Bone health in adolescents – Part 2

Bone Xray

Calcium Needs during the Growth Spurt and Low Bone Density

Last week we looked at skeletal growth and the factors that influence skeletal growth particularly during the adolescent stage. Two of the factors affecting skeletal growth are physical activity and nutrition, specifically the need for calcium. We looked at calcium sources, the amount of calcium needed and how calcium is absorbed.

In this article we help you understand that there may be reasons why sufficient calcium might not be taken in so that you can encourage your adolescents to follow appropriate nutritional practices.

Finally we provide you with a practical summary to maximise calcium intake and ensure optimal bone growth and health.

BARRIERS TO ADEQUATE CALCIUM INTAKE AND ABSORPTION

Young female athletes who restrict energy intake tend also to restrict dairy products and thus are at a risk of low calcium intakes. (1,3) This is based on the misconception that all dairy foods are fattening. The truth is however that low-fat or fat-free milk and milk products contain at least as much calcium as full-cream milk and can therefore be part of a successful weight loss or weight maintenance program. (2)

Adolescents usually start consuming greater amounts of soft drinks and other cold drinks at the cost of milk intake. Soft-drink consumption peaks in adolescence, at which time milk intake is at its lowest level. (2) In addition, high intakes of caffeine and salt (found mainly in processed foods) increase loss of calcium in the urine and thus prevent it being absorbed into the cells where it is needed and used.

Some individuals avoid the intake of dairy products due to lactose intolerance. The truth is although that many children with lactose intolerance can drink small amounts of milk without discomfort, especially when accompanied by other foods. Other alternatives include the use of fermented dairy products such as hard cheese and yoghurt, which may be tolerated better than milk. Lactose-free and low-lactose milks are available. Non-dairy food products (such as certain vegetables) or calcium-supplemented foods (including calcium-fortified soy milk) may be used as other calcium sources. (2)

Another potential barrier to calcium absorption is alcohol. Alcohol also increases urinary losses of calcium and can lead to decreased serum vitamin D levels, resulting in malabsorption of calcium from the small intestine. (4)

WHEN ARE CALCIUM SUPPLEMENTS REQUIRED?

Supplements should be considered for children and adolescents who cannot or will not consume adequate amounts of calcium from preferred dietary sources. Supplements most often contain calcium carbonate and supply 300 to 600 mg of elemental calcium per tablet. Remember to check the “elemental calcium” amount per tablet. Some mineral supplements also contain vitamin D.

Decisions about their use must be made on an individual basis, keeping in mind the usual dietary habits of the person, any individual risk factors for osteoporosis and the likelihood that the use of the supplement will be maintained. (2). Absorption of calcium reaches a plateau at doses of about 500 mg. Calcium carbonate tablets should ideally be taken with food. Doses spaced throughout the day appear to result in a greater total calcium absorption than if one large dose is taken only once during the day. (3)

PRACTICAL SUMMARY

  • The easiest way to achieve adequate calcium intake is to consume 3 servings of dairy products per day (4 servings per day for adolescents). One serving being 1 glass of milk (250 ml), 175 ml yoghurt or 30 g of cheese.(2)
  • If the parent is not achieving the recommended calcium intake, it is unlikely that the child is achieving the recommended intake. (2)
  • Knowledge of dietary calcium sources is important for increasing the intake of calcium-rich foods. The largest source of dietary calcium for most persons is milk and other dairy products (e.g. yoghurt and cheese). (2)
  • Alternative sources of calcium are important for children and adolescents who do not drink milk. Most vegetables contain low amounts of calcium. The bioavailability of calcium from green vegetables generally is high, especially if the oxalate content is low (e.g. broccoli and other greens). In spinach, the high oxalate content makes the bioavailability of calcium very poor. Some high-phytate foods such as whole bran cereals and soy foods may also have poor bioavailability of calcium. (2)
  • Breakfast cereals are frequently fortified with minerals, including calcium. A soy beverage, unless fortified with calcium, is not a good source, because the bioavailability of calcium is low. If one relies on calcium-fortified foods or non-dairy foods that are low in vitamin D, then another source of vitamin D is needed to provide adequate intake of 200 IU per day (0.5 µg per day). (2)
  • Be aware of the nutritional habits that may affect calcium intake and/or absorption.
  • Decisions about the use of calcium supplements must be made on an individual basis, keeping in mind the usual dietary habits of the person, any individual risk factors for osteoporosis and the likelihood that the use of the supplement will be maintained. (2)

REFERENCES

1. Bass S & Inge K. Nutrition for special populations: Children and young athletes in Bourke Clinical Sports Nutrition

2. Greer FR & Krebs NF. (2006) Optimizing Bone Health and Calcium Intakes of Infants, Children and Adolescents. Am Acad Ped 117:578 – 585.

3. Burke LM, Castell LM, Stear SJ et al. (2010) A – Z of nutritional supplements: dietary supplements, sports nutrition foods and ergogenic aids for health and performance Part 7. Br J Sports Med 44:389 – 391.

4. Micklesfield, L. Bone Physiology. Lecture notes, UCT.

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