During growth, undernutrition (including insufficient caloric intake and protein) can severely impair bone development. Low protein intake can be detrimental for skeletal integrity by lowering both the production and action of Insulin-like Growth Factor (IGF-1), which enhances bone formation. In addition, this growth factor stimulates both the intestinal absorption of the bone mineral elements calcium and phosphate, via an increase in the renal production of calcitriol (the hormonal form of vitamin D) and directly stimulates the tubular reabsorption of phosphate. Therefore, during growth and pubertal maturation, an impaired production and/or action of IGF-1 due to a low protein intake may result in reduced bone development. A positive correlation between protein intake and bone mass gain can be detected in children.

Variation in protein intake within the normal range for well-nourished children and adolescents can affect skeletal growth, and so impact on a child’s capacity to achieve their genetic potential for peak bone mass [1]Rizzoli, R., Nutrition: its role in bone health. Best Pract Res Clin Endocrinol Metab, 2008. 22(5): p. 813-29.

. Milk is a source of high-quality proteins which contain growth promoting elements. Healthy children who were given extra servings of milk in their diets, and hence extra protein, experienced significant increases in IGF-1 as compared to control subjects [2]Cadogan, J., et al., Milk intake and bone mineral acquisition in adolescent girls: randomised, controlled intervention trial. BMJ, 1997. 315(7118): p. 1255-60.

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Milk and other dairy products are the source of up to 80% of dietary calcium intake for children from the second year of life onwards. Studies have reported a downward trend in milk consumption since the 1970s, with some evidence suggesting that the decrease is related to the increased consumption of sweetened beverages [3]Dror, D.K. and L.H. Allen, Dairy product intake in children and adolescents in developed countries: trends, nutritional contribution, and a review of association with health outcomes. Nutr Rev, 2014. 72(2): p. 68-81.

.  Given that a 240 mL glass of milk, a cup of yoghurt or 42 g of natural cheese each provide around 300 mg of calcium, achieving the recommended daily allowance should not be difficult. However, less than 15% of adolescent girls in the United States consume the RDA for calcium, with the average intake being just 876 mg/day [4]Bailey, R.L., et al., Estimation of total usual calcium and vitamin D intakes in the United States. J Nutr, 2010. 140(4): p. 817-22.

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Reports from around the world suggest that low levels of vitamin D in children are highly prevalent [5]International Osteoporosis Foundation: Vitamin D Status Around The World.

. Vitamin D3 is synthesized in the skin when exposed to UV-B rays in sunlight, with synthesis dependent on several factors including latitude, skin pigmentation and use of sunscreen. It is only found in small amounts in certain foods such as oily fish and eggs, and in some countries in fortified foods such as margarines and breakfast cereals. A pan-European study found that adolescent consumers of ready-to-eat-cereals had favourable micronutrient intake, including vitamin D, as compared to non-consumers of such cereals [6]Michels, N., et al., European adolescent ready-to-eat-cereal (RTEC) consumers have a healthier dietary intake and body composition compared with non-RTEC consumers. Eur J Nutr, 2015. 54(4): p. 653-64.

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Infants are born with low vitamin D stores and are largely dependent on breast milk and supplements as sources of vitamin D in the first few months of life. As the vitamin D content of breast milk is dependent on maternal vitamin D status and is often low, and sun exposure is likely limited, they are particularly vulnerable to vitamin D deficiency. Vitamin D deficiency in infants can lead to bone malformation (rickets), seizures and difficulty breathing. 

Certain disorders which result in malnutrition and resulting failure to achieve peak bone mass during adolescence can have life-long implications for bone health. Read more in disorders that affect nutritional status.

• Anorexia Nervosa / excessive leanness

  Excessive leanness in adolescence leads to a low peak bone mass. Many girls struggle to stay thin in order to participate in dance or competitive sports, or to emulate the emaciated self-image reflected in fashion magazines. An obsession with thinness can lead to eating disorders such as anorexia which can irreversibly damage the skeleton. See Anorexia Fact Sheet.

• Inflammatory bowel disease (IBD)

  Several aspects of bone metabolism in children with IBD are inhibited, with both bone formation and bone resorption decreased by 30–50% as compared to normal rates [7]Pappa, H., et al., Skeletal health of children and adolescents with inflammatory bowel disease. J Pediatr Gastroenterol Nutr, 2011. 53(1): p. 11-25.
  
  . At diagnosis, children with IBD, particularly those with Crohn’s disease, often have stunted growth. Before the era of treatment with biologics, the use of glucocorticoids contributed to bone alterations. Control of inflammation, improved nutrition and regular physical activity are the key components of management of IBD in children with respect to bone health.

• Coeliac disease

  Fractures were almost twice as common in individuals with clinically diagnosed coeliac disease as compared to those without the disease [8]Heikkila, K., et al., Celiac disease and bone fractures: a systematic review and meta-analysis. J Clin Endocrinol Metab, 2015. 100(1): p. 25-34.
  
  . A position statement published in Canada in 2012 recommended that BMD should be assessed one year after diagnosis of coeliac disease in children if gluten-free diet (GFD) adherence is not strict, and that a GFD is the most important treatment for bone loss [9]Fouda, M.A., et al., Evaluation and management of skeletal health in celiac disease: position statement. Can J Gastroenterol, 2012. 26(11): p. 819-29.
  
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• Cystic fibrosis

  A significant proportion of children with cystic fibrosis (CF) have low BMD. In 2011, the European Cystic Fibrosis Society published comprehensive guidelines on the assessment, prevention and treatment of bone disease in CF sufferers [10]Sermet-Gaudelus, I., et al., European cystic fibrosis bone mineralisation guidelines. J Cyst Fibros, 2011. 10 Suppl 2: p. S16-23.
  
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