Wednesday, April 02, 2008

The More Protein You Eat, The More Calcium You Excrete

Below is the relationship between dietary protein intake and urinary calcium excretion from 26 studies.1

Doubling protein intake increases urinary calcium by about 50%.2

Why is that?

When we eat protein, our body breaks it down, or catabolizes it, in the process generating acids, e.g. sulfates (from sulfur-containing amino acids). Our bodies attempt to neutralize these acids by using buffering agents. One place it finds buffering agents is in bone:
"Buffers in the bone matrix neutralize the excess diet-derived acid, and in the process, bone becomes demineralized. Excess diet-derived acid titrates bone and leads to increased urinary calcium and reduced urinary citrate excretion. The resultant adverse clinical consequences are possibly increased bone demineralization and increased risk of calcium-containing kidney stones."3
One recent study that found that a diet high in protein (90g/d) "is compensated for by increased bone resorption".4 (Resorption means the loss of a substance, in this case bone.)

How does that 90g/d compare to recommended amounts?

The Dietary Reference Intake for protein is 0.8 grams per kilogram of body weight for adults, such that:
  • The recommended protein intake for a 120 pound adult is 44 grams/day.
  • The recommended protein intake for a 150 pound adult is 54 grams/day.
  • The recommended protein intake for a 180 pound adult is 65 grams/day.
Protein amounts in some foods:
  • McDonald's Double Cheeseburger - 26 grams
  • 3 oz. canned tuna - 22 grams
  • 2 oz. cheddar cheese - 14 grams
  • 1/2 cup cooked lentils - 9 grams
  • 1 cup broccoli - 6 grams
  • 1/2 cup green peas - 4 grams
Note above that some proteins are more acid-generating than others, for example, those containing amino acids with sulfur (methionine and cysteine - found primarily in foods of animal origin). These proteins can be especially detrimental to bone health.5

Below is the average acid-generating capacity, or the Potential Renal Acid Load (PRAL) of some food groups.6 Positive numbers indicate the food is acid-generating, negative numbers indicate the food is alkaline-generating (beneficial in this regard).

PRALs of food groups:

Milk and dairy products ... 1.0 to 23.6
Meat ... 9.5
Fish ... 7.9
Grain products ... 3.5 to 7.0
Fats ... 0
Vegetables ... -2.8
Fruits ... -3.1

PRALs of a few common foods:

2 ounces cheddar cheese ... 15
3 ounces salmon ... 8.0
3 ounces chicken, meat only ... 7.3
3 ounces beef, lean only ... 7.0
1 egg ... 4.0
1/2 cup white rice ... 3.6
2/3 cup corn flakes ... 1.0
1 slice white bread ... 0.8
1/2 cup peas ... 0.8
1 medium carrot ... -3.0
1 medium apple ... -4.0
1 medium banana ... -6.5

Since PRAL is defined as the mEq of (Cl + PO4 + SO4 – Na – K – Ca – Mg), foods high in, say, potassium (K: fruits and vegetables) tend to have a low PRAL or are alkaline-generating (beneficial). Foods that result in high levels of sulfates and phosphates (meat and cheese) tend to have a high PRAL or are acid-generating.

Interesting that cheese which is high in calcium has a very high PRAL. Perhaps its protein and other compounds outweigh calcium's effect. Speaking of which, guess who said this (I gave you a hint):
"Excess dietary protein, particularly purified proteins, increases urinary calcium excretion. This calcium loss could potentially cause negative calcium balance, leading to bone loss and osteoporosis. These effects have been attributed to an increased endogenous acid load created by the metabolism of protein, which requires neutralization by alkaline salts of calcium from bone."7
So they recognize that the acid-generating protein in their product detracts from its calcium content - and that the food which is advertised to "Build Strong Bones", could in fact lead to "bone loss and osteoporosis".

Why do I say all this? Because I think there are better ways to preserve calcium in bone than to take supplements ... or to eat dairy foods.

Also, given my recent posts on protein intake and cancer (e.g. Now You See It, Now You Don't: Liver Cancer) it would seem prudent to monitor protein intake.
1 Low Protein Intake: The Impact on Calcium and Bone Homeostasis in Humans, The Journal of Nutrition, 2003.
2 Dietary Animal and Plant Protein and Human Bone Health: A Whole Foods Approach, The Journal of Nutrition, 2003.
3 Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, Institute of Medicine, Food and Nutrition Board, 2004.
4 The Effect of a High-Protein, High-Sodium Diet on Calcium and Bone Metabolism in Postmenopausal Women and its Interaction with Vitamin D Receptor Genotype, British Journal of Nutrition, 2004.
5 A Positive Association of Lumbar Spine Bone Mineral Density with Dietary Protein Is Suppressed by a Negative Association with Protein Sulfur, The Journal of Nutrition, 2008.
6 Potential Renal Acid Load of Foods and its Influence on Urine pH, Journal of the American Dietetic Association, 1995.
7 The National Dairy Council.

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