Articles

Calcium Intake And Metabolism

B. Pretorius & H.C. Schönfeldt
ARC-ANPI, Private Bag X2, Irene, South Africa

1. Introduction

   Milk and milk products are valuable and usually well-tolerated sources of natural calcium that can help to prevent osteoporosis. It is disturbing that dogmatic arguments are used to blame a nutrient that is important for a healthy diet. Calcium intake does not mean the mere consumption of foods, beverages or supplements that contains calcium. The calcium that is in each particular foodstuff must be digested, absorbed and utilised while maintaining overall mineral balance.

2. Calcium sources

   Good sources of calcium include milk and other dairy products, kale, kelp, tofu, canned fish with bones, peanuts, walnuts, sunflower seeds, broccoli, cauliflower and soybeans. Fortified foods such as fruit juices, breads and cereals are also common sources of calcium. Calcium in hard water and some mineral waters may be important dietary sources for some people (Bookman Press 1998; 2000). Calcium supplementation is another alternative.

   Table 1: Calcium content of some food sources.

   Food	Calcium content (mg/100 ml or 100g)
   Almonds (dried, blanched)	247
   Bread (wholewheat)	57
   Broccoli (cooked)	46
   Cauliflower (cooked)	27
   Cereal (All Bran Flakes)	50
   Cereal (Pronutro, wholewheat)	460
   Cheddar Cheese	788
   Fish (Anchovy, canned in oil, solids only)	232
   Gouda Cheese	806
   Lentils (whole, cooked)	27
   Milk (full cream)	120*
   Milk (low fat)	122*
   Peanuts (roasted, unsalted)	88
   Potatoes (baked in jacket, flesh only)	5
   Rhubarb (cooked without sugar)	145
   Sesame seeds (dried, hulled)	131
   Soybeans (dried, cooked)	102
   Spinach (cooked)	136
   Sunflower seed (dried)	116
   Sweet potatoes (cooked without skin)	21
   Tofu (fried)	372
   Walnut (dried)	94
   White beans (cooked)	17
   Wholemeal wheat flour	34
   Yoghurt (fruit)	145*
   Yoghurt (plain)	150*

   (Langenhoven, et al.; 1991) *(ARC; 1998a) #(ARC; 1998b)

3. Calcium Absorption

   Calcium from foodstuffs may have acute effects on calcium and bone metabolism. These metabolic effects are important when different calcium dietary sources are discussed and recommended. The absorbability of calcium affects calcium utilisation (Kärkkäinen, et al.; 1997). Organic acids such as oxalate (found in green leafy vegetables) and phytate (found in grain products) (Kärkkäinen, et al.; 1997) forms insoluble complexes with calcium and may decrease calcium absorption, whereas lactic acid (Rasic; 1987) may enhance it. Calcium found in vegetables or other non-fluid foods cannot go into solution until at least some part of the vegetable (or food) has been digested. Calcium must be dissociated from its ligands in a foodstuff prior to absorption (Weaver and Liebman; 2002). Therefore foods with high fibre content are likely to be poorer sources of calcium than foods that contain less or no fibre but an equivalent amount of calcium (Bronner and Pansu; 1999).

   1. Calcium Bioavailability

      From a nutrition standpoint, both calcium content and bioavailability are important. Bioavailability is simply the proportion of an ingested nutrient that is available for metabolic processes and storage (Van Lieshout and West; 2004). Bioavailability is a major issue with dietary calcium, as it can vary significantly in different dietary sources. Intestinal calcium absorption from milk has been estimated to range between 32% and 35% (International Osteoporosis Foundation; 2004), but from spinach (rich in oxalates) it was a mere 5% (Birbeck; 2001). The results of a study done on four calcium-rich foodstuffs (i.e. sesame seeds, spinach, fermented cheese and milk) show that calcium from sesame seeds and spinach does not cause an acute effect on calcium metabolism, whereas milk and fermented cheese are better dietary sources of calcium (Kärkkäinen, et al.; 1997). Another study showed calcium from cow's milk was more efficiently absorbed by adult men than calcium from calcium-fortified soy beverages (Birbeck; 2001). In practical terms this suggests that although the label indicate the two beverages have the same calcium per serving, in fact the bioavailability, or the amount of calcium the body absorbs and uses from the fortified soy beverage, will be significantly less. Figure 1 shows the amount of calcium actually delivered from various foods into the blood. Its values represent the product of food calcium content and food calcium bioavailability (Heaney, R.P.; 2000).

      Figure 1 : The amount of calcium delivered from different sources of food in the blood.

