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Publishing in JAMA Pediatrics, a group of researchers investigated the association between mother’s consumption of fish while pregnant and growth in infancy and obesity in their children, and reported that high intake of fish (defined as more than three times a week) was associated with increased risk of rapid growth in infancy and also childhood obesity in some but not all age groups.
Dr Daghni Rajasingam, spokesperson for the Royal College of Obstetricians and Gynaecologists (RCOG), said:
“This study in JAMA Pediatrics suggests that eating fish three times a week in pregnancy is linked to childhood obesity at age six. However, this is an association and not necessarily causal. The authors of the study highlight a number of flaws which could explain the results. The total number of calories eaten during pregnancy were not measured, nor the amount of other foods monitored. In addition, the number of calories and types of food consumed by the children were not examined.
“Fish is an important source of protein, healthy fats and vitamin D, and pregnant women should try to eat two portion of fish each week. Oily fish such as tuna, salmon or mackerel can contain higher levels of mercury which can be harmful to fetal development and we advise women to avoid having more than two portions of these types of fish each week.”
Ms Catherine Collins, Registered Dietitian, and Spokesperson for The British Dietetic Association, said:
“This is an interesting study which pooled selected research from 15 studies, mainly European, to examine fish consumption in pregnancy to growth patterns in the offspring. Compared to eating fish less than 3 times a week, the 1 in 10 women who ate fish 3 times or more a week appeared 22% more likely to have babies who rapidly grew in their first two years of life (which we know may increase obesity risk as an adult), or to have children considered overweight at age 6. High maternal fish consumption wasn’t linked to being overweight or obese at age 4.
“The authors suggest environmental pollutants found in fish may disrupt hormones and body fat metabolism, increasing the risk of weight gain in the children of women who ate more than 3 portions of fish a week. If this was the case, then a greater preference of the mother for oily fish would be expected to show an enhanced effect on the child’s weight gain, given that environmental toxins like PCBs and methyl mercury are more concentrated in oily fish than in white fish. In the UK, pregnant women are advised not to have more than two portions a week of fresh oily fish (tuna, salmon, whitebait, sardines) and to avoid larger oily fish (like swordfish, merlin and shark) due to environmental pollutants including mercury and PCBs which accumulate in these fish species.
“Were women who preferred oily fish more likely to create overweight children than those who preferred white fish or shellfish? The authors examined this and found no association between any one type of fish and childhood weight, which adds doubt to the suggestion that fish, and/or its potential pollutants, influence childhood metabolism.
“Cravings and food aversion are common in pregnancy, which wasn’t considered as part of the questionnaire. These cravings are often short lived and so won’t necessarily influence throughout the whole pregnancy. Personally, I developed a craving for both fruit, and a particularly well-known brand of cod fish fingers in my first pregnancy which I hadn’t eaten since childhood. I managed three 60-fish finger packs before suddenly deciding I’d had enough of them around 20 weeks in, and have never eaten them since. Yet the growth patterns of both my kids were similar. Of course, this is an anecdote, but it serves to underline how difficult it is to isolate one food type as the cause of an outcome in offspring 6 years later, without considering the whole diet and the impact of pregnancy itself on usual food choices.
“Natural variation in dietary choices is related to food availability, food costs and local customs. In this survey for example, the average Belgian woman consumed one portion of fish every two weeks, whilst the average Spanish woman ate 9 portions over the same time. Were Spanish children 9x more likely to be overweight or obese than Belgian children? Unfortunately the data isn’t presented for comparison.
“So is a liking for fish destined to make your child overweight? The answer is no – not from this data. Lack of details on mum’s diet, their food choices and preferences all influence a child’s eating habits, given mum is traditionally the ‘nutritional gatekeeper’ for most children. That fish consumption of any type didn’t influence being overweight at age 4 also adds doubt to the fish story. The lack of consideration of the rest of mum’s diet makes this study a red herring, and risks mums-to-be cutting out this healthy food from their diet. I’d recommend they follow the UK advice given to mums by the NHS, and continue to enjoy fish as part of a varied diet.
“As a reminder, the UK guidelines advise pregnant women to eat no more than 2 portions of fresh or frozen oily fish a week and avoid the older, larger oily fish (such as swordfish, shark and marlin) that may have accumulated higher levels of toxins and mercury. They’re also advised to avoid raw shellfish (such as prawns, oysters and mussels) due to the risk of infection, although cooked shellfish are usually fine. Oily fish that ‘count’ within the two portions a week are salmon, trout, mackerel, sardines, pilchards, herring and whitebait. Other fish in this restriction include sea bass, sea bream, turbot, halibut, rock salmon (dogfish) and crab. White fish (such as cod, haddock, coley, plaice, skate, hake) aren’t limited in pregnancy.”
Prof. Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“It’s particularly important to be clear what this study does and doesn’t show. In my view, it raises some important questions but, despite the large numbers of women and children involved, it doesn’t on its own provide a firm basis for dietary recommendations. Even if eating fish more often during pregnancy did cause an increased risk of child overweight – and the study certainly doesn’t establish such a cause and effect relationship – the possibility that this has something to do with environmental pollutants is, in the researchers’ own word, “speculative”. It goes a very great distance beyond the data that the researchers actually have.
