Inflammatory bowel disease (IBD) is a chronic debilitating inflammatory disorder of the gastrointestinal tract characterized by a relapsing and remitting course. The etiology of IBD is incompletely understood, but includes an imbalance between pro-inflammatory and anti-inflammatory signaling arising from genetic susceptibility and environmental triggers 
. The two main subtypes of IBD are Crohn’s disease (CD) and Ulcerative colitis (UC). CD can affect the entire gastrointestinal tract from mouth to anus, with transmural inflammation affecting the full thickness of the bowel that can lead to fistulization, abscess formation, stenosis, and bowel obstruction. UC universally involves the rectum with variable involvement of the colon and does not involve the small bowel or upper digestive tract. The inflammation of UC is restricted to the mucosa, except in fulminant disease. The natural history of IBD is characterized by disease flares of variable duration and severity, with periods of remission. The prevalence of IBD is increasing globally with data from 2017 suggesting that more than three million people in the United States and Europe are living with the condition, and nearly seven million are affected worldwide 
IBD is often diagnosed in young adulthood or middle age, resulting in a large burden of disability-adjusted life years that is now improving as more effective treatments become available 
. Advances in immunotherapy have improved outcomes, but many of these medications are expensive, have undesirable side effects, and are not always available. Even with effective therapy, the risk of disease progression or developing complications requiring surgery remains high, with a ten-year surgical risk of 46.6% and 15.6% in CD and UC, respectively 
Malnutrition is a frequently under-recognized complication of IBD despite its high prevalence ranging between 20 and 85% [5,6]
. Although undernutrition and malnutrition are often used interchangeably, a recent guideline from the European Society for Clinical Nutrition and Metabolism (ESPEN) defines malnutrition more broadly as “a state resulting from lack of intake or uptake of nutrition that leads to altered body composition (decreased fat free mass) and body cell mass leading to diminished physical and mental function and impaired clinical outcome from disease” 
. Malnutrition can thus include those with normal or even elevated body weight.
In the case of IBD, malnutrition can result from a number of different mechanisms (see ) including decreased oral intake, medication-related nutrient interactions, malabsorption, gastrointestinal nutrient loss, bile salt wasting, surgical resections, active inflammation, small intestinal bacterial overgrowth, chronic dehydration with resultant renal insufficiency, micronutrient deficiency, and metabolic bone disease. Malnutrition may present differently in CD compared to UC, and can often be predicted based on the anatomical location and severity of disease. Small bowel involvement in CD may lead to greater protein-energy malnutrition and micronutrient deficiencies over time, whereas UC patients tend to present with rapid nutritional decline during an acute flare or hospitalization [8,9]
. Patients with fistulizing CD or those who have undergone bowel resection are at a particularly high risk of malnutrition [10,11]
Figure 1. Multifactorial mechanisms of malnutrition in inflammatory bowel disease (IBD).
Sarcopenia is a nutrition-associated disorder characterized by reduced muscle mass and strength. There are multiple contributors to sarcopenia in IBD, including chronic inflammation, malnutrition, and physical inactivity 
. Sarcopenia is present in approximately 40% of patients with gastrointestinal conditions 
. Patients with IBD who are affected by malnutrition have up to 60% reduced muscle mass compared to healthy matched controls, and the presence of malnutrition increases the risk of adverse outcomes 
. Given the increasing prevalence of obesity among patients with IBD, elevated body mass index (BMI) frequently coexists with low muscle mass, termed sarcopenic obesity. Sarcopenia in IBD is associated with functional decline and a high risk of disability and mortality 
There is a wealth of data pertaining to malnutrition in hospitalized patients as a whole, and while some IBD-specific data exist, this area is still a growing field of interest. The prevalence of malnutrition is high among patients admitted to hospitals in general, with studies demonstrating a prevalence of up to 45% [16,17]
. A large study by Tobert et al. found that only 4% of patients in American academic centers were given a diagnosis of malnutrition, suggesting significant under-recognition by clinicians [18,19]
2. What Are the Clinical Implications of Malnutrition in Hospitalized IBD Patients?
Malnutrition in hospitalized patients is associated with prolonged length of stay, higher readmission rates after discharge, greater rates of complications, infections, and increased mortality [18,20,21]
. Additionally, the nutritional status of patients often worsens during hospital admission for a variety of reasons including anorexia due to illness, lack of home support, and hospital protocols placing patients NPO for imaging and procedures. While these orders may be well-placed at the time, prolonged unintentional continuation of iatrogenic lack of feeding contributes to “post-hospital syndrome” [22,23]
. Malnutrition places a high burden on the healthcare system, with the overall costs of hospitalized malnourished patients being up to 34% higher than those of well-nourished controls 
Specific to IBD, malnutrition is disproportionately higher in those admitted to hospitals [11,24]
. Malnutrition in IBD has been shown to be an independent risk factor for prolonged hospital admission, increased mortality, admission after emergency room presentation, increased risk of infections, greater need for urgent or emergent surgery, and the development of venous thromboembolism during hospitalization [11,25,26,27,28]
. Malnourished IBD patients receiving immunosuppressive agents may be at the greatest risk for opportunistic infections 
Patients with severe IBD frequently have restricted oral intake due to their symptoms. Long-term avoidance of specific foods was reported to be as high as 77% in a recent study and can lead to micronutrient deficiencies even in the absence of weight loss 
. When IBD patients are admitted to a hospital, dietary restrictions happen frequently for imaging and endoscopic testing. In patients who are already malnourished, the negative implications on nutrition status are amplified, especially if hospital admissions are frequent or prolonged.