   2. Calcium to Phosphorus ratio

      In human milk the ratio of calcium to phosphorus is as much as 2:1 or slightly higher. In cow's milk the ratio is more like 1:0.8. It has been suggested that the extra phosphate somehow "leaches calcium out of the bones" into the urine, but this has not been proven. Phosphorus does inhibit calcium absorption to a small degree, but it also reduces calcium excretion (Birkbeck; 2001). Adequate intakes of calcium and phosphorus are required for normal bone mineralisation. There are indications that excess phosphorus in the diet (eg. high consumption of soda drinks) may adversely affect bone health, but it must be remembered that this is in the context of the overall diet, and that merely comparing ratios in individual foods cannot predict the effects in the body when other foods are also consumed. To date there is no clinical evidence to suggest that the ratio of calcium to phosphorus in milk has any adverse effect on bone health (Birkbeck; 2001).

4. Calcium and Health

   Persons who have built up large stores of calcium during childhood and young adulthood will have no problems upon aging, but those who have never attained peak bone mass risk developing osteoporosis. This is not only responsible for the dowager's hump of elderly women, but also for hip fractures and their often-fatal consequences. The best approach to osteoporosis would be its prevention by increased calcium intake early in life to build up calcium stores. Among older people, calcium supplements and calcium rich foods may be helpful (Clinic Health Education; 1998).

   Mild calcium deficiency can cause nerve sensitivity, muscle twitching, brittle nails, irritability, palpitations and insomnia. Signs of severe deficiency include abnormal heartbeat, muscle pains and cramps, numbness, stiffness and tingling of the hands and feet, and depression. Children can suffer from rickets, a disease characterised by excessive sweating of the head; slowness in sitting, crawling and walking; bone deformities and growth retardation. In adults, deficiency can lead to osteomalacia with symptoms of bone pain, muscle weakness and delayed healing of fractures. Those prone to calcium deficiency include the elderly, people who don't eat dairy products or other high calcium foods, athletes, those on high protein or high fiber diets, and those who drink a lot of fiber (Bookman Press 1998; 2000).

   Blood levels of calcium are tightly regulated by the hormones calcitonin, parathyroid hormone and vitamin D. When intake is inadequate, calcium is removed from storage sites in bone and used to keep blood levels constant (Bookman Press 1998; 2000).

5. Nutritional Habits

   The mean life expectancy of the Central African population is 40 to 50 years, meaning that an ageing population at risk of the disease is very small. Diagnostic tools for the detection of the disease are available for only a minority of the population (International Osteoporosis Foundation; 2004). Statements about the incidence of osteoporosis in Central Africa are thus unjustified. It is a fact, however, that fewer African Americans develop osteoporosis compared to Caucasian Americans. This has nothing to do with nutrition, but with the skeletal metabolism of African Americans, which appears to be more effective than that of Caucasians (International Osteoporosis Foundation; 2004). Asian countries where mean life expectancy is higher than in Central Africa, i.e. close to 70 in China and more than 80 in Japan, advocate the intake of calcium supplements for their population, because they acknowledge that calcium deficiency due to nutritional habits is a serious risk factor for osteoporosis (International Osteoporosis Foundation; 2004).

   Human beings have learned very early in their evolution to make the best use of nutritional sources. If it were illegal to drink cow's milk, human ought not eat eggs, meat or fish either.

6. Milk as Functional Food

   Since milk and milk products are regarded as convenient, as it requires no further preparation prior to consumption, and is described as natural and fresh. Epidemiological studies suggest that milk and milk products fit well into a healthy eating pattern. Milk and milk products are acceptable nutritious food for persons one year and older and make a significant contribution to daily nutrients as can be seen in Table 2.

   Table 2 : Contribution of milk and dairy products to the diet

   Nutrient	Contribution to the diet
   Calcium	75 %
   Riboflavin	31 %
   Protein	19 %
   Vitamin B12	21 %
   Potassium	19 %
   Zinc	19 %
   Magnesium	16 %
   Vitamin A	17 %

   During the last two decades, increasing research interest has been paid to identifying dietary components that have a measurable impact on human biology. Within this framework, the term 'bioactivity' has arisen as a loose definition encompassing 'food components that can affect biological processes or substrates and hence have an impact on body function or condition and ultimately health'.

   The range of physiological processes that can be affected by dietary bioactivity is vast and includes functions related to digestion, nutrient absorption and post-prandial synthesis; mineral uptake and assimilation; the functioning of the intestiflora; hepatic and pancreatic turnover; and effects on the brain, cardiovascular and immune defense systems. Table 3 is examples of bioactive substances in milk and dairy products and the associated health benefits.

   Table 3: Potential health benefits associated with dairy products.

