Viewpoint

Raising Calcium Awareness

A response to J.A. Grant-Mackie of Auckland [ A Challenge to Some Behaviour! Retorts, October].

Dr Caroline C. Horwath

Over the past decade, persuasive new studies have been published which point strongly to the importance of calcium nutrition in the maintenance of bone health and the prevention of osteoporotic fractures.

For example, in a 14-year longitudinal study, Holbrook and others found that hip fracture rates were lower in individuals with higher calcium intakes. In what is probably the largest and best-controlled study examining fracture outcomes, Chapuy et al demonstrated that a calcium (1200mg/day) and vitamin D supplement reduced fracture risk by more than 30% over an 18-month period.

What is most fascinating about this study is that subjects were 3,270 women aged 69 to 106 years (mean age 84 years!) living in French nursing homes. It would thus appear that it is probably never too late to prevent osteoporotic fractures!

Many New Zealand women do not meet the current recommended calcium intakes (800mg/day for women aged 19-54 years, and 1000mg/day for women over 54 years). They are even further from meeting more recent consensus recommendations which take into account the latest studies on the calcium intake-bone health link. For example, the 1995 US National Institutes of Health Consensus Development Conference on Optimal Calcium Intake recommended:

• 1000 mg per day for women aged 25-50 years and postmenopausal women on hormone replacement therapy
• 1500 mg per day for women over 50 years not on hormone replacement therapy and all women over 65 years
• 1200-1500 mg per day for pregnant or lactating women.

Similar levels were advised by the 1993 Consensus Conference of the Scientific Advisory Board of the Osteoporosis Society of Canada. Also consistent with these higher recommendations is an Australian report which suggests that "in the light of these recent trials it would be reasonable to consider increasing the Australian recommended dietary intake for postmenopausal women to 1500mg calcium per day".

Why do so many women fail to meet the recommendations for dietary calcium intake?

This is chiefly because many New Zealand women fall well short of meeting the Dietary Guidelines for milk product consumption. It is clearly established from our National Nutrition Surveys (Life in New Zealand Hillary Commission Report) that New Zealand women derive the highest proportion of their calcium intake from milk and milk products.

While it is true that we can obtain calcium from foods other than milk and milk products (the bones of fish, large servings of broccoli, soybeans), it is very difficult to meet the Recommended Dietary Intakes for calcium if one avoids milk and milk products (these figures are the result of consensus opinion of nutrition experts, rather than the work of Grant-Mackie's "Dairy Board economists and marketeers").

A diet lacking this food group is highly likely to be lacking sufficient calcium. Plant sources generally contain much lower amounts of calcium than dairy sources, and furthermore the calcium is less well absorbed. Vegetarians who avoid milk products are thus a group potentially at high risk of poor calcium intakes, and need to take care to obtain sufficient calcium.

First-year nutrition students are familiar with what happens when calcium needs are not met: the body begins to draw on calcium from its storage site -- our bones. Our bones are the only known instance of a nutrient reserve which has an important function in its own right (ie. the mechanical functions of our skeleton).

The angry anti-industry tirade of J.A. Grant-Mackie is a good example of what can result from ignorance of our basic dietary guidelines and calcium intake recommendations. The research which Grant-Mackie is so "utterly appalled" by takes as its basis our New Zealand dietary guidelines for milk product consumption. The guidelines published by the Ministry of Health recommend that women consume at least two servings of milk or milk products each day.

Such recommendations concerning milk product consumption are not unique to New Zealand, and in fact are common to the dietary guidelines of other Western countries. In dismissing any research based on this dietary guideline as "lacking in objectivity and scientific validity", Grant-Mackie is, without explanation, dismissing our dietary guidelines.

In our research, women's intake was assessed in relation to the NZ dietary guidelines, and also in relation to a higher goal of four servings of milk products per day as result of the higher calcium levels recommended by recent consensus panels. The dietary guidelines have not, as Grant-Mackie would have it, been developed by "Dairy Board economists and marketeers", but rather by a consensus of nutritionists around the world.

