Particularities of the salt and water balance during pregnancy

Pregnancy is a particularly vulnerable phase in human development. Many influences can be very beneficial and others seriously detrimental at this time. Nutrition is such parameter with bivalent potential. The following report will explain how a "balanced" nutrition, i.e. one that is adapted to the needs of the pregnancy will improve the well-being of the mother and the growth of the foetus - and how, on the other hand, bad nutritional counselling can cause damage.

Nutrition is of major importance for the salt and water balance in the human organism. Among other things it influences the volume and the composition of the entire extra cellular volume (ecv). An important part of this ecvis the intravasale blood volume, that is the circulating blood. Therefore suggesting that the quantity and composition of the blood are of importance for the well-being of the pregnant woman and the development of the foetus. There is constant exchange between the circulating liquid volume(blood) and the other extra cellular volume - the so called interstitial liquid volume. These volumes are parted by the walls of the capillary blood vessels which are not permeable to the corpuscular componentsof the intra-vasale volume(blood cells).

Adaptive change in the mother's organism

During pregnancy major changes occur concerning the volume and composition of the ecv, particularly in its intra-vasalecomponent. On the whole the ecv-volume rises by 4 to 6 litres. About 1,250 ml of this are due to the circulating plasma volume, therefore rising from a value before pregnancy of ~  2,600 ml to about 3 850 ml. It is interesting that in the case of a pregnancy of twins this increase in plasma volume is + 1,960 ml, in the case of triplets it is + 2 400 ml and in the case of quadruplets + 2,500 ml. In the last case the plasma-volume just about doubles. It becomes clear that there is a feedback between the requirements of the pregnancy (number of foetuses) and the expansion of the plasma volume.

The blood plasma is the liquid part of the blood. 90 % of it consists of water, with the remainder being electrolytes, carbohydrates, fats (lipids), amino acids etc.. In the electrolytes the cations are dominated by sodium (Na⁺) and the anions by chloride (CL^-). The concentration of Na⁺ ions per litre of plasma is 142 mmol and that of potassium (K⁺) a mere 5 (!) mmol. Equally dominant is chloride with 102 mmol/l in comparison to the other ions of the plasma.

This simplified calculation already shows the great importance water and sodium chloride have for the consistency (homeostasis) of the ecvand especially of the circulating plasma volume. This aspect holds a particular significance during pregnancy.

There is good reason why the plasma volume in graviditate increases markedly more than the cellular blood components. In case of non-pregnant women the latter comes to about 1,400 ml. In graviditatethe following increases are observed: for a single pregnancy +400 ml, for twins +730 ml, for triplets +950 ml, for quadruplets +1,400 ml erythrocyte volume. Despite these absolute increases, the haematocrite drops. The hematocrite is the proportion of erythrocytesto the total blood volume. The norm value for non-pregnant women is 42 % vol (SI units 0.42). At the end of pregnancyit reaches, after a continuous decline, 35-36 % (SI = 0.35-0.36).

Flow properties and oxygen transport capacity of the blood

With this important change comes a major improvement in the flow properties (rheological properties). The higher the hematocritethe higher the viscosity of the blood and the worse its capacity to flow, i.e. the higher the rheological resistance. This causes bad conditions for the perfusionof the capillariesand another important problem can occur:

With low hematocritevalues of about 30-34 % the oxygen transport capacity of the blood is much better than at hematocritevalues of e.g. 40 % vol or more. It is interesting that the hematocritecontinuously drops from values of e.g. 40-42 % vol before pregnancy to values of 34-46 % vol (SI: 0.34 %-36 %) during the course of pregnancy. This benefits the micro-circulation where material and oxygen are exchanged. Together with the hematocritethe blood's viscosity is lowered, a change which is beneficial for the perfusionof small and very small vessels. All in all pregnant women in the 2nd and 3rd trimester benefit most from this haemodilution (blood thinning). It is physiological and has nothing to do with an anaemia that would need treatment. Of course knowledge of the natural adaptation in the intra-vasale volumeis necessary if the lowered haemoglobin valuesin the 2nd and 3rd trimester are not to be interpreted wrongly. It certainly must not be corrected by blood infusions.

Preeclampsia, eclampsia

Preeclampsia is a disease during pregnancy with the symptoms: oedema, proteinuria(protein in the urine) and hypertension. Frequency is about 10% of all pregnant women. In case of eclampsia (which has become extremely rare these days in Europe) convulsions occur. The cause of pre-eclampsia is still unknown. The disease is connected with an increased perinatal mortality.

Nutrition low in sodium chloride during pregnancy?