   Bioactive substance	Potential health benefit
   Vitamins and pro-vitamins Vitamin A and Carotenoids Vitamin D Folic acid, vitamin B6 and vitamin B12 Vitamin E Vitamin C	Antioxidative Beneficial effect in blood pressure regulation Role in calcium metabolism Lower homocysteine levels to reduce coronary heat disease Antioxidative and radical scavenger Antioxidative and enhance iron absorption
   Lipids Conjugated linoleic acid (CLA) Phospholipids: Sphingomyelin Sphingosine Cholesterol	Anticarcinogenic, atherosclerotic and antioxidative Antioxidative Tumor-suppressing Tumor-suppressing Precursor in the synthesis of steroid hormones
   Saccharides Lactose Lactulose Lacto-oligosaccharides Oligosaccharides, glycoproteins & glycolipids	Enhance calcium absorption Promoter of probiotic bacteria Probiotic growth promoter Protective against pathogenic infections
   Proteins Total casiens ( -, - & -caseins) Total whey protein -lactoglobulin -lactalbumin Immunoglobulins (A, M & G) Lactoferrin Lysozyme Peptides Glutathione	Ion carrier, precursor of bioactive peptides Retinol carrier, binds fatty acids, antioxidative Ca2+-carrier, immunomodulation, Anticarcinogenic Immune protection Stimulate in vitro Bifidobacterium growth, iron absorption, anticarcinogenic, antioxidative, increased iron absorption, antiviral, anti-inflammatory, immune-modulating Antimicrobial, synergistic effect with immunoglobulins and lactoferrin Stimulate macrophage activity Anti-ageing, immunomodulation
   Minerals Calcium Magnesium Potassium	Decreased risk of colorectal cancer Blood pressure lowering effect Decreased incidence of osteoporosis Protect against the development of hypertension Beneficial effect on blood pressure
   Polyamines Spermidine, spermine & putrescine	Important for cell growth and differentiation Accelerate intestinal maturation
   Lactic acid bacteria	Enhance nutrient absorption Immune stimulating effect Positive effect on intestinal flora
   Nucleic Acids Ribonucleotides & ribonucleosides Orotic acid	Act as metabolites, immunemodulation, increase iron absorption in the gut Cholesterol lowering effect

   1. Research Focus
      1. The metabolic effect of some dairy products on calcium metabolism was investigated. Full cream UHT-milk, full cream plain yoghurt, a calcium supplement (such as calcium carbonate) and water as control was used as treatments.
      2. Extensive studies on the nutritional content of South African milk and milk products as well as the influence of locality and season on the nutrient content has been performed

      Despite the confusion caused by "experts" on the role of milk in human nutrition, the significant contribution of milk and milk products over centuries as a source of valuable nutrients cannot be ignored. Dairy products are significant sources of bioavailable calcium while also providing other nutritional benefits to consumers.

   2. References

      Agricultural Research Council (ARC); 1998a; The nutritional content of South African milk and liquid milk products.

      Agricultural Research Council (ARC); 1998b; The nutritional content of South African cheeses.

      Bookman Press 1998; 2000-02-08; Vitamins etc. : Calcium;

      Birbeck, J; October 2001; Is milk really a good source of calcium? Dialogue 39 : 9 - 11.

      Bronner, F.; Pansu, D. Nutritional Aspects of Calcium Absorption. The Journal of Nutrition1999, 129(1), 9 - 12.

      Kärkkäinen, M.U.M.; Wiersma, J.W. and Lamberg-Allardt, J.E.; 1997; Postprandial parathyroid hormone response to four calcium-rich foodstuffs; American Journal of Clinical Nutrition 65 : 1726 1730.

      Mayo Clinic Health Education; 1998-10-02; What is osteoporosis?; http://www.mayohealth.org/

      LANGENHOVEN, M, KRUGER, M, GOUWS, E & FABER, M. (COMPILERS); 1991; MRC Food composition tables. Third Edition. Tygerberg: Nutritional Programme: Nutritional Intervention, South African Medical Research Council.

      Rasic, J.Lj.; August 1987; Nutritive value of yoghurt; Cultured Dairy Products Journal : 6 9.

      Van Lieshout, M. and West, C.E.; 2004; Micronutrient Malnutrition Course for Southern Africa. Chapter 1: Introduction to Malnutrion. p 10; Course held at The University of Pretoria during September 2004.

      Weaver, C.M.; Liebman, M. Biomarkers of bone health appropriate for evaluating functional foods designed to reduce risk of osteoporosis. British Journal of Nutrition 2002, 88, Suppl. 2, S225 - S232.