Calcium Needs during the Growth Spurt and Low Bone Density
Calcium is one of the essential minerals in the body and has many functions. One of its key functions is in relation to the development of bone particularly during adolescents.
In this article we address the factors affecting skeletal growth and the role of nutrition in bone health.
SKELETAL GROWTH SPURTS
Skeletal growth involves an increase in bone length and an increase in bone mass due to bone mineral accrual. The patterns of skeletal growth are linked to the stage of development, gender and the skeletal region. Pre-pubertal growth is characterized by an early growth spurt at 6-12 months of age. After this initial growth spurt, there is a gradual decrease in the speed of growth through childhood with a mid-childhood growth spurt at 7 to 8 years in some. The growth spurt during puberty is characterized by a period of minimal growth preceding the growth spurt. Thereafter there is a period of maximal growth where peak height is achieved. Following the period of fast growth, there is a decrease in speed of growth where the epiphyses close and final height is achieved. Gender differences are evident during pubertal growth with girls generally starting their growth spurt two years earlier than boys. (1)
The age at which long bones increase in length occurs approximately one year earlier than the age at which maximal bone mineral content is achieved. During this period there can be an increase in bone fragility whilst bone mineral accrual is taking place. (1,2)
FACTORS INFLUENCING SKELETAL GROWTH
Physical exercise applied during childhood and adolescence impact on growth and maturation. The intensity, frequency and duration of exercise play a role. (3,4) Moderate weight-bearing activity, such as running or jumping, has a more positive effect on bone mass than do non-weight-bearing activities such as swimming, which have minimal physical strain on bone. (4) The benefit of weight-bearing activity is greater if exercise is initiated in the pre-pubertal growth period. (5) However, when increased exercise intensity is combined with poor nutrition, the risk for suboptimal skeletal growth and development is increased. (1,3,6)
Inadequate nutrition may also be a contributing factor to impaired growth and development. Energy and nutrient intake in many young athletes may be inadequate to support the nutrition needs for normal growth and the additional costs of an intense exercise program. A negative energy balance can compromise skeletal growth and development. (1,7)
Delayed growth and development can also be influenced by psychological and emotional stress associated with maintaining body mass, extensive training programs without resting time, frequent competitions, altered relationships with peer and demanding parents and/or coaches, and physical trauma damaging the epiphyses. (1)
NUTRITIONAL FACTORS AFFECTING BONE HEALTH
A diet that contains vegetables, fruits, low-fat milk and milk products, whole-grans and adequate levels of protein and energy to maintain a healthy body weight can provide the nutrients needed for the formation and maintenance of healthy bones. (4,8) Adequate levels of calcium throughout life are critical to bone health, although other nutrients are also important, namely, the minerals copper, iron, fluoride, magnesium, manganese, phosphorus, potassium, sodium, zinc and vitamins A, C, D and K. (8)
Optimizing calcium intake is particularly important during adolescence. (4) Calcium is the main mineral in bone mass. If therefore must be provided in sufficient quantity from ingested foods to optimize skeletal growth in children and adolescents. (2,4,9) Calcium requirements is the highest during childhood and adolescence, with the exception of pregnancy and lactation. (1,4,7) Peak calcium-accretion rate is achieved during adolescence at an average age of 12.5 years in girls and 14 years in boys. (4) Calcium recommendations are therefore based on the amount required to maintain calcium balance and promote optimum bone accretion rates. During periods of low calcium intake, there is increase calcium retention, but a very low calcium intake is detrimental for bone development and health. (10) The influence of the family’s diet on the diet of children is well established and therefore, adequate calcium intake by all members of the family is important to set an example for kids. (4)
Dairy products are the predominant source of calcium in the diets of young, healthy, non-athlete children and adolescents and the easiest way to achieve adequate calcium intake is to consume 3 servings of dairy products per day (4 servings per day for adolescents). Most vegetables also contain calcium, although at relatively low density. (4) Other sources of calcium includes fish with bones, dried beans and calcium-fortified foods such as breakfast cereals (4,8) A soy beverage, unless fortified with calcium, is not a good calcium source due to the low bioavailability of calcium. (4) Low-fat or non-fat versions of milk and milk products have the same amount of calcium as full-cream milk. (8)
Besides the amount of calcium in the diet, the availability of calcium for absorption is also critical for bone development and maintenance. (9)
The active absorption of Calcium is dependent on vitamin D and therefore optimal calcium and vitamin D is necessary to promote good bone health. The current adequate intake for vitamin D for infants, children and adolescents is 200 IU per day (5.0 µg per day). (4) Vitamin D is found naturally in liver, cod liver oil, fatty fish and egg yolks. Other dietary sources include fortified milk and margarine. (8) The body can although synthesize vitamin D through skin exposure to sunlight (UV radiation). The amount of sun exposure needed to produce vitamin D depends on skin pigmentation and age. Young people with light coloured skin need about 10 minutes per day of casual exposure of their face and hands. People with dark skin may need 20 minutes of sun exposure. Vitamin D deficiency is characterized by inadequate mineralization, or demineralization, of the skeleton. In children, severe vitamin D deficiency results in inadequate bone mineralization, and in adults it can lead to increased demineralization of bone. (9)
A number of food constituents can possible further increase calcium absorption, e.g. individual milk components and non-digestible oligosaccharides. (9) Dietary intake of potassium and bicarbonate obtained from fruits and vegetables will decrease urinary calcium excretion and therefore increase calcium status. (4,8)
Some dietary substances may although decrease retention of calcium, including alcohol, caffeine, oxalates (e.g. in spinach), phytates (e.g. in soy) and protein. Dietary sodium is also an important factor in the renal excretion of calcium. (4,9)
In next week’s post we will focus on the barriers that may prevent calcium uptake as well as provide you with a handy practical summary.
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