Wednesday December 21 2016
Most adults and children in the UK eat too much processed meat
"Regularly indulging in a bacon sandwich doubles the risk of an asthma attack," is the needlessly alarming headline in The Sun.
A French study suggests eating four or more portions of processed (cured) meats a week may worsen symptoms like wheezing – but this does not amount to an asthma attack.
An asthma attack is a debilitating and sudden loss of normal lung function that may require emergency treatment.
The study of just under 1,000 French adults (42% with asthma) found evidence eating sausage, ham or dried sausage four times a week worsened asthma symptoms.
The researchers think compounds called nitrites, which are used to preserve meat, might be the culprits, as they have been linked to inflammation in the airways.
Some of the effect may also be influenced by obesity. It's thought obesity increases inflammation throughout the body, which could also affect the lungs, so the researchers adjusted for this in their analysis.
Independent experts from the UK have questioned whether the study looked at diet in sufficient detail.
It's difficult to prove that one type of food causes symptoms. A person's diet is a complex mix of foods, and food is only one of the factors that can affect people's chances of developing asthma symptoms.
Processed meats have also been linked to bowel cancer. A healthy diet for people with asthma is likely to be the same as for people without – plenty of fresh food, including vegetables, and low in sugar, saturated fat and salt.
Where did the story come from?
The study was carried out by researchers from the Institut national de la santé et de la recherche médicale (INSERM) and Clinique Universitaire de Pneumologie in France, the Spanish National Cancer Research Centre and Centre for Research in Environmental Epidemiology in Spain, Instituto Nacional De Salud Publica in Mexico, and Harvard Medical School in the US.
It was funded by grants from Merck Sharp & Dohme, the Global Allergy and Asthma European Network, and the US National Hospital program of clinical research.
The study was published in the peer-reviewed journal, Thorax.
The Sun and The Daily Telegraph's news stories both start by considering what this French study might mean for UK meat eaters.
The Sun warns of the risk of "a regular bacon sarnie", while The Daily Telegraph advises readers that "four ham sandwiches a week" could increase the risk of asthma attacks.
Both say processed meat "almost doubles" the risk of asthma attacks, which is an overstatement and may needlessly frighten readers.
The researchers found a 76% increased risk of worsened asthma symptoms, which is somewhat less than double (100% increased risk) and does not necessarily imply an asthma attack.
Worsened symptoms may amount to increased wheezing or coughing, not a full-blown asthma attack.
The reporting by the Daily Mail and BBC News was more balanced, making it clear that the link is not necessarily causal, and included the views of other independent experts.
What kind of research was this?
This study used information from a prospective study of more than 2,000 people, about 40% of whom had been diagnosed with asthma at the start of the study.
The researchers analysed the data using a statistical model to estimate links between asthma, cured (also known as processed) meat consumption, and body mass index (BMI).
Prospective studies are useful ways to spot links between factors and their development over time, but cannot on their own prove that one factor (diet) causes another (asthma symptoms).
What did the research involve?
Researchers took data from adults who had filled in health and dietary questionnaires in 2003-07, and had follow-up information available from 2011-13.
This information was taken as part of a survey for another study known as the Epidemiological Study of the Genetics and Environment of Asthma.
The survey included people with asthma, their close relatives, and a control group of people without asthma.
After adjusting for potential confounding factors, the researchers looked for links between worse asthma symptom scores in the second survey and levels of consumption of cured meat in the first survey.
The researchers excluded people from this study if they didn't have full data, if they had dropped out of the study or if they were under 16, reducing the original 2,047 participants to 971.
The dietary questionnaire asked people to state how often they ate 118 foods. The cured meat question assumed one portion to be one sausage, two slices of ham or three slices of dried sausage.
The researchers compared people who ate the least (less than one portion a week) with people who ate four or more weekly portions.
They adjusted their figures to take account of:
- overall dietary patterns
- smoking status
- education level
- physical activity level
The study is slightly unusual in that it used a model to assess whether BMI mediates any link between cured meat and asthma – in other words, the extent to which processed meat intake might cause asthma symptoms indirectly by increasing obesity.
Most studies like this would separate out any potential effect of BMI altogether as a confounding factor.
The study also used people's asthma symptom score as the outcome measure – regardless of whether they'd been diagnosed with asthma – rather than asking whether or not they had asthma.
They say this captures changes to asthma over time, as well as new cases of asthma among those in the control group.
What were the basic results?
After an average seven years of follow-up:
- 53% of people had the same asthma symptom score as they'd had at the start
- 27% had improved asthma symptom scores
- 20% had worse asthma symptom scores
The researchers found eating cured meat four times a week or more increased the risk of having worse asthma symptom scores by 76% (odds ratio 1.76, 95% confidence interval [CI] 1.01 to 3.06).
They found BMI mediated this effect, so 14% of the increased risk from cured meat could be explained by the link between a high cured meat consumption and high body mass index.
How did the researchers interpret the results?
The researchers say their methodology avoided underestimating the effect of cured meat by treating BMI as a mediating factor rather than a confounding factor.
They say: "While the indirect effect mediated through BMI accounted for only 14% of this association, the direct effect explained a greater proportion, suggesting a deleterious role of cured meat independent of BMI."
In other words, because higher BMI only explained part of the increased risk, this suggests something about cured meat increases asthma risk, independently of its effects on weight.
This study adds to concerns that cured meats like bacon, ham and sausage may be harmful to our health.
Curing was long used to preserve meat before the days of refrigeration. It often involves the use of salt, which in itself may be bad for health, and produces compounds called nitrites.
Nitrites are thought to cause inflammation to the airways, which might worsen asthma symptoms.
This study does not in itself prove cured meats worsen asthma symptoms. Previous studies have not shown this, and this study does have some limitations.
For example, it may not have completely accounted for confounding factors, and the decision to treat BMI as a mediating factor might overplay the significance of cured meats.
One expert from the British Dietetic Association said the food questionnaire seemed to be insufficiently detailed to fully account for the effects of different foods – for example, high- or low-fat dairy products.
And we don't know whether the types of cured meat regularly eaten in France have the same effect on the body as those most often eaten in the UK.
We don't need a study to tell us that eating a lot of salty, high-fat processed meat is likely to be unhealthy, especially if we do it year-round.
A balanced, healthy diet with plenty of fresh food, vegetables, wholegrains, pulses and fruits, without a lot of saturated fat, salt or sugar, is the best option.