Tuesday November 25 2014
Many low-fat yoghurts are high in sugar
"Eating a small portion of yoghurt every day may reduce diabetes risk," The Independent reports.
This news comes from a US study that assessed the eating habits of more than 100,000 people and then followed them up every four years, looking for new diagnoses of type 2 diabetes.
Pooling the results of this study with 14 other studies, the researchers estimated each serving of yoghurt – 244 grams (g) – a day decreased the risk of developing type 2 diabetes by around 18%.
There was no significant link between total dairy intake or intake of other specific dairy products and type 2 diabetes.
A challenge facing this and similar studies is making sure all relevant outside influencing factors (confounders) have been accounted for, which is very tricky to do in practise.
If this has not been done conclusively, yoghurt consumption may be acting as a marker of a healthier lifestyle in general and has no direct influence on diabetes risk, which may be the case here.
We also don't know what sort of yoghurt the participants consumed. For example, many low-fat yoghurts are very high in sugar, which could contribute to weight gain.
It is therefore possible yoghurt may reduce the risk of developing diabetes, but may increase the risk of other diseases.
Current advice to reduce the risk of type 2 diabetes remains the same: eat a healthy diet, maintain a healthy weight, avoid smoking, moderate alcohol consumption, and take regular exercise.
Where did the story come from?
The study was carried out by researchers from Harvard School of Public Health and was funded by the US National Institutes of Health.
One of the study's authors declared a competing interest as he "holds membership of Unilever North America Scientific Advisory Board".
Unilever produces a huge number of commonly eaten yoghurts. It is not clear to what extent this conflict of interest may have influenced the study design, methodology or interpretation.
The study was published in the peer-reviewed medical journal BioMed Central (BMC) Medicine. It is an open access journal, meaning that anyone can read the full research publication for free.
Generally, the media reported the study accurately. But many sources chose to suggest that, "it might be a good idea to eat yoghurt regularly" without due consideration for the potential downsides of this advice.
For example, eating low-fat, high-sugar yoghurt may contribute to weight gain and weight-related diseases other than type 2 diabetes. It could also potentially increase the risk of tooth decay, particularly in children.
It also wasn't made clear what type of yoghurt was consumed, or that the association between yoghurt and diabetes may still be influenced by other factors.
What kind of research was this?
This was a meta-analysis combining the results of three large prospective cohort studies.
The researchers attest that the relation between the consumption of different types of dairy and the risk of type 2 diabetes remains uncertain.
They therefore aimed to evaluate the association between total dairy and individual types of dairy consumption and incident type 2 diabetes in US adults.
Type 2 diabetes is a condition where the person can't control their blood glucose, either because the body doesn't produce enough insulin or the body's cells don't react to insulin.
The rapid rise in the number of adults in Westernised nations such as the UK developing type 2 diabetes is caused by:
- increasing obesity levels
- a lack of exercise
- an increase in unhealthy diets
- an ageing population
Read more about risk factors for type 2 diabetes.
What did the research involve?
The research team used existing data on 41,436 men in the Health Professionals Follow-Up Study (1986-2010), 67,138 women in the Nurses' Health Study (1980-2010), and 85,884 women in the Nurses' Health Study II (1991-2009) to look at the links between diet and type 2 diabetes.
Diet was assessed by validated food frequency questionnaires and data was updated every four years. Incident type 2 diabetes was confirmed by a validated supplementary questionnaire.
Every two years, data was gathered and updated on risk factors for chronic diseases, such as body weight, cigarette smoking, physical activity, medication use and family history of diabetes, as well as history of chronic diseases such as high blood pressure and high cholesterol.
Among participants in the two nurse studies, information on menopausal status, post-menopausal hormone use and oral contraceptive use was also gathered.
The researchers analysed their results in three phases, with each phase adjusting for more and more potentially confounding factors.
The fully adjusted analysis took account of the following potential confounders:
- calendar time with updated information at each two-year questionnaire cycle
- body mass index (BMI)
- total energy intake
- physical activity
- alcohol consumption
- menopausal status
- menopausal hormone use (Nurses' Health Study II participants only)
- oral contraceptive use (Nurses' Health Study II participants only)
- family history of diabetes
- diagnosed with high blood pressure or high cholesterol at baseline
- trans-fat intake (a type of unsaturated fat often found in processed foods)
- glycaemic load (eating foods known to raise blood glucose levels)
As well as intakes of:
- red and processed meat
- sugar-sweetened beverages
- other types of dairy foods
The team extended their work by conducting an updated meta-analysis that combined the new results from the three large cohort studies described above with findings from previous studies.
