Weight Loss Surgery FAQs

Your Body Mass Index (BMI) is the ratio of your weight (in kilograms) to the square of your height (in meters). For example, if you weigh 140 kg (22 stones) and your height is 1.75 meters, your BMI=140/1.75×1.75=45.7 kg/m2. You may be more comfortable to think about your weight in stones and pounds (1 stone = 6.35 kg), and your height in feet and inches (1 meter = 3 feet and 3 inches).

Use a BMI calculator

You can have healthy weight, overweight or obesity based on your BMI. Your BMI is healthy if it is 18.5-24.9 kg/m2. If your BMI is 25-29.9, you are overweight. You are obese if your BMI is more than 30. You have morbid obesity if your BMI is more than 40, or if your BMI is more than 35 and you have comorbidity [Link to What is comorbidity? FAQ]. If your ethnic origin is Asian or African, the BMI limits are lowered because you are likely to have higher genetic risks than Caucasians.

18.5-24.9 Normal weight
25-29.9 Overweight
30-34.9 Class I Obesity
35-39.9 Class II Obesity
40-49.9 Class III or Morbid Obesity
> 50 Super Obesity

Weight loss after bariatric surgery is talked about usually as ‘percentage excess weight lost’. You may see this written as %EWL. Your excess weight is the weight that is over and above your ideal weight. Your ideal weight is taken as the weight that would give you a BMI of 25. Your excess weight is calculated by subtracting your ideal weight from your actual weight. For example, if you are 5 feet 6 inches tall and you weigh 19 stones, your BMI is 43. For your height, your ideal weight is about 11 stones (this would make your BMI 25). So, your excess weight is 19 -11 = 8 stones. If you were to lose 4 stones, then you have lost 50% of your excess weight. If you lose 6 stones, then you have lost 75% of your excess weight, and so on.

Comorbidity means any medical condition that is caused by obesity. Common comorbidities are high blood pressure, diabetes, dyslipidemia (high cholesterol), heart disease, obstructive sleep apnea, asthma, fatty liver disease, acid reflux, joint problems, polycystic ovarian syndrome (PCOS), and depression. Also, obesity increases the risk and the aggressiveness of several types of cancer. Obesity shortens your life expectancy. For example, if you are a Caucasian woman aged 20-30 years and your BMI is more than 45, you are likely to lose about 8 years of life. If you are man in the same position, you will lose about 13 years. The UK National Bariatric Surgery Registry shows about 60% of men and 40% of women having bariatric surgery have 4 or more comorbidities. About 45% of men and 25% of women have diabetes. About 40% of men and 15% of women are on treatment for a breathing disorder called obstructive sleep apnea.

Bariatric surgery is the proper name for weight loss surgery. The word ‘bariatric’ is derived from the Greek word ‘bar’, which means weight. ‘Bariatric’ is the branch of medicine that deals with problems of overweight and obesity.

Diabetes, high blood pressure and lipid disorder (high cholesterol) are together called the metabolic syndrome. The metabolic syndrome increases your risk for heart attacks. Bariatric surgery is also called Metabolic Surgery because it can reduce the severity of the metabolic syndrome, or even cure this condition, and reduce your risk for heart disease.

Almost all medical conditions (called comorbidity)[link to What is comorbidity] caused by obesity are improved or even cured by bariatric surgery. About 3 in 4 patients with diabetes get normal blood sugar levels and are able to stop taking anti-diabetes medicines. About 2 in 3 people with high blood pressure and disordered lipids (high cholesterol) will get normal blood pressure and lipid levels, and will not need to take drugs. For others, the conditions may not disappear but the severity will reduce and fewer medicines will be needed. Also, substantial improvements are seen in obstructive sleep apnea, polycystic ovarian syndrome (PCOS), asthma, joint pains, sexual dysfunction and quality of life. Many women with infertility are able to become pregnant. Several studies have shown that obese people who have bariatric surgery live significantly longer than obese people who do not have bariatric surgery.

