12 Nutrient Transfer During Pregnancy

Sabine Zempleni

By week 10 – 12 the placenta has matured enough to connect the tiny fetus to the mother’s blood vessel system. The laguna space between maternal and fetal side of the placenta can be filled with blood and the efficient transfer of nutrients to the fetus can begin. The tiny 0.5 ounce fetus from week 12 can now grow into a 7.5 pound newborn.

 

The Main Points:

  • The placenta mediates between fetus and mother.
    • The placenta has similar nutrient transport mechanisms than the small intestine.
    • Some nutrients are transported along a concentration gradient
    • Some nutrients can be accumulated in the fetus even if the mother has a low intake
  • Nutrient needs start increasing during the second trimester reaching a peak during the weeks before birth.
  • Glucose is the main energy source for the fetus.
  • Maternal metabolic conservation measures provide the additional protein for fetal growth.
  • Fat is a building block for the fetus and not an energy source.
  • Selected micronutrients need special attention: Iron, iodine, vitamin D and folate.
  • Does a mother need to eat for two?

 

 

The Placenta Mediates Between Fetus and Mother 

Before you learn about how nutrients are transported from the mother to fetus, it is important to understand that the mother’s and embryo’s blood vessel system are not directly connected. The placenta separates and mediates between mother and fetus.

The placenta is a spongy, pancake shaped organ with a weight  proportional to the fetus. If the fetus is small the placenta will be small and vice versa.

On the maternal side—right side of the circular cross-section of the placenta in the image above—the maternal arterial blood squirts from jet-like structures into the laguna (IVS), literally the placental lake. You can imagine this like a hot tub. The laguna is the tub and the jets protrude into the tub and squirt blood into it.

On the fetal side, branched villi reach into this tub of blood. Those villi are lined with absorptive cells. The villi are very similarly constructed like the absorptive villi in the small intestine you may be familiar with.

The blood vessels inside the villi are collected into the umbilical arteries and veins. Twisted around each other are the veins and the artery that form the umbilical cord.

Here is an interesting terminology difference. In the mother the arteries carry oxygen rich blood and nutrients away from the heart, and veins carry CO2 and waste toward the heart. This is how the terms artery and vein are defined.

The same definition applies to the fetus. Leading away from the placenta towards the heart of the fetus is the umbilical vein. This vein carries oxygen and nutrients coming from the mother (not CO2 and waste as we would expect).

The two umbilical arteries carry CO2 and waste away from the heart of the fetus towards the placenta. Think about that for a minute because this terminology can be confusing.

On the fetal side, the villi absorb nutrients and oxygen from the laguna and release CO2 and waste products into the laguna. The maternal side releases oxygen and nutrients into the laguna and takes up the CO2 and waste products into the maternal blood circulation carrying it away to the lung, liver and kidney for removal.

These would be the first two functions of the placenta: Shuttling water, oxygen, and nutrients to the fetus and removing waste products the fetus’s metabolism produces.

The placenta is also producing and secreting hormones and enzymes. Hormones secreted by the placenta prompt the adaptions to the maternal anatomy, physiology, and metabolism we talked  about. A well-functioning placenta is necessary to maintain the pregnancy and enables optimal growth for the fetus.

The placenta acts as an imperfect immunological barrier separating the genetically different fetus from the mother’s immune system (the maternal immune systems reacts normally to foreign genetic material).

The placenta also secretes antivirals and activates the maternal immune system against  bacterial infection to protect the fetus. This doesn’t mean that the placenta is sealing the fetus off perfectly—some viral diseases, for example, the zika virus can be transferred.

Around week 16 the placenta starts transporting immune globulins from the mother to the fetus. This transfer increases at the end of the pregnancy.  At term, the fetus has a greater serum concentration of maternally derived IgG than does the mother. The IgG acts like an early vaccination protecting against diseases the mother is exposed to.

Next to the barrier function the placenta is able to synthesize fatty acids, cholesterol and glycogen for its own use and for the fetus.

 

The Placenta Has Similar Nutrient Transport Mechanisms Than the Small Intestine

The transport mechanisms for nutrients on the fetal side of the placenta are very similar to what you learned about the absorptive mechanisms in the small intestine.

Those mechanisms are:

  • Passive diffusion
  • Facilitated diffusion
  • Active transport
  • Endocytosis

 

Some Nutrients Are Transported Along a Concentration Gradient

Water, electrolytes, free fatty acids, steroids, fat soluble vitamins alcohol, oxygen, and CO2 can diffuse from the laguna through the endothelial cells on the fetal side to be released into the fetal vein.

For glucose the common insulin dependent GLUT transporter is integrated into the epithelium enabling facilitated diffusion of glucose.

Nutrients transported by diffusion and facilitated diffusion require a concentration gradient to be transported to the fetus. For example, only if the mother supplies sufficient amounts of glucose into the laguna so the glucose concentrations in the laguna are higher than on the fetal side can transport take place (as you will later learn fetal insulin regulates this process as well).

