In my last several blog posts, I’ve been focusing on the fascinating and burgeoning field of microbiome research. As previously discussed, our diets and medication usage profoundly affect the composition and diversity of the teeming microbial communities that inhabit our gastrointestinal tract, which in turn profoundly affects our physical and psychological health.
But the human-microbiome interaction doesn’t just impact on you. If you’re a woman who is pregnant, or planning to become a mother, you need to know that the diet that you eat before and during conception also has a profound and long-lasting impact on the gut microbiome of your baby.
In particular, women who eat a high fat diet before and during pregnancy may be causing alterations in their developing babies’ microbiome that predispose their infants to diabetes and obesity, regardless of whether they themselves become overweight on this diet.
Until relatively recently, it was believed that babies were born sterile – in other words, that their first contact with bacteria was at birth. However, research in the last few years has established that the placenta (the organ that develops in a woman’s uterus during pregnancy, ‘housing’ the baby throughout pregnancy and providing oxygen and nutrients to fuel its growth) has its own microbiome.
Researchers believe that the placental microbiome ‘seeds’ the developing foetus with its first microbes, preparing it for the more intensive colonisation that occurs during birth and breastfeeding.
Although the placental microbiome is unique, it bears the greatest resemblance to the mother’s oral microbiome – the colonies of bacteria that inhabit the mouth and throat. It has somewhat less in common with the mother’s gut microbiome, and is very distinct from her vaginal microbiome.
How do bacteria from the mouth and gut set up shop in the placenta? Oral bacteria can enter the bloodstream, especially if the gums are inflamed (as is common during pregnancy due to hormonal and vascular changes), and travel to the placenta to colonise it. The gut membrane is also ‘leakier’ during pregnancy, allowing so-called transmigration of bacteria from the gut lumen (the ‘space’ inside the gut) into the bloodstream.
In addition, specialised white blood cells known as mononuclear cells have been found to transport gut bacteria to the mother’s mesenteric lymph nodes and mammary gland in late pregnancy and during lactation in mice, and almost certainly in humans too. So bacteria may travel to the placenta under their own system, or be intentionally delivered their by the mother’s immune system.
Incidentally, the discovery that oral microbiota can colonise the placenta may help explain the long-observed association between periodontal disease in pregnant women and an increased risk of premature birth. Periodontal disease is essentially a dysbiotic condition, in which the oral microbiome becomes unbalanced and dominated by disease-causing organisms.
The placental microbiome changes throughout the stages of pregnancy. It becomes less diverse but more unique to each individual, suggesting that a pregnant woman’s diet, lifestyle and other environmental factors may shape the bacterial ecosystem within her placenta – with profound implications for her developing foetus.
For example, there are significant differences in the placental microbiome of women who develop gestational diabetes mellitus (GDM). GDM is a type of diabetes resulting from severe insulin resistance that begins during pregnancy and resolves as soon as the woman gives birth, but leaves both her and her baby at increased risk of developing type 2 diabetes later in life.
The placental microbiome of women with GDM harbours more Proteobacteria and Alistipes and fewer Firmicutes than non-GDM placentas. Proteobacteria, as the name implies, preferentially metabolise protein, while Alistipes is a ‘bilophilic’ or bile-loving bacterial genus.
Both Proteobacteria and Alistipes are known to multiply in the gut microbiome of people who eat a high protein, high fat diet. (Bile secretion is increased by a fat-rich diet.) On the other hand, Firmicutes metabolise the carbohydrates in plants that are indigestible to humans: fibre, resistant starch, and non-starch polysaccharides, and consequently, Firmicutes numbers drop on a low-carbohydrate diet.
Another example: women who gain excess weight during pregnancy – regardless of whether they begin or end their pregnancy obese – have been found to have an altered placental microbiome, which correlates with an increased risk of premature birth.
How might weight gain affect the placental microbiome? Whereas oral and gut microbes primarily metabolise carbohydrates and amino acids as their fuel source, placental bacteria specialise in metabolising fats. Researchers have proposed that
“The placental microbiota may aid in efficient extraction of energy from circulating fatty acids and play a crucial role in supplying energy-yielding substrates to the fetus.”
An increased efficiency at extracting energy from fat and delivering it to the foetus is highly adaptive for pregnant women faced with food shortage. Unfortunately, in the modern environment, dietary fat is all too easy to obtain.
When female mice were fed a high fat diet (45% of calories from fat) from 8 weeks before they became pregnant, and continuing throughout gestation, their gut microbiomes were significantly different from those of mice who were fed standard lab chow (17% calories from fat) before and during pregnancy.
Specifically, their Firmicutes to Bacteroidetes ratio tipped in favour of Firmicutes, a pattern that has been associated with obesity in both non-human animals and humans.
Also, the animals’ gut microbiota overexpressed genes involved in the metabolism of fatty acids and ketone bodies, sulphur-containing amino acids (which are used in the production of bile), the breakdown of glucose (glycolysis), and the formation of glucose from fats and amino acids (gluconeogenesis).
In other words, the gut microbiota of mice fed a high fat diet before and during pregnancy became significantly more efficient at harvesting energy from food, which bodes ill for both the mother and baby mice.
These changes are all the more concerning in view of the fact that the calorie intakes of the two groups of mice were roughly the same, and the high-fat fed mice were not statistically heavier than the low-fat fed mice – a take-home point for women tempted to use a ketogenic diet to lose weight prior to becoming pregnant.
Similarly worrying outcomes were observed in macaques, primates whose normal gut microbiota is more similar to that of humans than that of mice. Female macaques who were fed a high fat diet (36% fat from animal fat, fish oil and corn oil) for at least 3 years before becoming pregnant developed dramatic shifts in their gut microbiome which were ‘inherited’ by their offspring. Even when the baby macaques were switched to a low fat diet after weaning, the maternal high fat diet-induced dysbiosis was not fully corrected.
Animals who were fed the low fat diet (13% fat from soy bean oil, but the same number of calories as the high fat diet) all tended to remain lean throughout the experiment whereas roughly two thirds of the macaques fed the high fat diet became obese and insulin resistant, and the remaining one third stayed lean and insulin sensitive.
However, all macaques fed a high fat diet showed similar changes in their gut microbiome, regardless of whether they were obese or lean, and all high fat-fed macaque mothers ‘passed on’ their dysbiotic gut microbiome to their infants. The researchers concluded
“Our data demonstrat[e] a role for the maternal diet, rather than maternal obesity per se, in shaping the offspring gut microbiome at 1 year of age.”
In summary, it is the mother’s diet that shapes her offspring’s gut microbiota and stacks the odds in favour of the offspring remaining lean or becoming obese, rather than whether the mother herself is lean or obese. Again, this implies that the use of a high fat ketogenic diet to achieve weight loss before or during pregnancy – even if successful – will increase the baby’s risk of becoming obese.
What does all this mean for human mothers, or intending mothers? If you want to doom your child to a lifetime of metabolic dysfunction and obesity, eat a high fat diet before and during pregnancy! If, on the other hand, you would like to gift your child with a health-promoting gut microbiome that will make weight management easy, and protect him or her from metabolic syndrome and type 2 diabetes, choose a diet rich in unrefined carbohydrate and relatively low in fat – that is, a wholefood plant-based diet.
Confused by contradictory diet advice? Need help to plan a preconception and pregnancy diet that is not just good for you, but sets your baby up for good health? Apply for a Roadmap to Optimal Health Consultation today!
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