Teorías del aprendizaje

Main characteristics of trials which showed a significant improvement following milk supplementation are summarised in table 2.5.

In early 20th century one of the first school milk based intervention studies was conducted in Scotland by Boyd Orr (Orr, 1928). It was a relatively simple and well-designed trial where school children were supplied with whole milk, skim milk, biscuits, or no supplementation at all. Both kinds of milk supplementation increased the weight and height by 20% in comparison to the other two groups (Orr, 1928). In addition to growth, at the end of the trial Orr also found that milk

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consuming children showed better growth outcomes, with children who were undernourished at baseline attaining the most benefit.

In 1980, socioeconomically disadvantaged British children who were receiving free milk (190ml) in schools were studied. Observations on these children showed that a slight but significant increment was found in height and weight gain within the milk supplemented group. The reason for such a small increase was most probably due to greater baseline heights of children in the control group which then diminished the difference of height in between the study groups (Baker et al, 1980).

A school milk intervention was performed in both rural and urban areas of Nigeria in early 1980s where all children were provided with powdered milk daily on school days (Nnanyelugo, 1984). The results concluded that both rural and urban children had increased calcium intake following the intervention, however, the positive effects of milk were more apparent in rural children. Only rural boys showed significant increases in height and weight probably because they had lower baseline characteristics. This study shows that children who are disadvantaged will benefit the most.

The impact of school milk feeding programme in children of various ethnicities was studied by Chen (Chen, 1989). The results of this study showed that after provision of milk the rate of prevalence of protein energy malnutrition (PEM) significantly reduced. Other than this, the attendance rates of children also improved over the span of this trial most likely due to improved general health as a result of consumption of milk as no other important developmental changes took place at that time period.

Cadogan et al (1997) conducted a randomised controlled trial (RCT) with adolescent secondary school girls. The intervention group was provided with either whole or skim milk based on preference. The controls received no milk. At the end of the trial, the milk supplementation group had significantly greater bone mineral density (BMD) and bone mineral content (BMC) in comparison to controls. Girls who received milk also had significantly higher serum IGF-1 levels and micronutrient intakes. The height, weight, body composition, and bone

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markers did not significantly differ between the girls who consumed milk and those who did not.

In China, a school milk based intervention was conducted to study the effects of milk supplementation on pre-pubertal girls who had low calcium and vitamin D intakes (Du et al, 2004). Participants were divided into three groups; (1) received 330ml calcium (560mg) fortified milk, (2) received 330ml calcium (560mg) and vitamin D (8ug) fortified milk, or (3) controls which received no milk. Both milk supplemented groups had significantly higher BMC, BMD, height, and weight compared to controls. When groups 1 and 2 were compared, group 2 had the highest increments in BMC and BMD. Serum PTH concentration was significantly lower in groups 1 and 2 in comparison to group 3. Furthermore, group 2 had significantly lower prevalence of vitamin D deficiency than group 1 and 3. A study with similar characteristics was performed to observe the effects of milk supplementation upon size adjusted total body BMD (Zhu et al, 2008). After implementation of the intervention it was observed that calcium supplementation positively affected BMD with the majority of benefit seen in the lower limbs. Another RCT was conducted in Asian girls where the first group received a high dose of calcium (1300mg) in the form of powdered milk, the second group received a low dose (650mg), and the third group acted as controls (Lau et a, 2004). The results showed that no significant changes in anthropometry were present but both intervention groups had significantly higher BMD in comparison to controls. When the high and low calcium dose groups were compared, no significant difference was present.

Lien et al (2009) provided stunted and underweight pre-pubertal children with fortified and normal milk. Bone parameters were not studied in this trial and height and weight also did not show a significant improvement. However, the results showed that overall health related quality of life and mental health significantly improved along with a significant reduction in stunting rates. Even though many trials have shown the benefit and efficacy of milk supplementation in children, there are a few studies that have shown no beneficial effects at all. A simple study by Cook et al (1979) concluded that children aged 6- 7 years did not show any significant improvements in health and growth outcomes

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after consuming milk from 1972 to 1976. Similarly a RCT conducted in pre- pubertal New Zealand children showed that 18 months of milk powder supplementation (1200mg/d) neither improved the bone status or the anthropometric variables (Gibbons et al, 2004). The authors concluded that children included in the trial already had high calcium intakes, so calcium supplementation would be more effective when targeted at children who have low habitual dairy consumption.