Friday September 17 2010
Dairy intake was measured using a food questionnaire.
“Two glasses of milk each day will help [you] lose weight,” reported the Daily Express. The newspaper said that “adults who drank the most milk – almost two glasses a day – and had the highest vitamin D and calcium levels, lost an average of almost 12lb after two years.”
The story is based on research that looked at vitamin D and calcium levels in overweight people who were taking part in a dietary trial. The participants were assigned one of three diets: low fat, Mediterranean or low carbohydrate. The researchers looked at whether there was an association between calcium in the diet, the level of vitamin D in the blood and subsequent weight loss. They found that higher vitamin D and calcium levels were associated with greater weight loss over the two-year trial.
However, the study was not designed to compare dietary dairy consumption with an alternative diet, and the participants in each group were neither restricted in the amount of dairy they could eat nor required to eat a minimum amount of dairy. This limits the conclusions that can be drawn from the research. Also, the study did not look at how many calories the participants consumed overall or how much exercise they did.
Dairy products are a good source of vitamin D and calcium and these are important nutrients. However, this study doesn’t offer enough evidence to suggest that dairy has a direct effect on weight loss.
Where did the story come from?
The study was carried out by researchers from the University of Negev, Israel, and was funded by the Israeli Ministry of Health and the Dairy Council. The study was published in the peer-reviewed American Journal of Clinical Nutrition.
The Daily Express focussed on dairy as a weight-loss aid, although this study does not give sufficient evidence that dairy directly causes weight loss. The newspaper did, however, highlight the importance of vitamin D for healthy bones, and that dairy products are a good source of this vitamin.
What kind of research was this?
This research was based on the results of a previous randomised controlled trial that looked at how three diets (low fat, Mediterranean or low carbohydrate) affected weight loss in overweight people over two years. The current study used the data from the original trial to see how calcium intake through dairy products affected vitamin D levels, and whether there was an association between vitamin D levels and a person’s weight loss in the trial.
Though a randomised controlled trial is the best way of assessing the effect of any intervention, this trial was not set up to look directly at dietary intake of dairy. Instead, it compared three different diets. The researchers re-examined this data to see whether dairy and calcium in the diet affected vitamin D levels and weight loss after the study was complete. A better way of assessing whether dairy and calcium in the diet affect vitamin D levels and weight loss would be a randomised controlled trial that looked at these factors directly.
What did the research involve?
The study recruited 322 men and women, aged between 40 and 65, who were overweight with a body mass index (BMI) of more than 27. They also included participants with type 2 diabetes or coronary heart disease, regardless of age or BMI.
The participants were assigned to receive one of three diets, which were provided in their work cafeteria. The participants followed a low-fat diet, a Mediterranean diet or a low-carbohydrate diet. The food was labelled according to which diet it belonged to and the number of grams of carbohydrate, fat and saturated fat it contained.
The researchers did not determine how many dairy products the participants could consume. The low-fat and Mediterranean diets allowed people to consume low-fat dairy products. The low-carbohydrate diet allowed participants to consume dairy products regardless of their fat content, as long as they were low in carbohydrates.
The participants were given a validated food questionnaire and were asked how many standard portion sizes of 127 food products they had consumed. The dairy group included 12 items, including low- and regular-fat milk, chocolate milk, low- and regular-fat yoghurts (with and without fruit and sugar), all sorts of low-fat and regular-fat cream, and yellow and white hard cheeses. Some of the dairy products were fortified with vitamin D or calcium, and this was noted. The participants completed the questionnaires at the start of the study and after 6, 12 and 24 months.
Vitamin D levels were measured in a blood sample taken at the start of the study and after six months.
What were the basic results?
The researchers found that people who had the highest BMI had lower levels of vitamin D in their blood, and that over 70% of all participants had lower than optimum vitamin D levels (less than 30ng per ml of blood).
They found that people who had a high dietary calcium intake or the highest blood vitamin D levels at six months tended to have lost more weight at 24 months.
They then looked at whether there was an association between the change in calcium and vitamin D levels from the start of the study to the six-month measurement, and weight loss at 24 months. They found that there was no association between change in calcium intake and weight loss.
Vitamin D levels decreased between the start of the study and the six-month measurement. This was probably because the six-month measurement was taken during winter and the measurement at the start of the study was taken during the summer months (our biggest source of vitamin D is sunlight on the skin). However, participants with a lower decrease in vitamin D at six months generally lost more weight at 24 months.
How did the researchers interpret the results?
The researchers concluded that higher dietary dairy calcium and increased vitamin D levels were associated with successful weight loss over the 24-month study period.
This study looked at whether there was an association between dietary calcium intake through dairy consumption, vitamin D levels in the blood and weight loss in people who were overweight. However, although the participants were assigned to one of three diets, the study was not appropriately designed to compare dietary dairy consumption with an alternative diet. The participants in all the groups were not restricted in the amount of dairy they could eat and did not have a minimum requirement for dairy consumption. The analysis did not assess weight loss per amount of dairy consumed. Rather, it looked at whether there was an association between calcium intake, blood vitamin D levels and weight loss. Although there was association between dairy consumption and calcium levels and vitamin D, the study cannot show a direct causal effect between eating dairy and weight loss.
This study did not calculate how many calories each of the participants on each of the diets consumed. The experimental diets were provided in a workplace environment and did not account for the additional food that the participants ate outside this environment. The participants were required to recall what they had eaten over the previous six months, and may have over- or under-estimated the amount of a particular food they consumed.
Additionally, the study did not assess how much exercise the participants did or how much time they typically spent outdoors, which may have affected their weight loss and their vitamin D levels.
Dairy products are a good source of vitamin D and calcium, but there is not sufficient evidence from this study to suggest that dairy has a direct effect on weight loss.