This previous research included prospective studies with cohort, case cohort or nested case-control design investigating the association between the intake of dairy products and the risk of type 2 diabetes. Literature was searched for up until October 2013.
In studies that reported the intakes by grams (g), they used 177g as a serving size for total dairy products, and 244g as a serving size for milk and yoghurt intake to recalculate the intakes to a common scale (servings per day).
What were the basic results?
During 3,984,203 person years of follow-up, they documented 15,156 cases of incident type 2 diabetes.
After adjustment for age, BMI and other lifestyle and dietary risk factors, total dairy consumption was not associated with type 2 diabetes risk.
The pooled hazard ratio (HR) (95% confidence interval [CI] of type 2 diabetes for one serving per day increase in total dairy was 0.99, 95% CI 0.98 to 1.01), so the this result was not statistically significant.
Among different types of dairy products, neither low-fat nor high-fat dairy intake was appreciably associated with risk of type 2 diabetes.
However, yoghurt intake was consistently and inversely associated with type 2 diabetes risk across the three cohorts with a pooled HR of 0.83 (95% CI 0.75 to 0.92) for one serving per day increment (trend analysis).
For added validity, they conducted a meta-analysis of 14 additional prospective cohorts with 459,790 participants and 35,863 incident type 2 diabetes cases.
The pooled relative risks (RRs) (95% CIs) were 0.98 (0.96, 1.01) and 0.82 (0.70, 0.96) for one serving of total dairy per day and one serving of yoghurt per day, respectively.
How did the researchers interpret the results?
The researchers' main conclusion was that, "Higher intake of yoghurt is associated with a reduced risk of T2D [type 2 diabetes], whereas other dairy foods and consumption of total dairy are not appreciably associated with incidence of T2D."
They added that, "The consistent findings for yoghurt suggest that it can be incorporated into a healthy dietary pattern. However, randomised clinical trials are warranted to further examine the causal effects of yoghurt consumption, as well as probiotics on body weight and insulin resistance."
This analysis of three large cohort studies, and a meta-analysis of 14 more, came up with estimates that each serving per day of yoghurt (244g) decreases the relative risk of developing type 2 diabetes by 18%.
It suggests other dairy foods and consumption of total dairy are not associated with type 2 diabetes. It was not clear over what time period this risk reduction was achieved, as follow-up times varied, but the maximum was 30 years.
The research team pointed out that their findings on total dairy intake were consistent with some, but not all, previous studies. Differences between this and previous studies may be because the current study used longer-term follow-up (more than 10 years).
The study had a number of strengths, including its large sample size, use of prospective data and ability to take account of a large number of confounding factors.
But, as with all studies, there are also limitations to consider.
What sort of yoghurt was consumed?
Firstly, what sort of yoghurt we are talking about here? Greek, natural or added sugar, low-fat or full-fat?
From the study data presented, there are few distinctions made and all types of yoghurt are lumped together in the analysis.
This means it is not possible to know which types of yoghurt are potentially beneficial. This may depend on the levels of sugar, fat and probiotic bacteria, or other constituents.
For example, many low-fat yoghurts are very high in sugar, which could contribute to weight gain and increase the risk of harms from other weight-related diseases.
Other health outcomes not considered
This study focused exclusively on the risk of developing type 2 diabetes. The effect of diet on other diseases was not studied, so any compensating effects would go unnoticed.
For example, those eating yoghurt may be at a reduced risk of developing type 2 diabetes, but at an increased risk of developing another disease.
Were all the confounders accounted for?
Also, despite adjusting for a number of potential confounding factors, it's difficult to know whether all relevant factors have been fully accounted for.
Yoghurt consumption may be a marker of a healthy lifestyle in general, which could be associated with reduced risk of this chronic disease.
This result seems to be consistently found across the three large cohort studies and 14 other studies, which gives it some credibility.
But a systematic review and meta-analysis would be the best way to assess the link. This would ensure that all relevant material is considered. There is no guarantee that important studies were excluded from the meta-analysis of the current study, which could influence its findings.
This type of study typically feeds into the development or updating of national guidelines, which consider all the available evidence before deciding on what dietary advice to give the public.
For now, current lifestyle advice to reduce the risk of type 2 diabetes remains the same: aim for a balanced diet high in fruit and vegetables and low in sugar, salt and saturated fats, take regular exercise in line with recommendations, avoid smoking, and moderate your alcohol consumption.
Read more about what eating a healthy, balanced diet entails.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.