Many studies have shown that iron deficiency exists already pre-operatively in obese patients and more particularly in female bariatric surgery patients of childbearing age due to heavy menstrual periods. Even more, all patients after bariatric surgery face a decrease in the production of hydrochloric acid, subsequently contributing to a lower iron uptake, since the Fe3+ to Fe2+ conversion is reduced, due to the resection of 75-80% of the stomach and the administration of PPIs (proton pumps inhibitors). Futhermore red meat, which is rich in iron, is less well-tolerated after BS ^{9, 10, 11}. As a result iron deficiency often is seen among bariatric patients who have undergone RYGBP as this procedure leads to iron malabsorbtion and reduced food intake but also after VSG due to lower acid content in the stomach. As a result, iron deficiency is one of the most common nutritional deficiencies after bariatric surgery Table 1^{10, 12, 13, 14, 15}. In 2014, Alexandrou et al, in their pilot study have reported that in 40 patients undergoing BS, 30% of them developed iron deficiency and 7% ferritin deficiency after a 4-year follow up ¹⁶ . During the same year, Van Rutte et al, found that in 200 patients who underwent VSG, one year post-operatively, 37 of them (18,5%) had iron deficiency and in 16 of them (8%) ferritin levels were below the normal range. Before the operation the prevelance of iron and ferritin deficiency was 38% and 7% respectively¹⁷ . In 2012, two studies reported iron deficiency from 0% - 10.5% 1 to 5 years postoperatively ^{18, 19} .

The main cause of anemia in pregnancy is iron deficiency. Pregnancy anemia can be aggravated by various conditions such as uterine or placental bleedings, gastrointestinal bleedings, and peripartum blood loss. In addition to the general consequences of anemia, there are specific risks during pregnancy for the mother and the fetus such as intrauterine growth retardation, prematurity, feto-placental miss ratio, and higher risk for peripartum blood transfusion ^{3, 7, 20, 21}.

Vitamin B12 deficiency is also very common before and after bariatric surgery (Table 2). Its preoperative prevalence ranges between 3-18%^{22, 23, 24, 25}. After bariatric surgery the absorption of the B12 may be impaired at different levels depending on the type of procedure. Absence of acid environment following gastric bypass surgery, inadequate secretion of intrinsic factor and malabsorption result in the decrease of vitamin B12 absorption in the terminal ileum leading to B12 deficiency ³. Two studies have found that cobalamin deficiency after BS was 11,5% and 5%, after a one year and a four year follow-up, respectively ^{16, 17} . Moreover, there is another study which has reported that in 41 patients having udergone BS, vitamin B12 deficiency was 20% post-operatively ²² . In addition, Pech et al, found that in their two different studies, the need for supplementation after BS procedure in a total of 182 patients was between 42 and 46,3% during the first two postoperative years, and vitamin B12 levels remained clearly within the lower third of the reference ranges during this period, which means that without the supplementation, a vitamin B12 deficiency would likely have occurred ^{23, 24} . In three more studies, vitamin B12 deficiency fluctuated between 0% and 17,2% one year after BS ^{18, 25, 26} and Gehrer et al, reported that cobalamin deficiency after a 3-year follow-up in 50 patients was 18% ²⁷ . Apart from the obvious hematologic effects, low concetration of cobalamin may result in elevation of serum homocystein. Hyperhomocysteinemia is known to be directly related to early loss of pregnancy. Moreover, inappropriate levels of B12 during pregnancy due to lower intake can result in neurobehavioral disorders and anemia in the infants ^{28, 29}.

Folate absorption occurs along most of the small intestine, mainly in the duodenum and in the jejenum, and depends on pH level, mechanisms of the intestinal wall which have to do with nutrient’s transport, and saturation points ^{30, 31}. After bariatric surgery,follic acid can be absorbed throughout the entire intestinal tract, through mechanisms of adaptation ¹⁵. Therefore, folate deficiency after BS ensues primarily due to low intake of folate-rich food, such as legumes, lettuce and green leafy vegetables, and/or due to vitamin B12 malabsorption ^{32, 33} . If maternal folate stores are insufficient prior to conception, the risk of folic acid deficiency is higher and is associated with fetal neural tubes disorders (NTDs), while also the risk of an adverse pregancy outcome such as a preterm delivery and birth defects in the following pregnancy increases. ^{20, 42}. According to the results of the reviewed studies (Table 3), pre-operative folate deficiency has been estimated to exist in up to 54% of the subjects, whereas recent studies from the USA support that this prevalence is much lower, about 0-6% ^{34, 35, 36}. Five studies found that the prevalence of follic acid deficiency one year after BS ranges between 0% and 15% ^{17, 19, 25, 26, 37} . Furthermore, Ruiz-Tovar et al. found that in 30 BS patients the prevalence of follic acid deficiency was 0% after a two-year follow-up. On the same wavelength, Toh et al. observed in their study that none of the patients developed folate deficiency one year postoperatively, too²⁶. On the other hand, Gehrer et al, found the highest prevalence of this deficiency in the literature, which was 22% in fifty patients 3 years after the BS ^{27, 38}. Finally, Aarts et al. and Hakeam et al. found in their studies that folate deficiency occurred in 15% and 9,8% of their patients respectively, one year after VSG ^{25, 37}.

