"Milk could be the key to beating crippling arthritis," reports the Daily Express, while the Daily Mail adds that, "A glass of milk a day keeps arthritis at bay".
Both headlines are potentially misleading. The study the papers were reporting on was about slowing the progression of osteoarthritis in the knee joints, rather than preventing it occurring in the first place.
The study focused on a single aspect of osteoarthritis, rather than making a general assessment of the effects of the disease. This single aspect was the gap in the knee joint (joint gap width) seen on X-ray, which is an established clinical measure of the progression of the condition.
The study found that for women who regularly drank milk, the joint gap did not reduce as much after four years compared with those who drank less or no milk – a good sign for the milk drinkers. The joint gap difference between those drinking the most milk (more than seven glasses per week) and no milk was 0.12mm after four years.
Some readers may be thinking, so what does that mean and is it important? One of the main limitations of the study is that it focused on a very narrow outcome: joint gap width. It is unclear if the smaller reduction in joint gap width reported was of any significant benefit to people with osteoarthritis on a practical level.
While the study's implications may be vague at this stage, on a broader level the study reminds us of the importance of getting the right amount of calcium in your diet, which helps strengthen bones. Read more about improving your bone health.
Where did the story come from?
The study was carried out by researchers from Harvard Medical School in the US and was funded by the US National Heart, Lung and Blood Institute.
It was published in the peer-reviewed medical journal, Arthritis Care and Research.
While the main body of the Daily Express' and Daily Mail's content was accurate, both newspapers' headlines were somewhat inaccurate.
The Express implied that milk can relieve the pain of arthritis. This is unproven, as the study did not examine whether milk consumption was related to arthritic knee pain.
The Mail stated that milk kept "arthritis at bay". But this study involved people who already had the condition, so this statement is untrue. The study was actually about whether milk helped stop the arthritis getting any worse, not whether milk prevented it from occurring in the first place.
What kind of research was this?
This was a prospective cohort study assessing whether milk intake affected osteoarthritis progression in the knee.
Arthritis is a common condition that causes pain and inflammation within a joint. In people affected by osteoarthritis, the cartilage (connective tissue) between their bones gradually wastes away, leading to painful rubbing of bone on bone in the joints. The most frequently affected joints are in the hands, spine, knees and hips.
Osteoarthritis often develops in people who are over the age of 50. However, it can develop at any age as a result of an injury or another joint-related condition.
Milk has long been known to be beneficial to bones, so the researchers wanted to find out if people who drink more milk might have slower worsening of their knee osteoarthritis.
A cohort study is a practical way of assessing whether milk could affect osteoarthritis progression.
However, the limitations of this study include the fact that there are many other factors (confounders) that could also influence this.
All of these need to be accounted for if the role of milk is to be isolated and assessed for its individual impact. Practically, this is very difficult and residual confounding often biases the results to some degree.
A randomised controlled trial (RCT) would be the ideal study design, but is often far more costly to perform.
What did the research involve?
The researchers used 2,148 participants (and tested 3,064 knees) with radiographically diagnosed knee osteoarthritis who were participating in the US Osteoarthritis Initiative. Each had dietary information gathered at baseline and were followed up at one, two, three and four-year time points to assess knee osteoarthritis progression.
Milk consumption was assessed using a food frequency questionnaire completed at baseline. Participants were asked how often they had drunk milk (any kind) in the past 12 months.
Progression of knee osteoarthritis was measured using quantitative joint space width (JSW) between the medial femur and tibia of the knee based on plain radiographs, a standard clinical measure.
Cartilage acts as a "shock absorber" in the knee, so when it is faulty or wears away – as is the case in osteoarthritis – the joint can become painful and stiff, and normal range of movement can be limited. Narrowing joint space indicates cartilage loss and worsening osteoarthritis.
Only participants with a Kellgren and Lawrence grade (a widely used diagnostic classification for osteoarthritis) of 2 or 3 knee osteoarthritis were included:
- grade 2 – definite osteophytes, definite narrowing of joint space
- grade 3 – moderate multiple osteophytes, definite narrowing of joint space, some sclerosis and possible deformity of bone contour
Standard statistical methods were used to test if there was an independent association between milk intake and a decrease in JSW over time. Researchers accounted for baseline disease severity, body mass index, dietary factors and a range of additional potential confounders.
What were the basic results?
The main finding was a significant dose-response relationship between baseline milk intake and adjusted mean decrease of JSW in women (p for trend 0.014) at four-year follow-up.
With increasing levels of milk intake (none, three or fewer, four to six, and seven and more glasses per week), the mean decreases of JSW were 0.38mm, 0.29mm, 0.29mm and 0.26mm respectively.
This showed milk drinkers had less narrowing, meaning their osteoarthritis was not deteriorating as much as those drinking less milk.
In men, there was no significant association between milk consumption and the decrease of JSW.
Analysis was adjusted for baseline disease severity, body mass index, dietary factors and other potential confounders.
How did the researchers interpret the results?
The researchers stated that their "results suggest that frequent milk consumption may be associated with reduced OA [osteoarthritis] progression in women. Replication of these novel findings in other prospective studies demonstrating the increase in milk consumption leads to delay in knee OA progression are needed."
The results indicated that women who regularly drink milk have slower knee osteoarthritis disease progression compared with those drinking less milk over a four-year period. This was found to be the case in women, but not in men.
The study strengths included its prospective design, the large number of subjects with knee osteoarthritis, and the use of an objective assessment of disease progression. The researchers measured this using a single clinical measure of knee osteoarthritis progression: joint space width.
Its main weakness was that it relied solely on joint space width to show disease progression. It was not clear if the small joint width differences reported actually made any positive differences to patients' lives or feelings about living with the condition.
The research also did not make a functional assessment of any change in knee mobility, pain or the ability to perform day-to-day activities. These would have been welcome additions to this study for those who have the condition. This was probably a result of the design of the study and the inherent restrictions of using an existing data set.
The study adjusted for many potential confounders, but this may not have been completely effective, so residual confounding could still have influenced the results in women. This means that this study cannot prove milk is the cause of the beneficial joint space outcomes, as it may be caused by other factors.