Again, as any first-year nutrition student is well aware, the omission of any food group from the diet increases the risk of nutritional inadequacy. Thus our dietary guidelines include the recommendation to consume a variety of foods each day from each of the food groups (milk products being one of these important groups, and an important source not just of dietary calcium, but also of magnesium, potassium and B vitamins).

The recommendation for women to consume at least two servings of milk or milk products each day is not, as Grant-Mackie claims, an unacceptable bias created by the influence of the NZ Dairy Board, but a key element of dietary guidelines. If we are to reach a situation where more women meet calcium intake recommendations, a key public health message must inevitably involve promotion of this dietary guideline.

Of course, there are valid reasons as to why a small proportion of women cannot consume milk products (e.g., lactose intolerance, allergy), and in these situations advice is needed on alternative sources of calcium. No-one would dispute that. However, it must also be acknowledged that the majority of women will not meet their calcium needs from broccoli and tofu consumption.

In fact, if Grant-Mackie cared to find out more about this research which he dismisses so angrily, he would know that one of the key goals of this research has been to better understand women's reasons for avoiding or cutting down on milk products. We have specifically included in our surveys many questions concerning barriers to following the dietary guideline on milk products. Specific questions on lactose tolerance and allergy were included in our surveys. This is because it is vitally important that these are acknowledged and addressed by any education program seeking to reduce osteoporosis risk.

Our surveys involve the collection of baseline data so that we may better understand the many influences on women's milk product consumption patterns. Such an understanding is vital to planning health promotion programs to address the important public health problem of osteoporosis. The Stage of Change model is a particularly attractive model for designing health promotion programs since it acknowledges and respects the attitudes and motivation levels of individual members of the audience. Our baseline study would enable us, in a subsequent nutrition program, to provide women with materials individually tailored to their particular needs.

For example, a woman avoiding or restricting milk products due to fear of weight gain, could, amongst other resources, receive material regarding how calcium needs may be met without unwanted weight gain, and other advice regarding body weight issues. A woman who is lactose intolerant would receive information focusing on alternative ways to meet calcium intake recommendations.

Grant-Mackie criticises our approach by saying: "This is not science, it is marketing!" However, the approach used in our research is exactly the approach used in the US National Cancer Institute-funded smoking cessation programs and dietary fat reduction programs. These multi-million dollar programs undertaken by the University of Rhode Island Cancer Prevention Research Centre are based on the "Transtheoretical Model or Stage of Change Model" approach. This approach, although dismissed outright by Grant-Mackie, is deemed credible and scientific by the US National Cancer Institute.

Grant-Mackie's comments on the suitability of milk products for children are irrelevant given that the population being studied in this research is adult women.

It is most telling that Grant-Mackie is so quick to dismiss research as biased, "unbalanced", "lacking in objectivity" etc. A reader who was unbiased, balanced, and capable of objectivity, would on reading an article which raised various concerns for him/her, have most likely contacted the author for an open discussion of the many interesting issues involved before launching into an uninformed and angry tirade. Had Grant-Mackie made any attempt to contact me, I would have been most happy to discuss these issues, but no such attempt was made. It is thus clear where the real bias lies.

Furthermore, Grant-Mackie reveals a total lack of understanding of the way in which the scientific process and scientists work when he accuses Otago University researchers of "allow[ing] ... sponsorship to dictate the research objectives". No reputable scientist would do this, and in fact, the dependence of our careers on the publication of our work in peer-reviewed scientific journals rules out this possibility. Fortunately, Grant-Mackie takes back these allegations of our science being corrupted by industry, by going on to contradict himself by adding: "I am not saying that this is what has gone on in this case, I am saying only that it looks like it, and that it is a good lesson to take note of! " This is illogical and lacking in objectivity.

Our research objectives, methods of investigation, and results are uninfluenced by our funding source. Grant-Mackie's writing is at best an excellent example of the sort of bias and lack of objectivity that he is so ready to make accusations of in others.

A fully referenced version is available upon request to the NZSM.

Dr Caroline Horwath is the Senior Lecturer in Human Nutrition at the University of Otago.