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A study published in JAMA Network Open looks at the Planetary Health Diet and body fat in foetuses.
Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:
“This is a report of fetal growth and development outcomes in women of mixed ethnicity in the United States. The authors estimated dietary intake from a food frequency questionnaire and calculated score giving adherence to the Planetary Health Diet (PHD) which is predominantly a plant based eating pattern. The main findings were that mothers whose had a high PHD score gave birth to infants that were heavier, had large head circumferences and had more body fat. The authors suggest the higher body fat in the infants from mothers who have a high dietary PHD score might be undesirable and lead to a a greater risk of obesity.
Limitation
“This is not a controlled trial but an observational study. The dietary assessment was made in early pregnancy and not later in pregnancy when the fetus deposits most adipose tissue. It is uncertain whether sufficient adjustments were made for maternal frame size given the large variation known to exist between different ethnic groups. There were no data provided on the proportion of infants with macrosomia (usually defined as a birth weight over 4 kg) which is associated with gestational diabetes.
Interpretation
“It seems to be wrongheaded to suggest that the accumulation of fat in the fetus prior to birth is harmful. Fat accumulation in the fetus has a biological advantage because it provides insulation reducing heat loss from the body, it is also an important energy reserve in the immediate postnatal period when fetal metabolism is adapting from nutrition via the placenta to milk. Fetal adipose tissue is enriched in long-chain polyunsaturated fatty acids which and are needed for normal brain and visual development in the post-natal period.”
Prof Kevin McConway, Emeritus Professor of Applied Statistics, Open University, said:
“I think the interpretation of these research findings needs considerable care. There are important limitations in what the study can tell us.
“I’ll comment on the limitations where my expertise as a statistician is relevant. I’ll also mention other limitations in areas where my own expertise doesn’t reach, but where information from scientists in other areas would be very useful (in my view).
“The research paper starts by mentioning child obesity in the US. I will mention here, though, that the researchers’ conclusion that some aspects of the mother’s diet suggest “potential implications for offspring metabolic health” don’t necessarily mean that fetuses who have more fat in certain parts of their bodies will become obese as they grow up. There are many other aspects of metabolic health than being obese. But my own expertise doesn’t allow me to comment more on this point.
“The researchers used data from a cohort of pregnant women in the US. The cohort was put together with a different primary purpose, not specifically to examine how mothers’ diets might be associated with how their babies grew on the womb. But data on diet in the early part of pregnancy was collected for a subset of the participating mothers, and that’s what allowed this new research to be conducted. During pregnancy, fetal growth was measured up to five times for each pregnancy, using ultrasound.
“For this particular study, the researchers calculated a score for how closely each mother’s diet matched the Planetary Health Diet (PHD), a dietary recommendation published originally in 2019.
The PHD is largely plant-based but (optionally) includes some foods of animal origin. However, the amount of red and processed meat that can be included is limited, as are processed foods generally, added sugars and some starchy vegetables.
“The researchers then looked for associations between the score for the mother’s closeness to the PHD, and measures of the fetus’s size and growth during pregnancy. For some, but far from all, of the fetal measurements, the researchers found a correlation between the measurement and the mother’s score comparing her diet with the PHD.
“Generally the statistical aspects of the research are well conducted. However, this is an observational study, and single observational studies always leave room for doubt about what causes what.
“The problem is that the women, who fell into different dietary groups in relation to the PHD, also differed in terms of a lot of other factors (as shown in Table 1 of the research paper, for example). It’s possible that the mother’s diet during pregnancy was the cause, at least in part, of the associations between diet and the fetal size measures. But it’s also possible that the differences in fetal size measures are actually caused by differences in one or more other factors, and that those other factors just happen, independently, to be correlated with diet.
“The researchers did make statistical adjustments for several possible factors relating to the mothers (including age, ethnicity, education, marital and job status and BMI before pregnancy), and for the baby’s sex. But one can never be sure that everything relevant has been adjusted for – possibly there was no data available on a relevant factor.
“So we just can’t be sure that the correlations between the mother’s diet in relation to the PHD, and the fetal measurements, arise because the diet causes fetal development to change. It’s plausible that that is the explanation, or at least part of it, but the true explanation of cause and effect could be something different.
“(The researchers acknowledge this in the paper, in the section on Strengths and Limitations, using the jargon term “residual confounding”.)
“I can’t comment on how appropriate or accurate the measures of maternal diet or of fetal size using ultrasound are – that’s beyond my expertise. I will point out, though, that although the so-called 2-dimensional fetal measures (lengths of some bones, and circumference of head and abdomen) were obtained for most of the fetuses included, that wasn’t true for most of the 3-dimensional measurements such as arm and thigh fat volumes. It seems to have been more difficult to make those measurements in some cases. Therefore the conclusions about those 3-dimensional measures will be subject to greater statistical uncertainty, generally.