All weight loss operations reduce the working size of the stomach. This is called the restrictive effect of bariatric surgery. But, there is much more than restriction. With the gastric bypass and the gastric sleeve, there are changes in the levels of many hormones that control your hunger, fullness and blood sugar levels. These hormones are released from special cells in the lining of the small bowel and the stomach. Because of these changes in hormone levels, you can feel as if a switch has been turned off in your head. Your hunger is reduced and you feel fully quickly after eating. With the gastric band, the effect appears to be through the nerve endings in the wall of your stomach. When the band is properly inflated, stimulation of the nerve endings controls your appetite and gives fullness with small meals.

It is a common worry that bariatric surgery may cause malnutrition because food will not get absorbed. In fact, there is little or no malabsorption of carbohydrate, protein or fats after a standard gastric bypass, gastric sleeve or gastric band (the only truly malabsorptive operation is called bilio-pancreatic diversion with duodenal switch). But, you can get malnutrition if you do not eat properly or if you do not follow-up [link to What is follow-up? FAQ] with your doctor. After bariatric surgery, it is vital that you follow the dietician’s advice, take the prescribed nutritional supplements and have regular tests.


Please bear in mind that bariatric surgery is not a cure for obesity. No weight loss operation is a magic wand, and it is not an easy way out of the problem. You will have to work hard to get the benefit out of the operation. Think of weight loss surgery as a powerful tool for weight loss. Like any tool, it will work only as well as you use it and maintain it. You will have support from our specialist dietician to help you to change your eating habit. But, ultimately, the responsibility is yours. Apart from healthy eating, regular physical exercise is very important for losing weight and for maintaining weight. You should make the time and put in the effort to exercise properly. You may not achieve satisfactory weight loss if you do not put in the effort that is needed. In the longer term, you can regain lost weight if you do not continue to take care.

Lifelong follow-up is essential after weight loss surgery. For the first 2 years after your operation, your follow-up will be with the hospital and with your GP. After 2 years, the follow-up will be primarily with your GP, but either your GP or you can contact us at any time for advice. You can think of the follow-up care in 3 parts:

Advice about diet and life-style

You will be given advice by Mr Sarela, dieticians, nurses and other healthcare professionals. It is vital that you listen to the advice and discuss any problems or concerns. The dietician is a particularly important person. She will give you detailed instructions. Please keep in regular touch with the dietician. If you have gastric band, you will need follow-up for band adjustment [link to Band Adjustment on Gastric Band page].

Nutritional Supplements

After a gastric band, gastric sleeve or gastric bypass, you should take a multi-vitamin and mineral preparation daily. We recommend Forceval™, one capsule daily. With a gastric sleeve or gastric bypass, you need to take some additional drugs: calcium-vitamin D tablets and iron tablets daily, and vitamin B12 injections (every 3 months). For the first 4 weeks after the operation, you should take medicines as syrup or dissolvable tablets or chewable tablets only. Afterwards, you can take whole capsules or tablets. You will need to see your GP to get a prescription for these medicines. A detailed discharge summary will be sent to your GP, explaining the medicines and follow-up tests. Please make an appointment to see your GP soon after your operation. The drugs need to be taken life-long. The dosage may need to be changed, according to results of tests. Also, you may need other medicines, based on test results.


You will need to have blood tests regularly at your GP’s centre. After gastric sleeve and gastric bypass, the tests should be done 3-monthly during the first year, and then one-yearly. After gastric band, the tests can be done one-yearly from the start. The tests may have to be repeated more frequently if some problem is found. Detailed information will be sent to your GP to request these tests. Please contact your GP’s centre to make appointments. For more information about follow-up medicines and tests, see http://www.bomss.org.uk/bomss-nutritional-guidance/

You will need to take a special diet, called a liver reduction diet, for 1-2 weeks before your weight loss operation. The liver overhangs the stomach, and it is necessary to move the liver out of the way during weight loss surgery. The liver can get quite heavy in obese people. The purpose of the liver reduction diet is to make the liver lighter and easier to handle, and so make the operation safer. The liver reduction diet is rich in proteins and low in carbohydrates and fats, and it will deplete a substance called glycogen that is stored in your liver. You will be given written instructions about the liver reduction diet.