 

Some Nutrients Can Be Accumulated in the Fetus Even if the Mother Has a Low Intake

Amino acids, water, soluble vitamins, calcium, iron, and iodine require an ATP dependent transporter. This means that even if the mother is deficient in those nutrients the placenta can filter out those nutrients and accumulate them on the fetal side to a certain degree.

Phospholipids, lipoproteins, and immunoglobulins are taken up by the villi via endocytosis. VLDL and LDL bind to their respective receptors. The receptor mediates endocytosis, the engulfing of the large molecule by the cell wall.

 

 

Nutrient Needs Start Increasing During the Second Trimester Reaching a Peak During the Weeks Before Birth 

During the first weeks of pregnancy the fetus will require very little nutrients. When does the mother need to eat for two or is this ever necessary?

After all organs are mapped out, the fetal growth starts to accelerate after 16 weeks.

Interesting, is that the growing fetus does not only gain weight (See the bend in the curve around week 24 in the lower left graph above when growth speeds up?), but the body composition changes as well.

An adult human has a water content of 50 to 60 %. A fetus at 25 weeks of gestation is mostly water—88%. Secondly, a fetus at 25 weeks has very little body fat. Only 1 %.

Throughout the remaining pregnancy the body water comes down to 75 %, fat free mass increases, and during the last weeks of pregnancy the fetus accumulates fat deposits.

Along with fat deposits the fetus will now accumulate fat soluble vitamin stores as well as stores for some minerals.

The important part here is that the need for some nutrients and energy increases during the second trimester. Other nutrients such as fat-soluble vitamins don’t have an increased need until the final weeks of the pregnancy.

I know, I haven’t answered the Eating for Two question. I’ll get to it.

 

 

Glucose Is the Main Energy Source For the Fetus

The main metabolic nutrients for the fetus are glucose, and to a lesser degree, amino acids. The fetus is not able to use fat as an energy source. Long chain fatty acids are needed, but as building blocks for growing tissues only.

The fetus does not produce glucose. Since glucose is the main energy source for the fetus and is also used as a building block a constant flow of glucose from the mother is necessary to support optimal fetal growth.

During late pregnancy when the fetus has higher energy needs the mother will develop low insulin sensitivity (=physiological insulin resistance). Insulin dependent maternal tissues such as muscles and liver take up less glucose. That way maternal glucose blood concentrations remain elevated longer after a meal. This is the same mechanism you learned about in the T2D chapter.

This allows for a higher glucose concentration in the placental laguna. The placenta takes some of the glucose as a source of energy.

How much glucose is absorbed by the fetal side of the placenta is regulated by the concentration of fetal insulin. If fetal energy need increases, insulin locks to its receptor and  the GLUT transporter is activated. Facilitated diffusion of glucose across the epithelial cells takes place. If fetal insulin concentrations are low, glucose is used as the predominant energy source for the placenta.

Once the glucose is circulating the fetal cardiovascular system, about 55% are directed to cells for energy use and 45 % are used as building blocks for non-essential amino acids, ribose (DNA), and glycerol (triglycerides).

 

 

Maternal Metabolic Conservation Measures Provide the Additional Protein For Fetal Growth

Since the fetus and placenta are growing rapidly during the second half of the pregnancy, one would think that the mother should increase her protein intake significantly. This is not correct.

Most of the additional need for fetal and placental growth is covered by maternal metabolic adaptions. Amino acids that are transferred to the fetus stem from the maternal diet, conservation measures, and if protein intake is low from the mother’s lean body mass.

Again, the placenta diverts some of the amino acids for its own growth and maintenance. The remainder is actively transported to the fetus. Most of those amino acids are used for tissue synthesis and some for energy purposes.

Here is an interesting fact: In today’s society more protein is often considered better. Protein shakes are part of many people’s daily routine.

A pregnant woman needs to rethink here. While insufficient protein intake, especially when essential amino acids are missing, is connected to reduced fetal growth, high protein intake is connected to slowed fetal growth as well. Studies show that a high intake of protein supplements is connected to newborns that are small for their gestational age. Why? Not clear yet.

 

 

Fat Is a Building Block For the Fetus and Not an Energy Source

Sufficient essential fatty acids translate into higher birthweight and larger head circumferences. Low supply of essential fatty acids is connected to lower cognitive capabilities. On the other hand  too much essential fatty acids increase peroxidation and therefore will require more antioxidants to avoid oxidative damage.

At this point it is unclear if American women that have a low w-3 intake would benefit from a general fish oil supplementation during pregnancy. W-3 fatty acid supplementation in the form of DHA is also discussed in the prevention of preterm birth.

Increased transfer of fatty acids in general takes place during the last weeks of pregnancy. This fat is not used as an energy source, but to build the fetal adipose tissue.

 

 

Selected Micronutrients Need Special Attention: Iron, Iodine, Vitamin D and Folate

Most mothers in the US have reasonable micronutrient intake to provide the fetus with sufficient amounts. A few minerals and vitamins need to be highlighted because intake in the US is not always optimal.

The first mineral is iron. You already learned in the pre-conception section that iron deficiency should be addressed before pregnancy because iron stores take time to recover.

Severe iron deficiency in pregnancy impacts the development of the brain and nervous system. The consequences are learning disabilities, reduced language ability, reduced fine-motor skills, and limited emotional control.