“This is an observational study – that is, it is based on data obtained by observing women and their children, asking the mothers what they ate during pregnancy and measuring the growth of their children. The researchers found the mothers who said they ate fish more than three times a week, when they were pregnant, were somewhat more likely to have children who grew fast in their first two years, and to have overweight children by the age of 6 years. (The association with overweight at age 4 is actually not quite strong enough to be statistically significant.)
“But this type of study can’t show that it was the mothers’ fish eating that caused the increased risk of overweight children. Probably the children of mothers who ate fish more often differed from the rest in other ways too, and perhaps it’s these other ways that increase the risk of overweight. This issue – known as confounding – is always a problem in interpreting observational studies, but perhaps particularly so with some studies of diet. The researchers did a decent job of making statistical adjustments to allow for potential confounding variables for which they had data – they allowed for such things as the mother’s age and education and whether she smoked during pregnancy, for instance. But the research report itself clearly points out that “the possibility of unmeasured confounding may still remain.” The researchers could not allow statistically for things like other dietary patterns during pregnancy, or child diet or lifestyle, because they had no data on them, and these unmeasured quantities may be distorting any possible association (or lack of association) between fish eating in pregnancy and overweight children.
“The average amount of fish-eating differed very considerably between the different cohorts of women in the study. The Spanish and Portuguese mothers, for instance, ate fish eight or nine times as often, on average, as the Belgian mothers. What were the Belgian mothers eating instead? Perhaps that had an effect on their children’s weight. We can’t tell from this study. We also can’t directly tell how much more fish the Spanish mothers ate than the Belgians, for instance, because the researchers had data only on how often they ate fish and not on how much they ate at each fishy meal. This may add to the difficulty of interpreting the results.
“Finally, despite the prominent mention in the press release of persistent organic pollutants in fish, the study itself provides no evidence for or against the possibility that these pollutants in fish can pass from the mother to the unborn child and affect child growth. The researchers explicitly point out in their report that they did not have enough data to distinguish between fish types and cooking procedures that would be relevant to exposure to these pollutants. As the report and later parts of the press release make explicit, the researchers’ hypothesis about a role for these pollutants “remains speculative”. That is, they really don’t know one way or the other.”
Prof. Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:
“This study compares the prevalence of overweight and obesity among the 26,184 children of women in several countries (including Belgium, Ireland, Italy, Netherlands, Norway, Poland, Spain, Portugal, and USA) over a period of several years and attempts to relate this to mother’s fish consumption during pregnancy, as assessed from a food frequency questionnaire. Generally, fish consumption was low in Belgium, Netherland, Ireland, USA and high in Portugal and Spain.
“The authors report that children were more likely to have rapid weight gain in the first few years of life if there mother had reported consuming fish more than three times a week when they were pregnant. However, the size of the effect was relatively small and only just significant, and was not statistically significant in boys. It is well known that rapid weight gain in the first year of life increases risk of overweight or obesity in children. However, infant feeding practice in the first year of life is the biggest factor influencing rate of growth. Breastfed infants grow more slowly than bottle-fed infants, who are more like to show accelerated growth because they are overfed. This analysis did not adjust for infant feeding practice, which is likely to be serious confounding factor.
“The study makes the assertion that the difference in weight gain may be the result of long-term effects of persistent pollutants. However, this is purely speculative and not based on measurement of exposure to these pollutants. Persistent organic pollutants (such as polychlorinated biphenyl and brominated fire-retardants) tend to accumulate in the livers of white fish (cod, plaice, etc.) rather than the flesh, but they are more likely to be present in higher amounts in the flesh of oily fish (e.g. herring, mackerel). So it is very important to know what type of fish is consumed – normally more white fish is consumed than oily fish, but this was not measured in this study.
“Generally, concentrations of persistent organic pollutants such a polychlorinated biphenyl and dioxins are low in fish in Europe with the exception of fish caught in the Baltic Sea. Some of the US Great Lakes have been polluted for many years. Previous research (1) has shown that high levels of persistent contaminants were found in the blood of pregnant American women who had consumed fish caught by sports fishermen from polluted US inland waterways; these were associated with reduced not increased birthweight. Mercury, however, is found in high amounts in some long-lived fish such as marlin and swordfish. Generally, levels of persistent pollutants are routinely monitored in the European community and the European Food Safety Authority sets safe limits for these contaminants. Much fish consumed in the UK is farmed and these are routinely monitored to ensure the levels of contaminants remain below the safety thresholds. Consequently, the risk of consuming contaminated fish remains low in the UK.
“Previous research (2,3) found that women who eat fish in pregnancy are less likely to have a pre-term birth and low birth weight infants. The Avon Longitudinal Study of Parents and Children (ALSPAC) was the first UK study to show favourable effects of eating fish on pregnancy outcomes, particularly cognitive function. In a much larger prospective study of pregnancy (151,880 mother-child pairs) in Northern Europe, which included Southampton Women’s Survey, a favourable effect on birth outcome was noted with fish intake. There have also been a few randomized controlled trials in which women have been advised to consume more fish during pregnancy and these do not find any evidence of increased weight gain in the offspring.
