“Eating hot food off plastic plates can increase the risk of kidney stones”, is the excitable and frankly inaccurate headline in the Daily Mail.
The scare-story actually comes from a small study involving just 12 people. They were split into two groups of six:
The two groups were then crossed-over and the experiement was repeated.
Melamine is an organic compound used to produce a type of resin widely used in the manufacture of kitchenware. Melamine tableware is popular, particularly for use with children (as it is almost unbreakable) and is generally regarded as safe, provided manufacturers’ instructions are followed.
However, it should not be used as a container when heating up foods in microwaves or conventional ovens.
The researchers found that people eating from melamine bowls had increased levels of melamine in their urine, compared to those who ate their soup from ceramic bowls.
The researchers speculate that prolonged exposure to melamine could alter the chemical make-up inside the kidneys, leading to the formation of kidney stones which are usually made up of one or more of the following - calcium, ammonia, uric acid, and cystine.
This small study does not lend any evidence to the speculation, meaning further research is necessary.
The study was carried out by researchers from Kaohsiung Medical University and Kaohsiung Municipal Hsiao-Kang Hospital, Taiwan. It was funded by The Taiwan National Health Research Institutes, the National Science Council, and Kaohsiung Medical University Hospital.
The study was published in the peer-reviewed Journal of the American Medical Association.
Its results were over-stated in the Daily Mail. The paper implied the study found that using melamine for hot food increases the risk of kidney stones. The study did not look at any association between melamine and kidney stones, although it does mention previous research purporting to show a link.
While high levels of melamine in foods have been associated with kidney stones and other problems, melamine products used in the UK have to conform to international safety standards to prevent adverse health effects.
The authors of this study refer to an incident in 2008 in which melamine was found in baby formula milk, resulting in six deaths and 50,000 hospitalisations – often referred to as the Chinese Baby Milk Scandal (which BBC News reported on). They also say that continuous low-dose melamine exposure has been linked to the formation of stones in the kidneys and other parts of the urinary tract, in both children and adults.
To test these supposed risks, the researchers carried out a randomised crossover study which aimed to find out if eating hot food off melamine tableware is associated with increased levels of melamine in the urine. In a crossover study, participants are randomly divided into two groups.
One group receives one treatment (in this case, eating soup from melamine bowls) and the second group the other treatment (eating soup from ceramic bowls). The effects are measured and then the treatments are reversed, so that both groups receive both treatments.
The researchers say they ran a pilot study on 12 volunteers which showed that after eating hot foods in melamine bowls, average melamine concentrations in the urine increased sharply, peaking between four and six hours later. They then set up a crossover study to compare melamine levels in the urine after eating hot soup from melamine and ceramic bowls.
The researchers recruited six healthy male and six healthy female volunteers, whom they randomly divided into two groups, with equal numbers of men and women in each. The first group was asked to fast before consuming 500ml of hot noodle soup (90°C) served in melamine bowls as a 30-minute breakfast. The second group followed the same procedures, but consumed the same soup from ceramic bowls. All participants were advised not to use any melamine tableware for three days before the experiment.
After a three week wash out period, the assigned treatments were reversed. Urinary melamine levels were measured immediately before and at two-hourly intervals, for 12 hours after consuming the soup. Total melamine excretion was also calculated.
Researchers found that for 12 hours after eating the soup, the average total melamine excretion was 8.35 micrograms when people ate from melamine bowls, and 1.31 micrograms when they ate out of ceramic bowls, a statistically significant difference.
There was no difference in urinary levels of melamine between either group after consuming soup from melamine. There was a difference in urine levels of melamine between the groups who ate from ceramic bowls.
The group who ate from melamine bowls first and ceramic second had higher average levels of urinary melamine. Researchers attributed this to the “carryover” effect as the estimated half-life (the time it takes a substance to fall to half of its initial level) of melamine is six hours.
They say that melamine tableware may release large amounts of melamine when used to serve hot foods and the consequences of long-term melamine use should be of concern.
This study found that when participants ate hot food from melamine bowls, their urine levels of melamine increased compared to melamine levels after eating from ceramic bowls. This suggests that melamine had migrated into the soup it contained. However, as the authors point out, it is unclear if melamine levels found in the urine would cause any health problems. Also, the study only looked at one brand of melamine tableware, so whether the findings would apply to other brands is uncertain.
While this crossover type study allowed the researchers to compare what happened to participants using melamine with those using ceramic bowls, the chief drawback is that the study design means that the effects of one ‘treatment’ (using melamine bowls) may carry over and alter the response to the second ‘treatment’ (using ceramic bowls).
Researchers usually try to prevent this by introducing a “washout” period between consecutive treatments that is long enough to allow the effects of a treatment to wear off – which for this study was three weeks.
Melamine is a chemical that has many uses, including plastic tableware. In small amounts, it can migrate from tableware into foods. Contamination with melamine above established safety limits may put people at risk of kidney and other problems. In the past, plastic tableware from China and Hong Kong has caused safety concerns. However, international standards now exist for the manufacture of melamine tableware to ensure its safety. There are also established safety limits for the migration of melamine into food.
It is unclear whether EU standards are any stricter than other global standards, and whether this would affect the likelihood of melamine exposure being a risk factor for kidney stones in the UK. Arguably, well-established risk factors for kidney stones (such as eating a high-protein, low-fibre diet and inactivity) are of greater concern.