One of the controversial issues around good nutrition for healthy bones is the question of whether or not
too much protein causes bone loss, and the related question of whether dairy products are a good source of
calcium. These questions arise from a body of research suggesting that body acidity is an important
determinant of bone health. As dietary protein is a contributor to body acidity, and animal protein
sources do so to a greater extent than plant proteins, then this relates to recommending dairy as a source of
calcium.
Protein is an essential macronutrient that is necessary for building and repairing all body tissue. Bone
is made up primarily of protein, in the form of collagen (about 50% of bone tissue is protein) and calcium.
The protein foods (meat, fish, poultry, eggs, dairy, legumes and nuts) contain amino acids that are the
building blocks for repair and maintenance. As well, protein foods contain vitamins and minerals that are
essential for good health. Many studies have shown that a diet low in protein can adversely affect bone.
When protein intake is very high, this causes an excess of acid production in the body; thus, an excess
protein intake is considered “acid-forming.” Body chemistry is very sensitive to what is called pH, or
the acid/base relationship. The body’s chemistry cannot work well unless the pH balance is maintained within
very strict limits. What this means is that if what you eat is acidic or generates acidity, then your body
has to neutralize the blood to keep the pH balance where it needs to be. Calcium is used to neutralize the
extra acid that is formed in the body, and this calcium is coming out of your bones: when blood is too
acidic, calcium inside the bones is released into circulation to neutralize the excess acidity and to
maintain a healthy pH level. When calcium intake is not adequate, a high-acid diet can lead to bone loss over
time.
The basis of the assertion that “drinking milk can cause osteoporosis” is derived from the logic that if
animal proteins contribute to body acidity, then surely milk, which contains animal protein, must cause
calcium to be leached from the bones, as discussed above. Groups such as PETA (People for the Ethical
Treatment of Animals) claim that North Americans, with a diet high in dairy products, are at greater risk of
osteoporosis and osteoporotic fractures than people who live in countries where dairy products are not a
large part of the diet. What is not taken into account by this seemingly logical conclusion is that a typical
North American diet does not meet the calcium intake requirements recommended by Osteoporosis Canada, and
that many people are not consuming dairy products. Thus the occurrence of osteoporosis in countries such as
Canada is not caused by drinking milk, and there are no studies in which drinking milk has been linked to
bone loss.
This argument also ignores the fact that osteoporosis is a "multifactorial disease." Calcium and
vitamin D are by no means the only factors that contribute to protection against bone loss and
osteoporosis. Other factors include genetics, physical activity, agility, and other dietary factors that
may influence the absorption and retention of calcium in the body. The "acid load" of a diet may
influence how much calcium is retained in the body versus how much is lost in the urine. In general,
diets with more fruits and vegetables tend to have a lower "acid load." But if a diet is deficient in
calcium, having a diet with a low "acid load" can't make up for that – we cannot synthesize calcium, and have
to obtain it in our diet.
To return to the point that osteoporosis is a multifactorial disease, one of the biggest problems with
studies that compare osteoporosis rates in different countries is that frequently other variables (in
addition to calcium and vitamin D) are different. So, for example, a country with higher rates of
habitual physical activity, more vitamin D exposure through sunshine, and without a snowy/icy winter (with
the associated risk of falls that lead to fractures) could have a lower fracture rate than a country like
Canada with less physical activity, less sunshine, and a snowy/icy winter – even if calcium intakes are
higher here.
The question arises: how much protein is too much? There is no true consensus as to what constitutes an
optimal intake of protein. Studies show that women and seniors do not consume enough protein daily. In fact,
in the elderly, protein deficiency may be an important problem for bone health. Excessive amounts of protein
are not a concern for most people and will only cause bone loss if calcium intake is inadequate.
- Canada’s Food Guide recommends two to three servings per day of 2-3 oz. (57 grams – 85 grams) of
meat or alternatives.
- The DASH diet (Dietary Approaches to Stop Hypertension), although designed for hypertension, is also good
for bone health. It recommends about 75 grams of protein per day.
- Dairy products provide 8 grams of protein per serving.
Dairy products are also a good source of calcium that is readily bioavailable. An eight-ounce glass of
milk – whole, 2%, 1%, skim or chocolate - will provide about 300 mg of calcium. Milk is also fortified with
vitamin D, as are some yogurts. A glass of milk provides about 100 IU vitamin D, which can increase the
absorption of calcium 30% or more. Meta-analyses - in which the results of all available studies are
considered in their entirety - have demonstrated that calcium and vitamin D have a small but potentially
important effect on reducing fracture risk and the risk of falling in older adults. The benefits are
observed at intakes that are consistent with those recommended by Osteoporosis Canada – 1500 mg calcium and
at least 800 IU vitamin D daily from all sources.
There are many factors that contribute to developing and maintaining healthy bones:
- A balanced diet with adequate calcium and vitamin D
- Physical activity, particularly weight-bearing and weight-resistant activities
- A lifestyle that avoids practices that deplete calcium from the bones, such as:
-
- Smoking
- Excess sodium
- Excess caffeine
- Excess alcohol
- Lack of fruit and vegetables.
Paying attention to all of these factors is the best way to reduce the risk of osteoporosis and
osteoporotic fractures.