The mechanisms which affect calcium metabolsim afterbariatric procedures are not clearly understood. Lower Vitamin D - mediated absorption of calcium due to Vitamin-D deficiency, changes in HCl (hydrochloric acid) secretion and intolerance of calcium-rich food after bariatric procedures are pathways which can influence calcium nutrient status. Moreover, hypovitaminosis D may occur due to less sun exposure, increased uptake of Vitamin D in adipose tissue and vitamin D malabsorption caused by bypassing segments of the intestine where it is naturally absorbed. Because of the inverse relationship between the levels of serum Vitamin D/serum calcium and the levels of serum PTH, vitamin D deficiency provoking secondary hyperparathyroidism is frequently observed post-operatively. However, serum calcium levels are usually within the normal ranges ^{11, 39, 40, 41, 42}. Four different studies with one year follow up reported that the prevalence of vitamin D deficiency fluctuated between 39 and 70,4 % and true secondary hyperparathyroidism occurred in 18 and 39% of the patients. However, it was found that calcium levels were within the normal ranges for almost all the patients in all these studies ^{17, 18, 25, 43} In addition, one study with 40 individuals reported that four years after BS, hypovitaminosis D was found in 56,3% of the patients and elevated levels of PTH in 43,3% of them ¹⁶. Furthermore, on the same wavelength was also the study of Saif et al., who found that 5 years after, BS vitamin D deficiency and hyperparathyroidism were present in 42% and 58,3% of 82 patients respectively. However, calcium levels were within normal values ¹⁹. On the other hand, Moize et al, did not find any vitamin deficiency D, 5 years after BS in 61 patients although, it should be noticed that the prevalence of hypovitaminosis D, 4 years after the procedure, was 44,4% ⁴⁸. The prevalence of calcium deficiency was 12,5% and hyperparathyroidism was present in 87,5% of them. ⁴⁴ Furthermore, in another study, the prevalence of Vitamin D deficiency and elevated PTH was only 3,3% for both markers in 30 patients 2 years after BS, whereas, pre-operatively it was 96,7% and 20% respectively³⁸. Finally, Gehrer et al, reported in their study that hypovitaminosis D and elevated levels of PTH was found in 32% and 14% of 50 patients, who underwent BS, after a 3-year follow-up, with calcium levels within normal ranges for all of them ²⁷ . Data regarding the frequency of vitamin D deficiency, calcium deficiency and hyperparathyroidism after bariatric surgery is presented in Table 4.

Pregnancy is characterized by many hormonological and metabolic changes ⁴⁵ and vitamin D deficiency is relatively common, especially in obese women ^{46, 47} . Bodnar et al. documented a significantly higher percentage of vitamin D deficiency in obese pregnant women. Bariatric surgery also can lead to a postoperative vitamin D deficiency in pregnant women ⁵⁵. Maternal vitamin D deficiency is associated with a variety of maternal and fetal complications, such as pre-eclampsia, gestational diabetes and a higher risk of asthma and schizophrenia in childhood ⁴⁸. The exact mechanism by which vitamin D can facilitate the onset of pre-eclampsia is unclear ^{49, 50}.. Another important observation is that maternal vitamin D levels can influence the bone status of the newborn. Viljakainen et al. showed a significant smaller tibia mineral content in the newborns of pregnant women with lower levels of 25(OH)D during pregnancy ⁵⁰ .

Zinc is a trace element which is only second to iron in concentration in the human body ⁵¹. Zinc plays a pivotal role in the cellular metabolism, being essential for more than three hundred enzymatic reactions ⁵². Prior to bariatric surgery, serum zinc levels in obese women are generally lower than in healthy ones ⁵³. Zinc deficiency after bariatric surgery should always be considered, especially for pregnant women, since many factors could contribute to it. These include bypassing intestinal segments of zinc absobtion duodenum and jejunum), intolerance to zinc-rich sources (red meat, fish), reduced gastric acid production (required for zinc absorbtion and bioavailability) and bacterial overgrowth (due to alterations of the intestinal tract) ^{54, 55, 56, 57}. In addition to these, zinc levels decline by about 30% during pregnancy. Low levels of zinc have been associated with premature deliveries, low birth weight, abnormal fetal development and neurological defects such as spina bifida. Cases of zinc deficiency during lactation are accompanied by skin rashes or dermatitis, often in combination with failure to thrive and irritability ^{58, 59, 60}. Zinc deficiency is not invariably reported after bariatric surgery (Table 5). Reportedly, it was calculated approximately 5%, one year after BS ¹⁷ . Hair loss, as a sign of zinc deficiency, was observed in 30% of the 43 individuals who underwent BS, one year postoperatively ⁴³ . Moreover, Moize et al, found that 5 years after BS, zinc deficiency was 12,5% in 61 patients, while 4 years postoperatively, in the same study, zinc deficiency was 47,6% ⁴⁴ . Furthermore, Gehrer et al, reported that zinc was below the normal ranges in 34% of the fifty patients who underwent BS, after a 3-year follow-up period ²⁷ and Salle et al, found in their study that zinc deficiency was 18,8% one year after BS ⁶¹. In addition, one study found that zinc deficiency was 14% two years after BS ²³, which was in agreement with another study which reported 14,3% zinc deficiency five years after the operation ¹⁹ .

