In an age where people’s days are increasingly busy, finding the time to incorporate a healthy lifestyle is paramount. When the focus is prevention, healthy eating habits and regular physical activity are the key factors that contribute to the health of populations, especially with regards to chronic disease ^[1][2][1,2]. Over the past decades, there has been a real explosion in the worldwide prevalence of obesity [3], with a significant increase in the amount of research being published, with over 390,000 papers on this topic. Unfortunately, despite the numerous health programs, campaigns and wide dissemination of information focusing on prevention, things are not improving. The World Health Organization (WHO) predicts that the number of obese people will continue to increase steadily over the next years [4]. It is estimated that the number of overweight and obese adults by 2030 to reach 2.16 billion overweight and 1.12 billion obese individuals [5]. That is, obesity has become the 21st century’s epidemic, with more than 100 million euros spent on health care costs over a lifetime for 1000 people with morbid obesity [6].

When conventional treatments for obesity result insufficient, bariatric surgery (BS) has been recognized as an effective alternative for weight loss in subjects with severe obesity and has become one of the most common surgical procedures performed worldwide, according to recently published data [7]. Between all the different surgical techniques, laparoscopic Roux-en-Y gastric bypass (LRYGB) and laparoscopic sleeve gastrectomy (LSG) are the most widely used and have shown similar long-term results on weight loss and resolution of comorbidities [8]. In addition, something that is often overlooked, is weight loss before surgery, also known as preoperative weight loss. It is important due a number of factors, including control of fatty enlargement of the left lobe of the liver, which acts to obscure the operating field, and the mitigation of the risk of bleeding during the operation, therefore reducing postoperative complications and mortality [9]. Even with surgery, however, sometimes the weight loss is less than expected, or comorbidities do not improve ^[10][11][12][10,11,12]. This has led researchers to investigate potential strategies to overcome such issues.

It has long been demonstrated that adherence to a Mediterranean diet (MD) is associated with longevity, and a lower risk of suffering from obesity ^[13][14][15][13,14,15]. It is one of the most recognized healthy dietary patterns, characterized by a high intake of fruits and vegetables, legumes, nuts, whole grains, extra virgin olive oil, a moderate consumption of fish and poultry, with a low intake of red and processed meats, sugar-sweetened beverages, and processed foods. Unfortunately, to the best of current knowledge, very few studies have been performed with regards to the effects of a Mediterranean diet on bariatric surgery outcomes.

2. Mediterranean Diet Adherence before Bariatric Surgery

In order to evaluate the effects of a Mediterranean diet in patients undergoing LSG, a group of researchers conducted an 8-week study with 37 obese male patients ^[16][17]. Up to this point few researchers had begun exploring the effects of preoperative eating patterns on short-term and mid-term weight loss ^[17][21], and there had been no studies investigating a preoperative Mediterranean diet. Patient characteristics for this descriptive prospective cohort were obese men, with a body mass index (BMI) ≥ 40 kg/m², aged 25–65 years, married, living at home with their parents, or living with a partner in a stable relationship for at least 1 year.

An important observation to make, is that this was a Mediterranean-protein-enriched diet (MPED), having a macronutrient distribution of 30% protein, 25% fat, and 45% carbohydrate, as opposed to a more traditional Mediterranean diet, which has a distribution of 15% protein, 35% fat, and 50% carbohydrate ^[18][22]. Each participant followed a diet for 8 weeks and was asked to complete a series of assessments. The way in which researchers made sure that all participants consumed a similar diet was by developing four meal plans, one for every two weeks, which were composed of pre-defined fruits, vegetables, pasta, milk products, herbs and spices, as well as meat and fish.

Dietary assessments consisted of 3-day food records completed for 3 consecutive days, a daily food diary, and 72-h recalls conducted by a trained dietician. To ensure compliance, urine samples were collected biweekly and used with urinary ketone reagent strips to determine a total ketone score. Qualitative methods were also used to measure diet acceptability using a five-point Likert scale. Lastly, weight, visceral fat, body composition, liver size, and biochemical and metabolic patterns were measured before and after the 8-week intervention.

Before the intervention, all participants showed an excessive intake of saturated fats, mostly in the form of red meat, a high intake of cholesterol-rich foods, such as eggs and cheese, and an extreme consumption of sweet-carbonated beverages. They were also observed to have a low consumption of nutrient rich foods, such as fruits, vegetables, legumes, whole grains, nuts, olive oil and fish.

After 8-weeks of following a Mediterranean diet, the authors observed significant reductions (p = 0.01) in weight (−16.7%), visceral fat (−27.4%), left and right liver lobe sizes (−29.1% and 25.2%, respectively), and total fat mass (FM) loss (14.1%), without a significant reduction (p = 0.0960) in fat-free mass (FFM). Clinical parameters such as liver enzymes, and lipid profiles also showed improvements. The total ketone score was found to be highly correlated with the total percentage weight reduction at the end of the intervention. This study demonstrated that a Mediterranean diet might be beneficial for bariatric surgery candidates looking to lose weight while maintaining FFM.

