Busting the myth of 'eating for two'

It may soon be officially confirmed that women don’t need to ‘eat for two’ during pregnancy — and for that you can blame the fruit flies!

Researchers from Imperial College London have shown that the body adapts to absorb more energy from the same amount of food, with a hormone released during pregnancy triggering the intestine to grow dramatically and stimulating the mother's body to store more fat.

“Previous studies have shown that eating for two during early pregnancy is unnecessary. Our research suggests that this is because the digestive system is already anticipating the demands that the growing baby will place upon our body," says lead researcher Irene Miguel-Aliaga.

“We normally think of our internal organs as being a fixed size, but the fact is that they are not. They can grow and change, and we show that this is important for making babies," she adds.

The findings may also help to explain why some women struggle to lose weight after giving birth. The research suggests that if hormone levels fail return to normal after birth, a mother's intestine may remain abnormally large, so she will continue to extract extra energy from her food. “Some women find it difficult to lose weight after pregnancy," says study co-author Jake Jacobson. “We may now have found a biological reason for this."

The experiments were performed in fruit flies, and the team is hopeful that the results will translate to humans.

“Many of the fly genes that we studied exist in humans, so our results are absolutely relevant," says Jacobson. “Flies also utilise and store fat like we do, and their metabolism is controlled by similar hormones."

Previous studies show that the intestines of many mammals — such as mice, rats and cows — grow during pregnancy, but it has not been clear exactly why this happens. The new research, which was published in eLife, has shown that a fly hormone, called 'juvenile hormone', triggers the changes to the intestine and fat metabolism. Juvenile hormone acts in a similar way to human thyroid hormones, which regulate the body's energy demands.

“We know that in humans the levels of several hormones change during pregnancy, and that these changes can affect how our digestive system works," says Miguel-Aliaga. “We expect that these human hormones act in the same way that juvenile hormone does in flies; to resize the intestine and thus help the mother to extract more energy from her food."

Scientists had previously thought that a woman's appetite changed in response to the baby's increasing demands for energy. Miguel-Aliaga says her team's research shows that this is not the full story. The team has shown that levels of juvenile hormone begin to rise in female flies surprisingly early — immediately after mating, in fact. This tells the intestine to rapidly adapt so that it is prepared to meet the energy needs of the fertilised eggs.

The researchers know that these metabolic changes do not happen because the mothers are eating extra food. They analysed a strain of sterile flies that cannot produce eggs and therefore do not become pregnant or experience a prompt, if one exists, that might increase their appetite. They found that even these flies grew visibly larger intestines after mating.

The changes in metabolism appear in part to determine the success of fly pregnancy. “If we prevent juvenile hormone from changing the intestine, the flies produce fewer eggs. This hormone is key to the fly producing as many healthy eggs as possible," says Miguel-Aliaga.

In the long term, these metabolic changes may have more sinister consequences. In flies, juvenile hormone stimulates the intestine to grow by activating stem cells in the lining of the intestine. Stem cells have the potential to replicate an infinite number of times and can cause cancer in humans. Scientists know that pregnant women are more susceptible to some cancers. If the stem cells remain activated after pregnancy, they will continue to grow — and may ultimately form a tumour.

The researchers will now use mice — whose organs are similar to our own — to find out more about the functions of pregnancy-regulated hormones in humans