You will be admitted to hospital on the morning of the operation. With a gastric band, you may be able to go home in the evening or you may need to stay overnight in hospital. With gastric bypass and gastric sleeve, most people need to stay in hospital for 2 nights after the operation. There is no restriction on activity after the operation. You can take light exercise, within the limits of your comfort. There is no hard and fast rule about returning to work or driving. It is mainly common sense, and you should do what you feel you are able to do.

Care of bone health is really important after bariatric surgery, particularly after gastric bypass. There is increased risk of fractures after bariatric surgery. Bones are living, dynamic organs in the human body and bone health is critically dependent on calcium, phosphorous, vitamin D and parathyroid hormone. This section describes, in some detail, the role of each of these elements and their interactions.

What is vitamin D?

Vitamin D occurs in two forms: vitamin D2 (called ergocalciferol) and vitamin D3 (called cholecalciferol). Both forms are equally effective and collectively are called vitamin D. Over 90% of vitamin D activity in the human body is provided by vitamin D3. Uniquely amongst the vitamins, D3 is made in the skin: by the action of Ultra-Violet B (UVB) rays of sunlight on a naturally occurring skin compound, called 7-dehydrocholesterol. In order to function effectively, vitamins D2 and D3 are chemically converted in the liver into a compound called 25-hydroxy-vitamin D (also called calcidiol), which is then further converted, in the kidney, into 1,25-dihydroxy-vitamin D (also called calcitriol). Calcitriol is the metabolically active form of vitamin D in the human body.

What are the functions of calcium and vitamin D in the human body?

Calcium is an important mineral in the human body. Calcium is a vital component of bones. Also, calcium is necessary for the normal functioning of nerves, muscles and the heart. Blood calcium levels are controlled by vitamin D and by the parathyroid hormone (PTH). If blood calcium levels drop, there is increased release of PTH, which stimulates the re-absorption of calcium stores from bones into the blood stream.

Vitamin D is critical for the absorption of calcium from the intestines. For dietary calcium to be absorbed across the lining of the intestine, a special calcium-transport protein is necessary. Vitamin D is essential for the activation (transcription) of the genes that encode the calcium-transport protein. Without vitamin D, only 10-15% of dietary calcium is absorbed. Vitamin D is needed also to regulate the excretion of calcium from the kidneys; without vitamin D, a large part of the body’s calcium store will be lost in urine. Thus, Vitamin D plays a vital role in maintaining healthy bones. Throughout life, there is constant re-modelling of bone that is dependent on well-controlled fluxes of calcium and phosphorous.

What are the normal requirements of vitamin D and calcium?

Vitamin D is measured in International Units (IU). The normal daily requirement of vitamin D in a healthy, young adult is 400-600 IU. The requirements are higher for the elderly, obese and those with intestinal, kidney or liver disease. The daily requirement of calcium is about 1000 mg. The calcium requirement is dependent on adequate intake in diet. In contrast, most of the vitamin D requirement is provided by natural synthesis in the skin. Few food items contain vitamin D: oily fish (e.g. salmon, mackerel, tuna, hilsa), egg yolk, cod liver oil and Shitake mushrooms are the limited dietary sources of vitamin D.

What is vitamin D deficiency?

Vitamin D is tested by measuring 25-hydroxy-vitamin D (calcidiol) in a blood sample. 25-hydroxy-vitamin D levels may be reported in terms of either nmol/L or ng/ml and the diagnoses, corresponding to various ranges, are shown in the table below:

nmol/L ng/ml Diagnosis
<30 <12 Deficiency
30-50 12-20 Insufficiency
50-125 20-50 Normal

How are calcium levels in the body tested?

A blood test for calcium reports on the level of calcium that is circulating in blood. Calcium is carried in blood in combination with proteins and also in a free state (called ionized calcium); commonly, total calcium is tested and is reported in terms of either mg/dl or mmol/L. In adults, the normal range of total calcium is 8.8-10.4 mg/dl or 2.2-2.6 mmol/L (the reference values may vary slightly amongst laboratories).

