“Chemicals found in fast food wrappers and clothes are linked [to] weight gain in women,” reports the Mail Online. Researchers in the US found that women who regained most weight after dieting in a weight-loss study had higher levels of a group of chemicals called perfluoroalkyl substances (PFAS) in their blood.
PFAS are a class of synthetic chemicals used in many industries. They are used in both food packaging and kitchenware due to their “non-stick” properties. A concern expressed in this study is PFAS may disrupt the normal balance of hormones in the body, leading to weight gain.
The study found no difference in people’s ability to lose weight, whatever level of PFAS they had in their bodies. People in the study lost an average 6.4kg during the trial.
However, the difficulty with dieting often comes after the initial 6 months of weight loss, when people try to keep to their new weight. The average weight regain in the study was 2.7kg in the 18 months after the weight loss period. The study found women who had the highest PFAS levels regained around 2kg more weight, compared to women with the lowest PFAS levels.
The study doesn’t prove that PFAS levels caused the weight regain, however. It could be that higher PFAS levels simply indicated that these people tended to eat more high-calorie packaged food.
Keeping weight off after a diet can be challenging. But with a long-term plan and the willingness to make some lifestyle changes, it is possible. Read more advice about how to keep the weight off.
Where did the story come from?
The study was carried out by researchers from Harvard TH Chan School of Public Health, Louisiana State University and Tulane University, all in the US, with funding from the US National Institutes for Health. It was published in the peer-reviewed medical journal PLOS Medicine, which is free to read online.
The Guardian and the Mail Online give reasonably accurate overviews of the study. The Times, however, overstated the study results, saying: “The frying pan may be as much to blame for your expanding waistline as the fry-up sizzling inside it.” Whatever the effect of PFAS chemicals on metabolism, weight gain comes from taking in more calories than the body uses up, so diet is the key way to manage weight.
What kind of research was this?
This study used the RCT data to examine the impact of one of the factors measured at the start of the study – levels of PFAS chemicals – on the study outcomes. That means the study can’t prove differences in PFAS levels are responsible for differences in weight loss or regain, because other potential confounding factors might be responsible.
What did the research involve?
Researchers measured levels of 5 types of PFAS chemicals in the blood of 621 overweight or obese men and women, who then took part in a clinical trial of different types of weight loss diet. They also measured people’s weight, body mass index (BMI), body fat, metabolic rate and thyroid function, among other measures.
People then spent 6 months on the diet, and were followed up for an additional 18 months. Body weight was measured again at 6, 12, 18 and 24 months. Metabolic rate and other measures were taken again at 6 and 24 months.
Researchers looked to see whether – after adjusting their figures to try to take account of potential confounding factors – people’s PFAS levels at the start of the study were linked to how much weight they lost, how much weight they regained, or changes in their resting metabolic rate.
Metabolic rate measures how quickly people’s bodies use up calories, so it has a potentially large effect on weight.
Metabolic rate usually goes down when people diet to lose weight, and up again when they return to their usual diet.
The researchers took account of the following confounding factors:
- ethnic background
- educational level
- smoking status and alcohol consumption
- physical activity (measured by questionnaire)
- the weight loss diet they were assigned to
- BMI at the start of the study
- menopausal status and use of hormone replacement therapy (HRT)
- thyroid function (the thyroid is a gland in the neck that produces hormones that can stimulate both weight gain and weight loss)
What were the basic results?
PFAS levels did not affect how much weight people lost in the first 6 months of the trial. People lost an average 6.4kg in this phase.
People with higher PFAS levels regained more weight in the 18-month follow-up period than those with lower levels. When the researchers looked separately at men and women, they found this difference only held true for women.
The difference was visible for all 5 chemicals measured. For example:
- women with the highest levels of one type, PFOA (perfluorononanoic acid), regained an average 4.3kg (plus or minus 0.9kg)
- women with lowest levels of PFOA regained an average 2.2kg (plus or minus 0.8kg)
The metabolic rates of people with higher levels of 3 PFAS chemicals slowed much more than the rates of people with lower PFAS levels during the first 6 months of the study. For the chemical PFOS:
- people with the highest levels saw their metabolic rate drop by 45.4 calories a day (plus or minus 15.5) during the 6-month weight loss trial
- people with the lowest levels saw their metabolic rate drop by 5 calories a day (plus or minus 16.3)
Metabolic rates were slower to recover for people with highest PFOS levels after the weight loss trial – rising by only 0.9 calories a day (plus or minus 26.2) from 6 to 24 months, compared to a rise of 94.6 calories a day (plus or minus 27.5) for those with lowest PFOS levels.
How did the researchers interpret the results?
The researchers were cautious in interpreting their results. They said the study “provides the first piece of evidence from a controlled weight loss trial that higher baseline plasma PFAS concentrations in adults are associated with a greater weight regain, especially in women, possibly due to suppressed RMR [resting metabolic rate] levels”.
They say the findings “suggest that environmental chemicals may play a role in the current obesity epidemic.”
The study found that PFAS may affect people’s metabolic levels, and that this might affect the ability of women in particular to manage their weight. However, there are limitations to the study which mean we can’t tell whether PFAS chemicals are responsible.
First, this type of study can’t prove that one factor causes another. For example, it may be that a high-calorie diet that includes lots of unhealthy foods exposes people to higher levels of PFAS through packaging. Higher PFAS levels may simply indicate that these people returned to a higher-calorie diet after the weight loss part of the study was over.
Also, the study didn’t measure what people ate after the weight loss part of the study, so we don’t know whether the weight regain was due to lower metabolic rate, or simply to people eating more.
And the study was designed to look at lots of different measurements. The more measurements you take, the more likely it is that some of them will turn up worrying results, simply by chance.
That said, the effect on metabolic rate and the link to weight regain is of concern, as these chemicals are widely used in manufacturing, from coating carpets and clothing to food packaging and cookware.
So, should people wanting to lose weight try to avoid PFAS? That would be difficult, and we don’t know whether it would help. We don’t know what levels of PFAS chemicals people in the UK have in their bodies. We don’t know whether using non-stick cookware, or avoiding food packaging made using PFAS chemicals, would reduce PFAS levels in the body. Without this information, attempting to avoid PFAS chemicals does not seem practical or advisable.
Researchers need to do more work into the effects of these chemicals on human health, and regulatory authorities need to consider whether their use should be restricted.
For people wanting to lose weight, the best course of action is to carry on with what we know works – a calorie-restricted, balanced diet.
Find out more about weight loss diets.