Since magnesium is an essential trace element for optimal metabolic function, magnesium deficiency may be a risk factor not only for neurological and cardiovascular complaints but also for metabolic syndrome ^{62, 63, 64, 65}. Futhermore adequate intake of the mineral is needed for normal embrionic and fetal development ⁶⁶. Magnesium deficiency is not reported as commonly as other nutritional deficiencies and accordingly there are only sparse data about postoperative hypomagnesemia (Table 6). One study, found that magnesium deficiency was present in 3% of 200 subjects having undergone BS (versus 2% preoperatively) one year after the procedure ¹⁷ . Furthermore, there are two different studies which agree that magnesium deficiency did not occur after BS ^{19, 22} , whereas Moize et al, reported that hypomagnesemia was observed in 12,5% of 61 patients, five years after BS ⁴⁴ . Finally, Coupaye et al, reported that cramps were observed in 3% of 43 individuals who underwent BS⁴³ . Although COPPER: Copper as a nutrient is essential in many enzymatic reactions and is involed in red cell production and function of the nervous system ^{76, 77}. It is absorbed in the stomach and the proximal duodeum ^{78, 79}. Patients undergoing malabsorptive bariatric surgery are at risk of impaired copper status due to hypoacidity in the remnant stomach pouch and bypass of the duodeum⁸⁰. Deficiecy seems to be rare after BS. The only data we found refers to Amar’s et. al. study in which they found that there was no deficiency in 41 patients after VSG ²². Although it is uncommon, clinicians should be vigilant in its early detection as iron and copper deficiency often coexist, especially in pregnancy. As a result, hypocupremia is often misdiagnosed until serious neurological symptoms, anemia and heart enlargement occurs ⁸¹.

There are no reports referring to chromium or vanadium deficiency in pregnant women after bariatric surgery.

it is not common, maternal magnesium deficiency has been associated with premature labor, small-for-gestational-age (SGA) neonates, pre-eclamsia and the pathogenesis of the sudden infant death syndrom (SIDS). Tayaka et. al., showed a significant lower level of Mg concentrations in umbilical cord blood in SGA neonates compared to that of control group ^{66, 67, 68}. Nevertheless, further studies should investigate the impact of Mg deficit in fetuses.

The above mentioned research data have shown that obese women in reproductive age suffer quite often from several nutritional deficiencies, even prior to bariatric surgery, and there is a high risk of aggravation of these defeciencies after either RYGBP or VSG for various reasons. Pregnancy can further amplify some of these nutritional deficiencies either by increasing the demand or by decreasing the intake ⁹⁹.Therefore, life-long supplementation of vitamins and trace elements is highly recommended after bariatric procedures ^{10, 100, 101, 102}. For pregnant women the importance of routine nutritional screening is heightened. They should be closely monitored and clinicians should be vigilant for the detection of any deficiency that may occur, in order to prevent adverse outcomes, maternal or fetal. Table 11 lists the nutrient supplements which are recommended for patients after BS, for women in normal pregnancies and during pregnancy after BS. These suggested supplementations should be regarded as general recommendations, since individual requirements may vary depending on the results of frequent testing that may detect a state of deficiency or deficiency symptoms.

Further testing should be conducted for the evaluation of more nutrient markers, namely magnesium, PTH, copper and other water-soluble vitamins ^{10, 103} . Overall, in the management of obesity, teamwork has proved to be beneficial, and a comprehensive group of well-trained professionals is vital for the efficient and effective nutritional care of the patient.bariatric surgery is associated with several nutritional deficiencies. As a result, it is recommended that a thorough, customized nutrition assessment should be conducted for every woman of reproductive age, potential candidate for bariatric surgery, pre-operatively, in regular post-operative intervals, and even more intensively during pregnancy. The most common deficiencies include iron, folate, vitamin B12, vitamin D and calcium deficiency. Most women should be supplemented even before conception and especially during pregnancy . Furthermore, many other nutrients, including vitamins of the B-complex (thiamine, pyridoxal), other fat-soluble vitamins (A, E, K) and minerals (copper, zinc, selenium), although rarely deficient, should be tested periodically, to ensure their levels remain within normal range.