Normal calcium in a blood test does not indicate adequate dietary intake or satisfactory bone stores of calcium. When calcium intake is insufficient, there is increased release of parathyroid hormone, which maintains normal blood calcium levels by increasing the re-absorption of calcium from bones. Thus, there could be considerable depletion of bony calcium, despite a normal blood calcium level. Calcium in bones is measured by testing bone mineral density, using Dual energy X-ray Absorptiometry (DXA; often called DEXA scan).

Why is there a risk of calcium deficiency and vitamin D deficiency after bariatric surgery?

The main sites for absorption of dietary calcium are in the upper small intestine (called the duodenum and jejunum). After a gastric bypass, food will not pass through the duodenum and the upper part of the jejunum; hence, calcium will not be properly absorbed. Calcium tablets (1000-1500 mg) should be taken regularly after gastric bypass surgery. With extra calcium intake, enough calcium should be absorbed from the lower part of the small intestine. Calcium carbonate is easily available in chewable forms and is better tolerated immediately after surgery; it should be taken with meals to improve absorption. Calcium citrate is the preferred for long-term usage because it is better absorbed in the absence of gastric acid.

There is a high incidence of vitamin D deficiency in the general population, irrespective of bariatric surgery. The main cause of vitamin D deficiency is inadequate exposure to ultra-violet B rays of sunlight because of darkly pigmented skin, clothing, atmospheric pollution and long winter months (in countries like the UK). Obese individuals need 2-3 times the normal daily requirement of vitamin D and are likely to have undetected vitamin D deficiency prior to bariatric surgery.

What are the consequences of calcium deficiency and vitamin D deficiency?

The main impact of calcium or vitamin D deficiency is on bones. In adults, vitamin D deficiency causes improper mineralization of bones, resulting in a condition called osteomalacia. Typical symptoms of osteomalacia are bony pain, mainly in the hips, pelvis, ribs and feet, and muscular aches. Also, deficiency of calcium and vitamin D can cause demineralization of bones, resulting in osteoporosis, with increased risk of fracture.

What tests are required to detect calcium or vitamin D deficiency?

Following gastric bypass and sleeve gastrectomy, blood tests for calcium, vitamin D and parathyroid hormone (PTH) should be done at regular interval. Raised PTH levels indicate a condition called secondary hyperparathyroidism, in which normal levels of blood calcium are maintained by increasing re-absorption of calcium from bone. Dual energy x-ray absorptiometry (DXA) may be done to keep check on bone mineral density.

What is the treatment of vitamin D deficiency?

Vitamin D deficiency is usually treated with Vitamin D3 (cholecalciferol). A blood test is then repeated to ensure that the 25-hydroxy-vitamin D level has come within the normal range.

Unlike carbohydrates and fats, the human body does not have a mechanism to store proteins. Hence, regular intake of a protein-rich diet is essential. The normal protein requirement is 30-60 grams per day. Bariatric surgery sometimes leads to intolerance for protein-rich food, resulting in insufficient protein intake. Regular assessment and counselling by a specialist dietician is very important to ensure adequate protein intake. If proper dietary advice is followed, protein malnutrition is rare. It is very important to prioritise food intake after bariatric surgery: the emphasis is on protein intake; carbohydrates and fats are of secondary importance.

Deficiency of iron is the main cause of anaemia after bariatric surgery (another common cause of anaemia is vitamin B12 deficiency). Anaemia means a decreased level of haemoglobin: the oxygen-carrying protein in blood. The commonest symptom of anaemia is fatigue from minimal exertion or even at rest. Iron is an important mineral in the human body. About two-thirds of iron is contained in haemoglobin; the remaining is in muscles and some enzymes. Iron in diet is in two forms: heme iron in meat and non-heme iron in plant foods e.g. lentils, beans, spinach, tofu. Non-heme iron is not as well absorbed as heme iron; hence, vegetarians need almost twice as much iron in their diet as compared to non-vegetarians. Healthy adults can absorb only 10-15% of dietary iron. Vitamin C enhances the absorption of iron; hence, iron tablets should be taken along with a vitamin C-rich drink e.g. orange juice. Tanins (present in tea) and calcium decrease the absorption of iron and should not be taken together with iron tablets.