“Oily fish (salmon, herring, mackerel) are an excellent source of long-chain omega-3 fatty acids and a recent analysis suggests a favourable effect of these fatty acids, which are provided mainly by oily fish, on risk of allergies (4). Current UK dietary advice is that women are encouraged to eat fish in pregnancy up to 2 serving per week but to avoid marlin, swordfish and albacore tuna as well as raw shellfish.”
Prof. Jean Golding, Emeritus Professor of Paediatric and Perinatal Epidemiology, University of Bristol, said:
“This paper has taken data from 15 European and US studies to assess whether the amount of fish eaten by the mother in pregnancy has any effect on the subsequent weight gain of the child. They showed that the children of women who ate more than 3 portions of fish per week during pregnancy were slightly more likely than those who ate less than 2 portions per week to be overweight or obese. However it should be noted that neither social conditions, the number of the child’s older siblings nor any other features of the mother’s diet were taken into account.
“It is important that this finding is tested in other studies before it is considered to indicate a causal effect. Meanwhile it is important to emphasize that eating fish in pregnancy is of benefit to the neurodevelopment of the child.”
Prof. Andrew Whitelaw, Emeritus Professor of Neonatal Medicine, University of Bristol, said:
“While eating fish in general are high protein and fatty fish provide valuable omega-3 fatty acids, there is now a question as to how much harm might result from accumulation of marine pollutants such as methyl mercury and organic compounds. Pregnant women have been encouraged to eat some fish but warned to limit consumption of fish to three times per week, particularly of salmon. Other previous laboratory research has suggested that feeding mice on salmon containing organic pollutants can induce type 2 diabetes.
“A strength of this new study is that a large number of pregnant women in 11 countries with widely varying fish intakes were surveyed and body-mass index (BMI) in their babies followed for 6 years. They found that consumption of fish more than three times per week in pregnancy was associated with an increase in childhood overweight or obesity at 4 years, which did not quite reach statistical significance at 95% confidence, even with 17,676 subjects, but did just reach 95% confidence at 6 years on 15,939 subjects. This is a detectable but not a powerful effect, nothing like the effect of smoking on many health outcomes.
“An observational study like this has to consider the possibility that there are differences between groups of women in addition to consumption of fish, and the authors have tried to compensate for possible sources of bias such as differences in pre-pregnancy BMI, weight gain in pregnancy and maternal smoking.
“What is striking is that the effect of fish consumption differs between countries. There was a big effect in Italy but no effect of eating fish more than three times a week on children in Norway. It is also important to consider the whole range of growth and developmental outcomes in children, not just obesity.
“We need information on whether pollution varies between different seas and oceans, between different species and between farmed and wild fish. Fish provide valuable nutrition and it may be a mistake to lump all fish together when it comes to risk and benefit.”
‘Fish intake in pregnancy and child growth: A pooled analysis of 15 European and US birth cohorts’ by Nikos Stratakis et al. published in JAMA Pediatrics on Monday 15 February 2016.
Declared interests
Dr Daghni Rajasingam: “Nothing to declare.”
Ms Catherine Collins: “No conflict of interest declared.”
Prof. Kevin McConway: “I have no conflicts of interest to declare on this.”
Prof. Tom Sanders: Prof Tom Sanders was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain. Tom has previously acted as a member of the Global Dairy Platform Scientific Advisory Panel and Tom is a member of the Programme Advisory Committee of the Malaysian Palm Oil Board. In the past Tom has acted as a consultant to Archer Daniel Midland Company and received honoraria for meetings sponsored by Unilever PLC. Tom’s research on fats was funded by Public Health England/Food Standards Agency. Tom is also a Scientific Governor of the charity British Nutrition Foundation, member of the scientific advisory committee of the Natural Hydration Council (which promotes the drinking of water), and honorary Nutritional Director of the charity HEART UK. Tom is now emeritus but when he was doing research at King’s College London, the following applied: Tom does not hold any grants or have any consultancies with companies involved in the production or marketing of sugar-sweetened drinks. In reference to previous funding to Tom’s institution: £4.5 million was donated to King’s College London by Tate & Lyle in 2006; this funding finished in 2011. This money was given to the College and was in recognition of the discovery of the artificial sweetener sucralose by Prof. Hough at the Queen Elizabeth College (QEC), which merged with King’s College London. The Tate & Lyle grant paid for the Clinical Research Centre at St Thomas’ that is run by the Guy’s & St Thomas’ Trust, it was not used to fund research on sugar. Tate & Lyle sold their sugar interests to American Sugar so the brand Tate & Lyle still exists but it is no longer linked to the company Tate & Lyle PLC, which gave the money to King’s College London in 2006. Tom also used to work for Ajinomoto on aspartame about 8 years ago.
Prof. Jean Golding: “I have no interests to declare.”
Prof. Andrew Whitelaw: “I have no conflict of interest.”