In the article published in 1991, E.A.P. Steegers et al. admit: "In the Netherlands, obstetricians, general practitioners and midwives generally continue to prescribe low salt diets as the prophylaxis of hypertensive disorders in pregnancy. Their use was prompted by the belief that excessive retention of sodium and water is the main cause of preeclampsia and eclampsia in particular ... " (Dr. J. Obstet. Gynaecol. 98 (1991) 980 - 987).

Surely the recommendation of a diet low in sodium chloride with regular fruit and rice days was (and is) not only given abroad. This is why it is worth looking deeper into this subject:

The older etiological idea was governed by toxic (toxin = poison) from the foetus. This is why in the English speaking world the terms "toxemia" or "toxicosis"were used for a long time. Equivalent to the "poisoning theory" were views according to which the disease was caused by food. At the beginning of the 20th century the idea was born that the decisive cause of hypertension and oedema during pregnancy and eclampsia was the sodium chloride content of the food. It is still – if the prenatal practice is viewed without prejudice – the guiding line today in many places. Therefore a hypothesis has kept its acceptance for more than a century although it has never been exactly proved! It is based on the observation that an increased salt content of food increases oedema and a reduced salt content reduces them. Presumably H. Cramer was the first, in 1906, to recommend salt restriction for treating prominent oedemain graviditate and as a prophylaxis for eclampsia. Known as the pioneer of the low sodium chloride diet, however, is the Dutch obstetrician De Snoo (1877-1949). His publicising skills made the philosophy about the danger of sodium chloride for pregnant women be carried around the whole world and became the dominant doctrine. Already in 1913, he had published a hypothesis according to which a low-salt diet was a priori beneficial for the pregnancy. In De Snoo's opinion oedema never are a physiological feature of pregnancy but an expression of excessive salt and water retention and always pathological, the reason being a significantly reduced tolerance to sodium chloride.

Even when De Snoo was not employed himself with the prevention of preeclampsia, his recommendation to withhold sodium chloride from pregnant women was still applied.

The approach which is in absolute contrast to De Snoo's are the publications of the English gynaecologist Margaret Robinson (1958), who observed an amelioration of the preeclampsiasymptoms when additional sodium chloride was given to normally salted food. Several international observations and studies (some experimental) followed observing the effects of nutrition either rich or low in Na⁺. From tests measuring the heart and blood-circulating systemand the renin-angiotensin-aldosterone systemit can be told that low sodium chloride nutrition turn against the physiological adapation during pregnancy and is therefore harmful. For example Na⁺ restriction leads to an increase of the peripheral resistanceof the blood vessels. This is either the result of an increased sensitivity of the vessels or of the growing concentration of pressoricactive substances (e.g. angiotensin II). This suggests an activation of the renin-angiotensin-aldosterone system. The volume the heart transports per contraction or per minute is reduced when Na⁺ is restricted, as is the blood plasma volumeand the total blood volume. All these changes are judged as harmful in gravitate.

In 1972 Atkinson commented on the bad influence of a reduction of the total blood volumefor preeclampsiaand eclampsia. His concept was that only a sufficient (=adequate) supply of sodium chloride taken with about 3 litres of water per day can build a total blood volumeappropriate for pregnancy.

Most recent diet recommendations aim in the same direction, as the one published in the Technical Bulletin 179 of the ACOG of 1993: "There is no clinical benefit in restricting sodium intake during pregnancy, and there is the potential for harm." The sufferers self-support group "AG Gestose-Frauen e.V. " voices in all its publications its opposition to restriction of sodium chloride and even recommends additional NaCl to be taken.

All in all one needs to come to the following conclusion considering the aspect of sodium chloride: Na⁺ restriction is neither sensible during normal pregnancy nor in the case of preeclampsia. This is why it has to be rejected today. Among the dangers of a sodium chloride restriction are reduction of the circulating blood plasma volume (hypovlaemia), haemoconcentrationand consecutively a worsening of the flow properties of the blood and an increase in the rheological resistance and increase in the peripheraltotalresistance (vasoconstriction). These are notable harmful consequences for pregnancy. Whether or not, however, additional sodium chloride (see above) can improve an existing hypovolaemiaand haemoconcentrationwith its negative effects for the micro-circulation, relative oxygen transportcapacityand flow properties of the blood (rheology) remains to be proven in studies. First clinicalimpressions of additional NaCl in the women's hospital in Suhl are, however, positive.

Results:

1. The old idea about the alleged advantages of low sodium chloride diet during pregnancy must finally and for ever be given up.
2. Low sodium chloride diet is dangerous both for the pregnant woman and for the unborn baby.
3. Rather food rich in proteins and always salted to taste should be recommended.
4. Whether or not additional salt is sensible has to be decided by further studies. First results in the women's hospital in Suhl are positive.
5. When a low salt diet is replaced by sufficiently salted food the desire to consume water increases which should be encouraged by nutritional counselling.

translation by Dr. Ute Clackson, revised by Sabine Kuse