3. Mediterranean Diet Adherence after Bariatric Surgery

When it comes to the effects of adhering to a Mediterranean diet both in the pre- and post-operative stage, in 2014 a group of Spanish researchers published a prospective observational study which evaluated the adherence of a Mediterranean diet in morbidly obese patients before and after undergoing sleeve gastrectomy ^[19][18]. A total of 50 patients, 44 women and 6 men, with a mean BMI of 50.4 kg/m² were included. Of these, 50% presented with dyslipidemia, 30% with hypertension, 28% with type 2 diabetes (T2D), 24% with osteoarthritis, and 16% with sleep apnea syndrome. The overall aim of the study was to analyze the association between Mediterranean diet adherence and postoperative weight loss, resolution of comorbidities, as well as cardiovascular risk factors.

The preoperative tests performed on each patient included abdominal ultrasound, upper gastrointestinal endoscopy, spirometry, and blood analysis, which also included nutritional parameters. Patients were prescribed a Mediterranean diet of 1200 Kcal preoperatively and were asked to follow itfor 2 months to achieve a weight loss of at least 10%—a requirement to proceed with the surgery. All patients completed the KIDMED test before the operation and 1 year after the intervention. The KIDMED test measures adherence to the Mediterranean diet, analyzing Mediterranean diet patterns such as daily consumption of fruits and vegetables, weekly intake of fish and legumes, as well as patterns contrary to the Mediterranean diet, such as the frequent consumption of meat, processed foods, and sweets. The test is composed of 16 questions, answered by a simple “yes” or “no”. A total score between 0–3 reflects poor adherence, between 4–7 moderate, and between 8–12 good adherence.

One year after surgery, patients had an average of 81.3% weight loss, a significant reduction in blood glucose levels, with a p value of 0.003 (mean decrease of 33.2 mg/dL), decreased levels of glycated hemoglobin, (p = 0.08), decreased triglycerides values (p = 0.001), and an insignificant decrease in low density lipoprotein (LDL) cholesterol with a significant increase (p < 0.001) in high density lipoprotein (HDL) cholesterol. Additionally, while the preoperative mean Mediterranean diet adherence score was 4.5, the postoperative mean score was 7, with a mean increase of 2.5 points (p < 0.001). This means that before surgery, only 6% of patients had a good Mediterranean diet adherence, a number that changed to 40% after 1 year (p = 0.02).

An inverse correlation was observed between KIDMED scores and weight loss, meaning patients who showed better adherence to the Mediterranean diet presented greater weight loss. The same was true for total and LDL-cholesterol, which decreased as KIDMED scores increased. On the same note, a direct correlation was established between KIDMED scores and HDL-cholesterol levels, where patients with higher scores had increased HDL values. Lastly, although not statistically significant, all comorbidities such as T2D, arterial hypertension and dyslipidemia were improved, with patients with better adherence showing greater improvements. This was the first study to evaluate adherence to a Mediterranean diet in patients with severe obesity both before and after bariatric surgery, and to analyze its effects on weight loss and comorbidities.

The second, and only other study which evaluated the effects of a Mediterranean diet after bariatric surgery was published in 2020 ^[20][19]. This was another prospective observational study aimed at assessing how changes in dietary food pattern and physical activity affect changes in weight, BMI, quality of life, and food tolerance after surgery. A total of 78 participants, 59 women and 19 men, aged between 18 and 66 years, with a BMI ≥ 35 kg/m2 were included. After surgery, researchers provided leaflets with general dietary recommendations on a healthy diet. Although the study measured both Mediterranean diet adherence and physical activity, for the purposes of this review, only outcomes related to a Mediterranean diet were explored.

Adherence to a Mediterranean diet, as well as weight, quality of life, and food tolerance were measured at baseline, 3 weeks before surgery, and at 3, 6, 9, and 12 months after. Mediterranean diet adherence was estimated using a validated 14-point method called Mediterranean Diet Adherence Screener (MEDAS), composed of 14 questions, which scores one if the answer adheres to a Mediterranean diet, and zero if it does not. Therefore, final scores range from 0–14 and are subdivided into: very low adherence (0–4), low adherence (5–7), medium adherence (8–11), and high adherence (12–14). Scores were dichotomized into an increase, a decrease, or maintenance of adherence to a Mediterranean diet.

At baseline, 59.2% of patients had a low or very low adherence to the Mediterranean diet. After a 12-month follow up, participants who increased adherence had a significantly higher weight loss (−48.7 kg vs. 43.9 kg), and percentage of excess weight loss (−71.2% vs. −63.1%) than those who decreased or maintained adherence (p = 0.036). This study also demonstrated that individuals who adhere to a Mediterranean diet after undergoing bariatric surgery present with greater weight loss than those who do not. However, a possible limitation of this study is that it did not use biomarkers to assess adherence, something that the above studies did.