For adults males, the recommended daily intake (RDA) of iron is 8 mg. For adult females in the menstruating age group, the RDA is 18mg; after menopause, the RDA is similar to that for men (8 mg). Iron intake is negatively influenced by low nutrient density foods (e.g. sugar sweetened sodas, deserts, potato chips), which are high in calories but low in vitamins and minerals. Basically, an unhealthy diet is likely to deficient in iron and, consequently, some obese patients may have anaemia even prior to bariatric surgery.

Iron absorption is facilitated by an acidic environment and occurs mainly in the upper part of the small intestine (duodenum and upper jejunum). Iron deficiency occurs in about 50% of patients after a gastric bypass because the food enters the small gastric pouch, which produces very little acid, and then passes directly into the lower part of the jejunum. Women in the reproductive age group are at highest risk of iron-deficiency because of the monthly loss of blood during menstrual periods.

In the human body, iron is stored within cells by a protein called Ferritin. With iron-deficiency, the body stores of iron get depleted before anaemia develops. Ferritin levels can be tested in blood and low ferritin (usually less than 15 micrograms/L) implies iron-deficiency. Hence, ferritin levels are tested in addition to the full blood count (FBC).

It is recommended that iron supplements should be taken daily after a gastric bypass. Most multi-vitamin & multi-mineral supplements contain some iron (e.g. one Forceval® capsule contains 12 mg of elemental iron) but that is not sufficient to prevent deficiency. Elemental iron is the amount of iron in a supplement that is available for absorption.

For treatment of iron-deficiency anaemia, 50-60 mg of elemental iron (contained in a 300 mg tablet of ferrous sulphate) should be taken twice daily for three months. The amount of iron absorbed decreases with increasing doses; hence, iron supplements should be taken in two or three equally spaced doses per day. Such doses of iron can cause gastrointestinal side effects like nausea, vomiting, cramps, diarrhoea or constipation.

The effectiveness of iron supplements is measured by the reticulocyte count (levels of newly formed blood cells) and blood levels of ferritin and haemoglobin. With iron supplementation in anaemic patients, the reticulocyte count begins to rise within a few days and the haemoglobin level rises after 2-3 weeks.

Iron supplements should be taken with caution because iron overload can damage the liver and heart, particularly in people with an underlying genetic condition called haemochromatosis (believed to affect about 1 in 250 people of Northern European descent). Haemochromatosis is usually not diagnosed until after organ damage has already occurred.

Vitamin B12 is necessary for proper formation of red blood cells and also for proper working of nerves. Vitamin B12 is naturally found in animal products but is not present in most plant foods. Vitamin B12 in food is bound to proteins and is released in the stomach by the action of acid and digestive enzymes. Vitamin B12 then binds to a substance called the Intrinsic Factor, which is a protein secreted by the lining of the lower stomach. The Vitamin B12-Intrinsic Factor complex travels to the lower part of the small intestine (called the distal ileum), where this complex is absorbed into the blood-stream.

Gastric bypass patients are prone to develop vitamin B12 deficiency because there is minimal acid, to cleave vitamin B12 from dietary protein, and because there is diversion of food away from the Intrinsic Factor-producing area of the lower stomach. With the sleeve gastrectomy, the dietary vitamin B12 will come into contact with Intrinsic Factor in the lower stomach; however, the effect of excision of the majority of the acid producing area of the stomach on B12 absorption is not known.

Deficiency of vitamin B12 is one of the main causes of anaemia after bariatric surgery. Anaemia means a decreased level of haemoglobin: the oxygen-carrying protein in blood. Vitamin B12 deficiency is characterised a form of anaemia called megaloblastic anaemia, in which the red blood cells become abnormally large. The commonest symptom of anaemia is fatigue from minimal exertion or even at rest.

In addition to anaemia, B12 deficiency may cause neurological symptoms, such as numbness and tingling in the hands and feet; difficulty to maintain balance, depression, confusion and poor memory can also occur.