Scientists also suspect that a low maternal iron status without full anemia might be connected with a more mild version of the consequences of inadequate iron supply. Here is the problem. It is difficult to link mild cognitive impairment and less emotional control directly to iron deficiency because many other factors can play a role as well.

Iodine has been a problem nutrient during pregnancy in the past and is still a problem in developing countries. Endemic iodine deficiency was prevalent in the landlocked regions of the USA, the Great Lakes, Appalachians, and Northwestern regions, known as the “goiter belt”. In this geographic area 26%–70% of children had clinically apparent goiter due to iodine deficiency. Starting in 1924 iodized salt can be purchased in the USA. Today iodine deficiency is rare in the USA.

If  a mother is iodine deficient the lack of iodine will lead to a goiter in the mother, but severe brain damage in the infant.  Sufficient amounts of iodine are necessary for the neurological development of the fetus. Deficiency leads to cretinism. The child will have severe mental retardation, loss of hearing and speech ability, short stature, and muscle spasticity.

If the mother has a suboptimal calcium intake, the fetus will not experience negative effects. When calcium blood levels fall below the normal level maternal calcium in the bone reservoir will be activated. In addition, the calcium absorption rate is increased. The negative impact will be experienced by the mother since she loses bone mass and this will increase the risk for developing osteoporosis later in life. Therefore it is important that pregnant women have an adequate calcium intake.

 

 

The main vitamins we need to look at are folate and vitamin D.

As you already know, adequate folate intake is essential for the development of the spinal column. Inadequate folate intake increases the risk for spina bifida and anencephaly. An infant with spina bifida might be born with severe disability depending on the severity. Infants with anencephaly will not survive.

Vitamin D deficiency or suboptimal status does not affect all women. Low vitamin D status is more pronounced in winter for all women, especially in Northern latitudes. Other groups that are at risk are women who cover their skin for religious reasons, have a dark skin, or are obese.

Americans have a sufficient intake of vitamin A, but high intake of vitamin A—not carotenoids—during the pregnancy is teratogenic and will lead to issues with neurological development of the fetus. Sources of high vitamin intake are accutane treatment for acne, multivitamins (prenatal vitamins only contain carotenoids, no retinoids), and liver consumption. Most Americans do not eat liver on a regular basis, but working with ethnic minorities this needs to be kept in mind. Liver is the storage site for vitamin A and naturally high in vitamin A. In addition animal feed is often supplemented with vitamin A and this causes the high concentrations you see in the image above for pork and chicken liver.

 

 

Finally, Does a Mother Need to Eat For Two?

So far you learned that during the first trimester and part of the second trimester the focus is much more on mapping out and developing organs. This process does require very little additional energy and nutrients.

During the second semester the fetus and the placenta start growing rapidly. The question is now how much energy and nutrients does this growth require?

Looking at the graphs above you will see in the top graph that the total energy expenditure is increasing during the second semester. You also see that the additional need for energy varies a lot from woman to woman.

Energy expenditure has several components, and looking at the resting metabolic rate—which includes the extras for pregnancy— and physical activity will give you a much clearer understanding.

You see that the RMR increases. By the end of the pregnancy the increases 10 – 20 % . But, at the same time the physical activity portion declines for most women. The decreasing physical activity portion of the energy expenditure cancels out some of the increase in RMR. Since physical activity varies widely in pregnant women the total energy expenditure will too.

Overall the extra need for energy is small. No mother needs to eat for two.

The energy recommendations are based on the additional tissue accumulation—fetus and placenta—during pregnancy. During the second trimester an energy increase of  340 kcal is recommended and during the third trimester this recommendation increases to 452 kcal per day. If you think in terms of food this is not even an entire meal but more a substantial snack.

 

The second question is if pregnant women have a disproportionally high need for micronutrients. If you look at two graphs below you will see that only a few micronutrients need special attention.

The two graphs below show you the recommended additional nutrient requirements for the third trimester in percent. The blue line is the additionally recommended energy. The 452 kcal would be roughly a 23 % increase if the woman ate 2000 kcal before pregnancy.

   

If we assume that the woman eats a healthy diet and adds some healthy foods worth 452 kcal then most nutrients would be covered by eating slightly more food. The additional nutrients needed is not disproportional for most nutrients with exception of

References: iodine, iron and zinc plus the vitamins B6 and folate. American women have more than sufficient vitamin B6 intake that covers this additional need easily. Folate, as you already learned, should be supplemented. Iodine can be supplied by iodized salt intake. The only true problem nutrient is iron. Pregnant women are prescribed pre-natal vitamins that contain iron and folate.

With the knowledge about fetal nutrient needs you will easily understand the nutrition recommendations for pregnancy. You will also be able to see why some popular eating patterns, such as the keto diet, are less than ideal for pregnant women.

 

Editor: Gabi Ziegler

NUTR251 Contributors:

  • Spring 2020: Eugene Baraka, Dario Henry, Eli Havekost, Kenyon Gaar
  • Fall 2020: Morgan McCain, Carly Schwager, Felicity Bowers

 

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