Are dairy products effective sources of calcium?

Question

Tradition says that dairy products are good sources of calcium just because they have it; while some modern studies (see below) show that protein digestion produces acidic environment, and body uses calcium from the bones to re-establish the correct pH. These opinions say that dairy products could even be detrimental to bone density and osteoporosis.

So in the end calcium balance is positive or negative from the consumption of dairy products?

References

Protein metabolism and calcium loss

It was asked in the comments to provide some reference about the "modern studies" relating calcium loss to protein intake.

The specific amino acid profile—especially of amino acids containing sulfur—determines the calciuretic effect of protein. Sulfate generated from the metabolism of these amino acids increases the acidity of the urine, causing greater amounts of calcium to be excreted in the urine. The proteins of many plants, especially legumes, have lower amounts of methionine and cysteine than do animal proteins.

Weaver, C. M., Proulx, W. R., & Heaney, R. (1999). Choices for achieving adequate dietary calcium with a vegetarian diet. The American Journal of Clinical Nutrition, 70(3 Suppl), 543S–548S

To date, the accumulated data indicate that the adverse effect of protein, in particular animal (but not vegetable) protein, might outweigh the positive effect of calcium intake on calcium balance.

Amine et al. (2002). Diet, nutrition and the prevention of chronic diseases: report of a Joint WHO/FAO Expert Consultation.

Answer 1

Dairy products are not the best source of calcium for many reasons.

• Lactose Intolerance

  • Lactose intolerant people can't have dairy products as they will usually cause various symptoms like diarrhea or cramping

• High Saturated Fat Content

  • Most dairy products are high in saturated fat, which can be a risk factor for heart disease
  • Milk has been able to cut down on this with reduced fat and fat-free options, but other dairy products, like cheese and ice cream are still very high in fat

• Cancer Risk

  • Ovarian Cancer - Studies have shown that high intakes of lactose (equal to 2-3 servings of milk) can increase the risk of ovarian cancer¹

  • Prostate Cancer - Studies showed inconclusive evidence that some men with a higher calcium intake might have a higher risk of prostate cancer; Evidence was not conclusive; Could also be linked to just calcium rather than dairy products²

Though dairy products are a convenient source, and they are high in protein and vitamins A and D, they are not the best source for calcium. In small amounts, dairy is fine, but there are good alternative sources.

Other sources

• Leafy green vegetables - kale, spinach, lettuce
• Beans, legumes, and almonds
• Calcium supplements

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_{[1] Dairy products and ovarian cancer}

_{[2] Dairy products, calcium and prostate cancer risk}

_{Should You Get Calcium from Milk?}

Answer 2

The issue whether “excess” dietary protein intake adversely affects bone in humans is a subject of current controversy in nutrition^(1),(2),(3) with one group concluding that “excess protein will not harm the skeleton if the calcium intake is adequate”⁽³⁾ and group that “excessive dietary protein from foods with high potential renal acid load (e.g., animal foods) adversely affects bone, unless buffered by the consumption of alkali-rich foods (e.g. vegetable foods)”⁽¹⁾ (quoted by Frassetto 2000).

Other studies reporting better bone health in women with greater protein intakes^(4),(5) (high protein intake prevents BMD decrease). Further, the Munger study reported that higher intakes of animal sources of dietary protein were associated with a 70% reduction in hip fracture, even after controlling for major confounding variables (Hannan 2000).

Read more: Calcium, BMD and Osteoporosis

In overall it's difficult to say whether calcium intake has positive or negative effect from the consumption of dairy products on our bones, based on the sample size and subject characteristics of the studies, so more studies needs to be conducted.

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References:

1. Barzel US, Massey LK. Excess dietary protein can adversely affect bone. J Nutr. 1998;128:1051–1053.
2. Massey LK. Does excess dietary protein adversely affect bone? Symposium overview. J Nutr. 1998;128:1048–1050.
3. Heaney RP. Excess dietary protein may not adversely affect bone. J Nutr. 1998;128:1054–1057.
4. Freudenheim et al.(46, Freudenheim JL, Johnson NE, Smith EL 1986
5. 47, Munger Cerhan JR, Chiu BC 1999 Prospective study of dietary protein intake and risk of hip fracture in postmenopausal women.

Answer 3

First of all, I'm not sure whether the question is answerable at all without a more detailed scenario.