Evidence suggests that identifying malnutrition and introducing a nutritional intervention can improve outcomes in hospitalized patients as a whole [30,31]
. While there are limited data regarding the impact of nutritional interventions specifically in hospitalized IBD patients, supervised interventions with nutrition-trained health professionals are likely to improve outcomes and carry minimal risk of harm. Therefore, screening and, when appropriate, interventions for malnutrition, should be undertaken in all hospitalized IBD patients.
3. How Can Clinicians Diagnose Malnutrition in IBD?
Defining the malnutrition construct as it applies to IBD remains challenging, largely due to a lack of validated tools for its diagnosis. The prevalence is high, but estimates are variable due to use of different tools. An ideal gold standard would include elements of history, physical examination, anthropometric measurements, laboratory markers, body composition, and assessments of physical and mental functioning. While such a comprehensive assessment would capture the spectrum of malnutrition in IBD, it is not practical and could not be universally applied. In clinical practice and even in research settings, malnutrition in IBD is usually diagnosed using nutritional assessment tools (NAT); however, many of these are not yet comprehensively validated in IBD.
Various NAT are available and validated in general medical conditions, including bedside global assessment, bedside muscle measurements, bedside body composition analysis, and cross-sectional measures of body composition. Bedside global assessments include elements of clinical history, diet history (including food exclusions, restrictions), physical examination, and laboratory values. The Subjective Global Assessment (SGA) is one of the most widely used bedside global NAT, and has been well validated in general populations 
. In IBD, the SGA shows significant associations with length of stay in hospitals; however, assessments missed a large proportion of patients who had decreased body cell mass as determined by bioimpedance analysis 
Bedside muscle measurements include hand grip strength (HGS), mid-upper arm circumference (MUAC), and mid-upper arm muscle circumference (MUAMC). HGS has been shown to be an effective, quick, and convenient parameter to predict functional status and muscular health in CD patients compared to healthy controls 
. HGS correlated well with the strength of other muscle compartments with good reliability and reproducibility, and can be utilized in hospital and community settings for IBD patients 
. MUAC is an easily obtained anthropometric measure of the right upper arm at the midpoint between the tip of the shoulder and tip of the elbow. MUAMC is more complicated and requires additional steps, as it incorporates MUAC and tricep skinfold measurements into a formula to determine the amount of muscle and bone in the upper arm. Mijac et al. identified that both MUAC and MUAMC were significantly lower in IBD patients compared to control subjects 
NAT focused on identifying altered body composition include bioelectrical impedance analysis (BIA), which can be conducted at the bedside, or radiologic tools such as dual-energy x-ray absorptiometry (DEXA), computer tomography (CT), and magnetic resonance imaging (MRI). Body composition analysis estimates lean body mass and can be used to determine the presence of sarcopenia. The practical use of DEXA and BIA are limited in a hospital environment due to extra resources including equipment and time, or potential adverse effects such as radiation. Currently, body composition analysis tools are primarily being used for research. While CT and MRI are frequently performed on patients with IBD for assessment of their disease, software for calculations of body composition from such imaging is not universally available. Thus, with resource or time constraints, HGS and MUAC are more portable and easily obtained in a hospital and outpatient environment compared to DEXA or BIA, which require specialized equipment. However, clinicians may not have the tools and time to perform these bedside measurements, which are likely most useful when taken serially.
ESPEN criteria to diagnose malnutrition require either a low BMI of <18.5 kg/m2
or the combined finding of unintentional weight loss with reduced BMI (<20 or <22 kg/m2
in subjects younger and older than 70 years, respectively) or reduced fat free mass index (FFMI) (<15 kg/m2
for females and <17 kg/m2
for males) 
. Fat free mass (FFM) is often calculated using BIA or DEXA with FFMI obtained by dividing FFM/height2
. The American Society of Parenteral and Enteral Nutrition (ASPEN) criteria for malnutrition require two or more characteristics including insufficient energy intake, weight loss, loss of muscle mass, loss of subcutaneous fat, and localized or generalized fluid accumulation that may sometimes mask weight loss or diminished functional status as measured by HGS 
However, ESPEN and ASPEN criteria may not capture all components of malnutrition in IBD, especially micronutrient deficiencies that may arise due to malabsorption, selective avoidance of food groups, or high losses. Until a gold standard for malnutrition in IBD is created, we recommend that a diagnosis of malnutrition in IBD not require a low BMI nor unintentional weight loss. We suggest if an IBD patient has a minimum of two criteria below, a diagnosis of malnutrition should be considered. These include food restrictions/avoidance, active luminal disease, symptoms of nausea/vomiting/diarrhea/poor appetite beyond one week, unintentional weight loss >5% in 3 months, HGS < 20% percentile based on age/gender, or low FFMI < 15 kg/m2
for females and <17 kg/m2
for males 
. These broad criteria aim to identify malnutrition in IBD and capture evidence of malnutrition that may not always be reflected via weight loss or low BMI alone.
