Posted on 08/28/2012 at 08:29:40 AM by Student BloggerBy Stefano Vendrame
The way I see it, there's something fishy about the new MyPlate nutrition guide icon—and it has nothing to do with fish, or the idea of strawberries on the same plate as my fettuccine alfredo. Come to think of it, it's not even on the plate. Nope, there's something fishy on the top-right side and it is a blue circle labeled dairy. Surprisingly separate from the protein slice, in what appears to be a glass, I can only deduce that what the USDA really meant to write in the blue circle is ‘drink a glass of milk.'
One of the most popular beverages in the United States, milk is hailed as an essential part of a nutritious diet from early childhood. After years of consuming the beverage that “does a body good,” should we accept that milk is good? In recent years, evidence is leading some health professionals to reconsider whether adults should be drinking milk at all. So for this blog entry, I thought I'd summarize the good and the bad of milk, what recent studies have shown.
The Good. Okay, we all know it. Milk is a nutrient-rich beverage packed with all the macronutrients, high quality proteins, and a good pool of vitamins and minerals, including vitamins B and A. It is a source of good-absorbable calcium which can help maximizing peak bone mass during adolescence and maintaining bone density during adulthood. On top of that, it has a fascinating chemistry that allows for technological wonders, such as fat globules separation to obtain cream, kappa-casein precipitation to form all kinds of cheeses, whey separation to make ricotta, and microbial fermentations to make yogurt, kefir and other probiotic drinks (1).
The Bad. Half of the calories in whole milk come from saturated fat, which could be a problem considering that excess saturated fat is associated with cardiovascular risk. A cup of whole milk has about 8 g total fat, of which 5 g is saturated fat. To make things worse, milk has significant amounts of middle-chain saturated fatty acids, such as lauric and miristic, which are amongst the most atherogenic (2). For this reason, many health professionals recommend selecting low-fat dairy options, such as skim milk, for regular consumption (1).
When milk is consumed in excess and protein intake exceeds protein need, it leads to increased calcium excretion, no matter the type of milk. Although controversial, the most plausible explanation of this fact is that excess proteins need to be catabolized since our body doesn't have a way to store proteins. But milk is rich in sulfur containing amino acids, whose catabolism generates organic acids that need to be quickly buffered to maintain pH stable in our bloodstream. When the kidney's physiological homeostatic ability is exceeded, calcium is stolen from our bones to buffer these acids and subsequently excreted via the urines (3). This leads to the paradox that consumption of a food universally thought as a good source of calcium may end up flushing out more calcium than it actually brings in.
Indeed, a famous outcome of the Harvard Nurses' Health study, a 12-year prospective study on 77,761 adult women, was that higher milk consumption does not reduce risk of bone fractures. Of note, the relative risk of fractures for women drinking two or more glasses of milk per day was actually higher than the risk for women drinking milk only once a week or less (4). It has also been noted that in most parts of Asia and Africa, where people don't drink milk as adults at all, the rate of hip fractures is dramatically lower than in western countries (3). All of which suggests that drinking liters of milk and eating tons of cheese, besides increasing cardiovascular risk, could be a disadvantage to our bones.
The Questions. What about a potential link between milk consumption, insulin signaling and inflammation? The culprit appears to be casein, the most abundant protein in milk, whose signaling may lead to alterations in the hormonal axis insulin-growth hormone, with overproduction of insulin-like growth factor 1 (IGF1) and overstimulation of pancreatic beta-cell proliferation, senescence and apoptosis (5). Elevated IGF-1 have been associated with high levels of subclinical inflammation, increased cancer rates, risk for type II diabetes mellitus, cardiovascular disease, infertility and neurodegenerative disease (6).
In recent years, the results of the Boyd Orr cohort study further sparked the milk debate. The study was started in 1937 and followed 5,000 English men from age 5 to 75. It was found that subjects with higher milk and dairy intake from early in life had a 3-fold higher colon cancer incidence, after controlling for intake of red meat and fruit and vegetables (7).
It remains too soon, in my opinion, to draw any conclusion on whether we should drink milk as adults or not. Did Hippocrates get it right in 377 B.C. when he stated: Milk of one's own species is beneficial, others' harmful? That remains to be seen. Like scientists always say: more research is needed. I am certainly going to keep enjoying my morning lattes until more conclusive evidence is available. But just in case, when the barista at the counter asks me ‘whole, skim or soy' every once in a while I'll go for soy.
2) Månsson HL. Fatty acids in bovine milk fat. Food Nutr Res. 2008;52:10-34.
3) Abelow BJ, Holford TR, Insogna KL. Cross-Cultural Association Between Dietary Animal Protein and Hip Fracture: A Hypothesis. Calcif Tissue Int. 1992;50:14-18.
4) Feskanich D, Willett WC, Stampfer MJ, Colditz GA. Milk, dietary calcium, and bone fractures in women: a 12-year prospective study. Am J Public Health. 1997;87(6):992-7.
5) Melnik B. Milk consumption: aggravating factor of acne and promoter of chronic diseases of Western societies. J Dtsch Dermatol Ges. 2009;7(4):364-70.
6) Melnik BC. Milk signalling in the pathogenesis of type 2 diabetes. Med Hypotheses. 2011;76(4):553-9.
7) Van der Pols JC, Bain C, Gunnell D, Smith GD, Frobisher C, Martin RM. Childhood dairy intake and adult cancer risk: 65-y follow-up of the Boyd Orr cohort. Am J Clin Nutr. 2007;86(6):1722-9.