Introduction

Does dietary counseling to increase milk intake produce useful changes to calcium status in older adults? And what, if any other nutritional changes might be produced? Researchers conducted this multi-center, randomized study to answer these questions and explore the practicality and efficacy of increasing calcium intake using food. There's general agreement that most North Americans have less than optimal calcium intake and that increased calcium intake can reduce the risk for osteoporotic fractures. However, most of the studies demonstrating this benefit use calcium supplements. Of 30 randomized, controlled studies, only three actually used food as a calcium source. Additionally, there is concern that recommending food sources to achieve increases in calcium intake is done at the expense of increased weight or fat intake.

Study Design

Two hundred and four (204) healthy men and women between the ages of 55 and 85 were recruited for the study. Criteria for inclusion included a body mass index (calculated as kg/m2) of between 16 and 36.5, five or more years since menopause, consumption of 1.5 or fewer dairy food servings a day, and the ability to consume three additional 8-ounce servings of fluid milk daily.

Participants were randomly assigned to a milk or control group for a 12-week intervention following four weeks of baseline observation. During the baseline observation, participants maintained their usual diets, completed two 3-day food records, and had baseline measurements taken. During the 12-week intervention:

• The milk group participants were instructed to add three 8-ounce servings of nonfat or 1% milk to their daily diet. The control group was instructed to maintain their usual diet.
• Energy and nutrient intake were assessed from milk intake logs collected at weeks 4, 8 and 12 and from 3-day food records collected at weeks 8 and 12.
• Calciotrophic hormones were measured at baseline and weeks 8 and 12. Urinary calcium and N-telopeptide excretions were measured at week 12.
• A repeated-measures analysis of variance model was used to analyze differences between baseline and treatment periods.

Results

Dietary Effects
At baseline, the milk and control groups did not differ appreciably in energy and nutrient intakes. Descriptive data indicate that the 204 participants were typical of free-living, healthy adults in the specified age range. Dietary changes were as follows:

• Subjects in the milk group consumed significantly more calcium, vitamin D, protein, phosphorous and magnesium. They reported increased in calcium and vitamin D (weighted averages of the men and women = 729 mg/d and 5.8 microg /d respectively).
• The reported increase in daily energy intake was less than half the 220 to 260 calories provided by the added milk, indicating that to a substantial extent subjects substituted milk for other foods.
• Fat intake did not change significantly, and there were no significant changes in total serum cholesterol level.
• Protein intake increased substantially, indicating the addition of milk only produced a limited reduction in intake of other protein sources.
• The milk group gained .68kg (1.5 lb.) more than the control group (P<.01). Almost all the gain was in the first four weeks, possibly reflecting the Thanksgiving and Christmas holiday that occured during the intervention. There were no significant weight increases in the milk group between weeks 4 to 12.

Skeletal Effects
The following changes, which were significantly different from baseline values in the milk group and from corresponding changes in the control group, were observed:

• In the milk group, serum parathyroid levels decreased significantly by approximately 9%. Urinary excretion of N-telopeptide, a bone resorption marker decreased by approximately 13%.
• Urine calcium in the milk group increased by 21 +/- 7.6mg/day (mean +/- standard error), and the difference between the groups was highly significant (P<.01). This is less than half the amount predicted to be absorbed from the increment in calcium intake, suggesting the difference is being stored in bone.
• Serum insulin-like growth factor-1 (IGF-1) level rose about 10% (+12 +/- 2.4 ng/mL; P<.001; mean standard +/- error)in the milk group. The level of insulin-like growth factor binding protein-4 (IGBFP-4) fell slightly in the milk group (1.9%) and rose significantly in the control group (7.9%; P<.05). The difference between groups for IGF-1 was highly significant (P<.001) For IGFBP-4 the difference between groups was significant (P<.05).
• In the milk group, serum 25(OH)D rose by 3.8+/-0.9ng/mL (mean standard error; P<.05), consistent with the increase in vitamin D intake from milk. Serum 25(OH)D was unchanged in the control group (P<.05).

By contrast, bone specific serum alkaline phosphatase levels (a bone formation marker) fell by approximately 9% in both groups, and the difference between them was insignificant.

Discussion

This study demonstrates that the addition of milk to the typical Western diet results in overall dietary improvement. This change can be accomplished without an undesirable increase in fat intake and a small effect on body weight. The extra calcium resulted in changes in bone remodeling and the calcium economy, indicating positive bone balance. The decline in the resorption marker in the milk group and no between-group difference in the formation marker are indicative of a shift in the direction of a more positive bone balance.

Urinary calcium increases about 65 mg for every 1,000 mg increment in calcium intake. The expected increase for this study would be about 47 mg, however it was about half that amount, suggesting the difference is being stored as bone. The increase in IGF-1 and the decrease in IGFPB-4 level are new observations that are beneficial for bone health.

It's recommended that for people who consume an adequate diet, increases dairy product intake should substitute for other protein sources. The researchers also suggest that calcium-to-protein ratio is a better way to characterize the bone-related nutritional value of a diet.

The authors conclude "important improvements in skeletal metabolism can feasibly occur in older adults by consumption of food sources of calcium. Dietitians and public health officials need to emphasize ways to meet optimal calcium intake recommendations through increased intake of low-fat dairy products.