4. What Tools Can We Use to Screen All IBD Patients for Malnutrition?
While a formal nutrition assessment in all inpatients would appear to be the optimal approach, this is not feasible due to limited resources. Nutritional screening tools (NST) are rapid and simple evaluation tools that can be completed by any healthcare team member, and even the patient. Screening tools are designed to detect risk for protein and energy malnutrition, and/or predict whether malnutrition is likely to occur under present or future conditions 
. A positive NST result should prompt further investigation through validated NAT ideally administered by nutrition professionals. Major nutritional guidelines suggest that all patients admitted to the hospital should undergo screening for malnutrition 
. ESPEN 2020 guidelines suggest that all newly diagnosed IBD patients should be screened for malnutrition 
. In practice, this does not always occur due to under-recognition of malnutrition by healthcare professionals, lack of standardized routine screening protocols, self-perceived lack of knowledge and skills, lack of validated tools, and the lack of assignment of responsibility [40,41,42,43,44]
Familiar and validated NST include the Malnutrition Universal Screening Tool (MUST) and Nutrition Risk Screening (NRS-2002). MUST was initially developed for outpatient settings where serious confounders of the effect of malnutrition are relatively rare 
. However, many institutions utilized MUST in hospitalized patients with good inter-rater reliability 
. NRS-2002 was designed to detect the presence or risk of malnutrition in hospitalized settings by assessing disease severity as this may increase nutrition requirements 
Both the NRS-2002 and MUST factor in BMI, amount of weight loss in a specified time, any reduced nutritional intake, and assessment of whether the patient is acutely ill. Unlike MUST, the NRS-2002 incorporates an age adjustment score of ≥70 years old, outlines specific medical conditions to stratify disease severity, and quantifies ranges of reduced food intake in a one-week span. Kondrup et al. outline the specific scoring parameters of these two screening tools 
. A recent multicenter randomized controlled study that identified hospitalized patients screened positive with NRS-2002 who received nutritional intervention found that these patients had a reduction in mortality, as well as improvements in functional status and quality of life 
. A study by Raslan et al. showed that NRS-2002 was able to predict unfavorable clinical outcomes best in hospitalized patients 
. Meanwhile, other studies found that MUST has good predictive validity for mortality and length of stay for hospitalized elderly patients 
A 2019 systematic review explored various NST used in IBD patients including MUST, NRS-2002, Malnutrition Inflammation Risk Tool (MIRT), and Saskatchewan IBD-Nutrition Risk (SaskIBD-NR tool) 
. The systematic review supported the association of NST and NAT with relevant outcomes, but the heterogeneity called for further studies before an optimal tool could be recommended. To date, there are only two studies assessing NST in hospitalized IBD patients which both used NRS-2002 [50,51]
. Takaoka et al. outlined how NRS-2002 significantly predicted hospital length of stay but did not predict need for surgery 
. Two outpatient IBD studies were successful in screening for malnutrition using patient-administered MUST compared to healthcare provider MUST screening [52,53]
. The SaskIBD-NR tool is an IBD-specific outpatient screening tool validated against a Registered Dietitian (RD)-led global nutrition assessment that does not capture disease severity. This tool was specifically designed to consider food restriction or elimination diets, and micronutrient deficiencies frequently observed in IBD patients 
. MIRT incorporates BMI and unintentional weight loss over three months, and unlike other screening tools, incorporates the serum inflammatory marker c-reactive protein. Jansen et al. discussed how MIRT showed significant associations with CD-related days in hospital, number of flares, complications, and CD-related surgeries 
We recommend that all IBD patients with a new diagnosis, flare, or complication associated with their IBD, including hospitalization, infection and surgery, be screened for malnutrition. Patients with active disease have the highest risk of malnutrition, especially in newly diagnosed IBD 
. Identifying ideal and/or validated NST for IBD patients, both for the inpatient and outpatient setting, remains a priority. Until then, using a simple yet well-validated NST with high sensitivity is most important in the acute care setting 
. While MIRT and SaskIBD-NR show promise for future IBD nutrition screening, further validation and inter-rater reliability are required before they can be suggested for both outpatients and inpatients. Given the success of hospitalized in-patient malnutrition screening in other populations that led to nutritional interventions and reduced patient mortality and readmissions, we suggest using either the MUST or NRS-2002 to screen hospitalized IBD patients [46,48]
Hospitalized IBD patients screened for risk of malnutrition should have comprehensive nutritional assessments involving a dietitian and a nutritional management plan that is well-developed. Focus groups highlighted that physicians do not screen for malnutrition frequently enough and miss opportunities to intervene early in malnourished IBD patients 
. Given the constraints in time and resources, care teams should involve a dietitian within 24 h of admission if screened for moderate to high risk of malnutrition for nutrition assessment and intervention.