So we have:

• Ca²⁺ excretion increasing with excess protein
  This seems to be linked to elimination of excess acids rather than directly to protein according to http://jn.nutrition.org/content/141/3/391.long.
  https://www.ncbi.nlm.nih.gov/pubmed/15546911 finds the additional excretion coming from the increased intake under high protein diet.

• Ca²⁺ uptake/loss in bones (which is typically the main point of the Ca uptake/loss discussion, and bone is a large researvoir of Ca in the human body):
  bone (re)building needs:

  • Ca²⁺
  • protein
    (we're talking here high/excess protein diets, so let's assume this is not a limiting factor)
  • mechanical stimulation (excercise)

• if you are in a situation with (primarily) net loss of bone because of any of the 3 ingredients to bone growth above (e.g. lack of excercise even though Ca and protein are available), you'll observe a net Ca loss, because Ca released from the bone that (naturally) broken down is excreted.

• Ca²⁺ uptake from food depends on

  • bioavailablility, which depends on presence/absence of other substances.
  • e.g. oxalate will precipitate Ca as insoluble and bioinavailable Ca-oxalate (in 1:1 mol ratio).
  • there are studies indicating that high-protein food increases Ca uptake, see e.g. http://jn.nutrition.org/content/141/3/391.long

  • vitamin D status of the recipient: vitamin D is needed to transport Ca²⁺, and roughly, the higher the vitamin D level, the higher the Ca uptake
    Not only rachitis in children but also osteoporosis in old people is correlated with vitamin D deficiency.

  • http://ajcn.nutrition.org/content/75/4/609.full outlines a mechanism how urinary Ca loss may trigger increased Ca uptake.

Taking those points together, we may construct 2 extreme lifestyles that would react quite opposite to the linked intake of protein + calcium in milk:

• On the one hand, imagine a sedentary indoors couch potato. No sun (low vitamin D), no excercise (no bone growth stimulus). Without adequate vitamin D levels, the Ca in the milk (although in principle bioavailable) is not even absorbed in the gut. Even if it were (or: even the little that is) is not used for net new bone growth as the excercise stimulus is missing. With a high protein diet (or, a diet causing excess acid urinary excretion), Ca is lost. It may have been lost even without a high protein diet because of bone loss due to lack of excercise. Or, with a low excess acid diet, bone would have been maintained even with the low level of excercise.

• On the other hand, consider a physically active outdoors person. Excercise gives a bone growth stimulus, sunlight does its share in supplying vitamin D. In that scenario, milk gives the protein as well as the Ca to actually grow bone. Result is a net gain in Ca, even though some Ca is excreted via urine.

Slightly off topic: this whole scenario feels a bit like a 101 in how to lie with statistics without actually lying: we have at least 4-5 factors that need to be at the right setting (and for 4 of them it is comparatively easy to be off) in order to achieve bone growth/net Ca gain. Now any study looking at a single factor will not find an effect unless they make sure all other factors are right. And if one factor is worse with the treatment group than the controls, a study may even seem to observe the opposite of what is really going on.

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I tried to find numbers for Ca²⁺ excretion per gram of protein. A rough guesstimate based on http://jn.nutrition.org/content/128/6/1051.full would be up to 200 mg Ca²⁺ urinary excretion for 150 g of protein consumption. Based on that rough guesstimate, food with a ratio > 1.33 mg Ca/g of protein would be considered as "gaining Ca²⁺".

https://en.wikipedia.org/wiki/Milk#Nutrition_and_health gives cow milk with 30 - 35 g protein / l and 1200 mg Ca²⁺/l, yielding a Ca : protein ratio of > 30 mg Ca/g of protein. Even assuming just the average absorption rate for Ca in food of 30 % (http://ajcn.nutrition.org/content/35/4/783.extract - didn't find numbers for milk, though it is frequently cited as highly bioavailable - but then it will also depend on other factors) that leaves almost an order of magnitude to the side of more Ca uptake than increase in excretion.

So overall, this back-of-the-envelope calculation is in favor of milk being a net source of Ca. Note however, that not all dairy products have the same Ca:protein ratio as milk. E.g. Whey has even more Ca:protein, and consequently the cheese of which whey is a leftover/side product has a somewhat lower Ca:protein ratio.