Friday January 11 2013
An increasing number of children are now overweight
The Daily Mail tells us that eating the wrong type of fat while pregnant ‘increases likelihood of having overweight children’. The ‘wrong type of fat’ in this case is omega-6 polyunsaturated fatty acids.
Omega-6 is what is known as an essential fatty acid – it cannot be produced by our bodies, but we rely on it to help with certain functions, such as brain development – so we need to get it from dietary sources such as sunflower oil.
This study looked at the mother’s blood levels of long-chain polyunsaturated fatty acids (PUFA) when she was 34 weeks pregnant, and then looked at measures of the child’s body fat when they were four and six years of age. The researchers were specifically interested in the effects of two types of PUFA:
- omega-6 fatty acid
- omega-3 fatty acid – another essential acid, found in many fish
The researchers found that maternal consumption of these omega-6 fatty acids was associated with their children’s weight, body fat mass and lean mass at both four and six years of age. No association was found with omega-3 consumption.
It should be stressed than an association is not proof of direct cause and effect. Arguably, there are far more important factors that affect a child’s weight – such as diet and activity levels of the child – than maternal fatty acid consumption.
As such, this study does not change current dietary advice for women during pregnancy.
Where did the story come from?
The study was carried out by researchers from the University of Southampton. This research received various sources of financial support including the Medical Research Council, British Heart Foundation, Arthritis Research UK and the National Osteoporosis Society.
The study was published in the peer-reviewed medical journal, Endocrine Research.
While the Daily Mail’s reporting of the methods and results of the study were accurate, it did jump to the unsupported conclusion that having higher levels of fat at the ages of four or six, automatically means that a child will become obese.
What kind of research was this?
This was a cohort study which aimed to see whether there was a link between the mother’s blood levels of long-chain polyunsaturated fatty acids (PUFA) during late pregnancy and her subsequent child’s body measurements, including fat mass and lean body mass, at ages four and six.
The researchers say that there is increasing evidence that the nutrition the developing baby receives while in the uterus influences their body composition during childhood and adulthood. They say that there is evidence that individual constituents of the diet may also have a role, and that in particular, PUFA levels may influence the development of fat tissue.
They did a preliminary test of the theory, but did not intend to test if the PUFA levels in pregnancy were directly responsible for the current obesity levels of children. For this, other factors – most importantly overall diet and activity levels in the child – would need to be considered.
What did the research involve?
The research used data from a population-based mother-child cohort study, known as the Southampton Women’s Survey (SWS). The SWS included more than 12,500 non-pregnant women aged 20-34 living in the Southampton area. These women had lifestyle and dietary assessments and body measurements taken at the time of recruitment to the study (between 1998 and 2002), and if they became pregnant, again at 11 and 34 weeks of pregnancy. At 34 weeks of pregnancy the women had blood samples taken for PUFA levels. They also completed food frequency questionnaires on their diet during the previous three months.
There were 1,987 births of single babies to women in the cohort. Children were followed up from birth onwards, including looking at their breastfeeding history. At three years their diet was assessed using a food frequency questionnaire. At four and six years children were also invited to attend detailed assessments of body composition, which included their weight, and a whole body scan which gave information on fat mass, lean mass and bone mineral content.
The researchers assessed the link between maternal PUFA levels during late pregnancy and the child’s body composition at four and six years. Looking at associations to see whether:
- maternal blood n-6 PUFA concentration (omega-6) was linked to offspring fat mass at four and six years
- maternal blood n-6 PUFA concentration was linked to offspring lean mass at four and six years
- maternal blood n-3 PUFA concentration (omega-3) was linked to offspring fat mass at four and six years
- maternal blood n-3 PUFA concentration was linked to offspring lean mass at four and six years
They adjusted their analyses for various factors, including whether the child was breastfed, child’s height and various maternal factors, including:
- mother’s pre-pregnancy body mass index (BMI)
- socioeconomic status
- smoking status
- walking speed in late pregnancy
- overall energy intake in late pregnancy
What were the basic results?
Of the 1,987 eligible mother-child pairs, 293 had full data available for analysis. After adjustment for the other factors measured, they found that late pregnancy levels of n-6 PUFA were positively associated with the child’s fat mass at both four and six years (so higher levels of n-6 PUFA were associated with higher levels of fat mass in the child).
However, pregnancy n-6 PUFA levels were not associated with lean body mass at either age. Nor were n-3 PUFA levels associated with either fat mass or lean mass at either age.
How did the researchers interpret the results?
The researchers concluded that the mother’s blood levels of n-6 PUFA during late pregnancy may influence the subsequent child’s body fat levels.
This study of 293 mother-child pairs finds that the mother’s blood levels of n-6 PUFA during late pregnancy influence her child’s body fat levels, with higher blood levels equalling higher child fat levels. The researchers say that n-6 PUFA, derived from plant oils are known to have an influence on fat development. Therefore, they say the observed association may suggest ‘prenatal PUFA exposure could be linked to risk of offspring obesity’.
However, though this study has merits, including a representative population sample and carrying out detailed assessment of both the mothers and children, it is difficult to draw any reliable conclusions from it.
The study adjusted for child height and various maternal factors during pregnancy, but even with this adjustment it is highly likely that the current body fat levels of the child are influenced by a number of factors, including the diet and activity levels of the child. These in-turn are likely to be influenced by the diet and activity levels of the child’s mother and other parent or family members.
Because of this, it is difficult to prove that the mother’s PUFA intake during pregnancy is associated with child obesity.
In fact, the study hasn’t actually measured obesity in childhood, it has just looked for associations between pregnancy PUFA levels and child’s lean and fat mass.
This study does not change current dietary advice for women during pregnancy.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.