Vitamin B12 levels are assessed by a blood test. In adults, values below 170-250 pg/ml (the precise value will vary amongst laboratories) indicate deficiency. The recommended daily allowance (RDA) for Vitamin B12 in adults is 2.4 mcg. Vitamin B12 supplements should be started within 6 months of bariatric surgery. The body’s ability to absorb oral vitamin B12 supplements is limited. Hence, intra-muscular injections (3-monthly) are the most reliable form of vitamin B12 supplementation.

Vitamin B1 (also called Thiamine) is important to support the activity of enzymes that are involved in the release of energy from carbohydrates. One of the main vitamin-B1-dependant enzymes is called transketolase (hence, vitamin B1 sufficiency is commonly measured by a blood test for the “transketolase activity co-efficient”). Vitamin B1 is important also for maintaining the covering (myelin sheath) of nerves and proper neurological function.

The body does not have a mechanism to store vitamin B1 and is dependent on an adequate daily intake. A daily multi-vitamin tablet is routinely prescribed, after any form of bariatric surgery, and should be taken regularly. The vitamin B1 contained in such supplement, along with that in a healthy, balanced diet that has been charted by a bariatric dietician, will be normally sufficient. Routine blood tests for thiamine activity are not necessary.

Mild deficiency of vitamin B1 may present with muscle pain, typically in the calves, after slight physical activity (because of accumulation of lactic acid) and tingling, numbness or weakness in the arms and legs (called peripheral neuropathy). Symptoms suggestive of mild thiamine deficiency can be easily corrected by taking two or three daily doses of a vitamin B-complex preparation.

Severe deficiency of vitamin B1 can develop if there is protracted vomiting after bariatric surgery, irrespective of the type of the operation. Severe deficiency can cause unstable gait, impaired consciousness and memory loss. Hence, it is vital that bariatric surgery patients should not ignore prolonged vomiting. Urgent hospitalisation, with intravenous administration of vitamin B1, and correction of any underlying mechanical problem (e.g. stomal stenosis with gastric bypass or slippage of gastric band) that is responsible for the vomiting, is necessary to prevent any permanent neurological damage. Glucose-containing intra-venous fluids will worsen thiamine deficiency and should not be used.

Obesity is a risk factor for development of gallstones [link to page on Gallstones]. Obese patients have altered proportion of bile salts and cholesterol in bile. Rapid weight loss after bariatric surgery causes further changes in the composition of bile. Previously silent gallstones can become symptomatic or new stones can form in the gallbladder. It has been estimated that about 1 in 4 patients will need treatment for gallstones after weight loss surgery.

Dumping refers to a set of unpleasant symptoms that can occur after gastric bypass, particularly if food choices are poor. Sugar, fat and fried foods are most likely to trigger dumping. In most cases, the symptoms of dumping occur within 30-60 minutes of eating (called early dumping): sweating, flushing, palpitations, tummy cramps, nausea and diarrhoea. Dumping occurs because the rapid entry of food into the small bowel, as a result of gastric bypass, triggers the release of various hormones (gut peptides). In the majority of patients, dumping is mild and may be beneficial because it is gives a disincentive to unhealthy eating. Dumping becomes less prominent with time. Avoiding liquid intake with meals and increasing intake of starchy foods, fibre and protein can minimize dumping. Rarely, gastric bypass patients may experience sweating, shakiness, loss of concentration or fainting at one to three hours after a meal. Such a presentation is called late dumping and is because of low blood sugar (called reactive hypoglycaemia). Taking a small amount of sugar, about one hour after the meal, is usually effective.

Constipation can be problem after any bariatric operation. It is usually because of insufficient intake of water and dietary fibre. Also, constipation can be a side-effect of calcium or iron supplements. Occasionally, laxatives may become necessary.

As weight is lost, stretched skin does not necessarily regain its tone. You can develop loose folds of skin. The lower abdomen (tummy), upper arms and thighs are common ares for the development of skin folds. For some people, skin folds cause only minor discomfort and no action is necessary. For others, larger skin folds can be a source of embarrassment and may cause poor hygiene or skin-fold infections. Plastic surgery may be required to deal with loose skin.

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