“The researchers are, rightly, careful to point out in their conclusion that some of the differences they found “should be considered exploratory in nature”, and that “they may not be clinically meaningful and are hypothesis-generating rather than indicative of clinical outcomes.” Because of that, they recommend further study.
“It’s also important to note that the researchers can’t go beyond saying that their findings “suggest potential implications [of adherence to the PHD] for offspring metabolic health.” They don’t say what those implications might be, and you’d had have to ask about that to a scientist with different expertise than mine. The very first sentence of the paper mentions child obesity, but I don’t think there’s any implication that the finding that fetuses whose mother adhered more with the PHD had more fat in certain parts of their body will result in fatter children.”
Dr Katie Dalrymple, Lecturer in Nutritional Sciences, King’s College London, said:
“This prospective cohort reports associations between early-pregnancy adherence to the Planetary Health Diet and fetal body composition. The findings are interesting but should be interpreted with caution. Maternal diet was self-reported and measured only once during the first trimester, which may not capture changes in intake later in pregnancy and could introduce exposure misclassification and selection bias.
“There are also limitations in the analytical approach taken. Several confounders included in the models are known to be correlated, and some continuous measures have been simplified into categorical variables. This reduces statistical power and undermines confidence in the robustness of the results. Furthermore, important confounders, known to influence both diet and fetal composition (such as smoking) have not been included. Although some associations reached statistical significance, they lack clear clinical significance. As the authors acknowledge, further research is needed. In the meantime, I would not advise pregnant women or those trying to conceive to change established healthy plant-based dietary patterns based on this single observational study.”
Prof Dimitrios Siassakos, Professor in Obstetrics and Gynaecology, UCL, said:
“This is an interesting study that shows the importance of studying the effect of diets before widespread implementation, particularly during pregnancy. A key limitation is the reliance on ultrasound-derived measurements, which have significant limitations and quite often fail to identify foetuses at risk of complications. Novel approaches with MRI may be able to identify fetal biometry, including fat deposition, much more accurately.
“It is important to study diets robustly – but this study has additional limitations:
“Women who adhered to the diet were more likely to be older; demographics were different between groups with different adherence level; diet and adherence was self-reported which might not be reliable; reporting of adherence might be influenced by demographics rather than reflect true differences in diet and nutrition; diet and adherence were measured at the end of the first trimester, and fetal growth measured in the second and third trimester would correlate with nutrition at that (later) stage, which might be significantly different from the first trimester depending on how women respond to pregnancy changes – the placenta notoriously influences metabolism, and placental hormonal production increases significantly in the 2nd and 3rd trimester.”
‘Adherence to the Planetary Health Diet Index and Fetal Body Composition’ by Priscilla K. Clayton et al. was published in JAMA Network Open at 16:00 UK time on Wednesday 17 December 2025.
DOI: 10.1001/jamanetworkopen.2025.44153
Declared interests
Prof Tom Sanders: “Relevant COI: I was Chair of the British Nutrition Foundation Report on Nutrition and Development: Short and Long term consequences for Health.
Full list:
I have received grant funding for research on vegans in the past. I have been retired for 10 years but during my career at King’s College London, I formerly acted as consultant for companies that made artificial sweeteners and sugar substitutes.
I am a member of the Programme Advisory Committee of the Malaysia Palm Oil Board which involves the review of research projects proposed by the Malaysia government.
I also used to be a member of the Scientific Advisory Committee of the Global Dairy Platform up until 2015.
I did do some consultancy work on GRAS affirmation of high oleic palm oil for Archer Daniel Midland more than ten years ago.
My research group received oils and fats free of charge from Unilever and Archer Daniel Midland for our Food Standards Agency Research.
I was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain.
Member of the Science Committee British Nutrition Foundation. Honorary Nutritional Director HEART UK.
Before my retirement from King’s College London in 2014, I acted as a consultant to many companies and organisations involved in the manufacture of what are now designated ultraprocessed foods.
I used to be a consultant to the Breakfast Cereals Advisory Board of the Food and Drink Federation.
I used to be a consultant for aspartame more than a decade ago.
When I was doing research at King’ 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.”
Prof Kevin McConway: “I have no relevant interests to declare.”
Dr Katie Dalrymple: “I have none to report, although I did work in the food industry for Danone between 2012-2016 (I did their grad scheme and worked in sales, marketing and medical affairs – mainly on the medical/nutricia products).”
Prof Dimitrios Siassakos: “Co-investigator of the Mibirth study using MRI.”