed, are there any reasons why consumption of dairy products may be harmful? There are many. Perhaps the most important is the association between dairy product consumption and some of our major cancer killers. Some but not all studies have shown a link between dairy product consumption and lymphoma, ovarian cancer, and prostate cancer. The explanations are hypothetical, but some researchers attribute the association with prostate cancer to higher calcium intakes. The sugar galactose, which is formed when the body breaks down lactose, or milk sugar, has been studied as a possible culprit with regard to ovarian cancer, but the results are inconclusive. Some have hypothesized that leukemia viruses in cows and in cow's milk might explain the association with non-Hodgkin's lymphoma, a cancer of the lymphatic system.

Conclusions

Given the lack of tangible benefits and the possible risks associated with dairy product consumption, it should not be part of an optimal diet. Care should simply be taken to consume at least five servings of calcium-rich foods a day. Such foods might include dark, leafy green vegetables such as kale, broccoli, mustard greens, turnip greens, and bok choy (but not spinach, from which calcium absorption is poor); almonds; figs, tofu processed with calcium sulfate; calcium-fortified soy milk, soy yogurt, or rice milk); blackstrap molasses; calcium-fortified orange juice; and the alternative grains amaranth, teff, and quinoa. For the nutritional reasons mentioned earlier, the emphasis should be on the dark, leafy greens.

¹Committee on Nutrition, American Academy of Pediatrics. The use of whole cow's milk in infancy. Pediatrics 1992;89:1105-1109.

²Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. N Engl J Med 1997;336:1117-1124.

³John JH, Ziebland S, Yudkin P, et al. Effects of fruit and vegetable consumption on plasma antioxidant concentrations and blood pressure: a randomized controlled trial. Lancet 359;1969-1974.

⁴Appleby PN, Davey GK, Key TJ. Hypertension and blood pressure among meat eaters, fish eaters, vegetarians and vegans in EPIC-Oxford. Pub Health Nutr 2002;5:645-654.

⁵Wu K, Willett WC, Fuchs CS, Colditz GA, Giovannucci EL. Calcium intake and risk of colon cancer in women and men. J Natl Cancer Inst 2002;94:437-446.

⁶Gunther CW, Legowski PA, Lyle RM, et al. Dairy products do not lead to alterations in body weight or fat mass in young women in a 1-y intervention. Am J Clin Nutr 2005;81:751-756.

⁷Rajpathak SN, Rimm EB, Rosner B, Willett WC, Hu FB. Calcium and dairy intakes in relation to long-term weight gain in US men. Am J Clin Nutr 2006;83:559-566.

⁸Berkey CS, Rockett HRH, Willett WC, Colditz GA. Milk, dairy fat, dietary calcium, and weight gain. Arch Pediatr Adolesc Med 2005;159:543-550.