**2. Searching Methods and Results**

#### *2.1. Searching Methods*

To answer the three aims of this narrative review, the existing literature was screened in an unsystematic way by three reviewers (O.E., K.B., and H.-J.D.). Each author used three online databases; PubMed, Web of Science, and Google Scholar and ensured the search was conducted no later than the 1 August 2022. Three different groups of search terms were used for each of the three study aims. Search terms can be found in Table 1. Additionally, the authors conducted forward and backward tracking of the included articles to identify studies via the reference lists or citations. Both experimental and observational studies published in English were included in the review. Studies published prior to 2000, abstracts, posters and flyers, conference proceedings, and unpublished papers were not included in the study. Studies where the full text was unavailable were also excluded.

In addition, an unsystematic search of two databases: Medline and CINAHL, as well as the European Society of Clinical Nutrition and Metabolism (ESPEN) guidelines and Practice-based Evidence in Nutrition (PEN), was conducted in July 2022 by K.B. The aim of this search was to identify the existing guidelines relating to the nutritional management of chronic pain. Search terms included the following MESH headings ("practice guideline or guideline"; "diet, food and nutrition") and key words ("chronic pain, fibromyalgia, arthritis, back pain, musculoskeletal pain, and migraine disorders").

#### *2.2. Searching Results*

Once the search on the PubMed, Web of Science, and Google Scholar databases was completed, a total of 1400 articles were identified. In total 112 eligible articles were identified and included in the paper. The full texts of two eligible studies were not available and the corresponding authors did not respond to the request for the full text [15,16]. The findings were analyzed based on the three study aims: the nutrition-related health of people experiencing pain, the underlying potential mechanisms that explain the interaction between nutrition and chronic pain, and the role of nutrition screening, assessment, and evaluation for people experiencing pain, and the scope of nutrition practice in pain management.


#### **Table 1.** Search Terms.

\* Wildcard represents unknown characters and identify word combinations by filling in automatically.

Following the searches on Medline and CINAHL for the guidelines related to nutrition and pain management, 112 articles were identified. None of the identified articles provided relevant information about the guidelines related to nutrition and pain. One article provided conditional recommendations and an evidence based decision aid for the use of specific dietary ingredients in chronic musculoskeletal pain [17]. However, the population of interest in this article was the United States military, which limits the generalizability of the findings to the general population. The search results from Practice-based Evidence in Nutrition (PEN) identified 32 practice guideline toolkits, of which seven corresponded to pain-related conditions such as osteoarthritis, musculoskeletal/connective tissue disorders, irritable bowel syndrome, rheumatoid arthritis, interstitial cystitis, inflammatory bowel disease, and spinal cord injury but there were no guidelines for chronic pain [18]. Of the 54 ESPEN guidelines, none were related to chronic pain [19].

## **3. The Nutrition-Related Health of People Experiencing Pain**

Identifying the nutrition-related health and clinical features of people experiencing chronic pain is important for effective pain management. Malnutrition, or poor nutrition, is a health condition that occurs when an adequate nutrition cannot be acquired. An insufficient supply or consumption of nutrition (undernutrition) can lead to a person becoming underweight, while an oversupply or excessive consumption of nutrition (overnutrition) can lead to a person becoming overweight or obese. In this section, the associations between malnutrition, weight, dietary habits, and chronic pain will be discussed.

#### *3.1. Underweight, Overweight, and Obesity*

Population-based studies suggest that there is a higher prevalence of chronic pain amongst people with an unhealthy weight (i.e., underweight, overweight, or obese), compared to those who are of a healthy weight [20–22]. Underweight is defined as a body weight below the healthy weight range, while overweight and obesity are defined as an excessive and abnormal increase in white adipose tissue. The body mass index (BMI) is a weight-for-height index (kg/m2), which is commonly used to classify the weight status in adults. A BMI of less than 18.5 kg/m2 is defined as underweight, between 25.0 kg/m<sup>2</sup> and 29.9 kg/m<sup>2</sup> is defined as overweight and over 30 kg/m2 is considered obese. Obesity also

consists of three subclasses: class I (30–34.9 kg/m2), class II (35–39.9 kg/m2), and class III (≥40 kg/m2). There are limitations associated with the BMI as it does not take into account ethnicity or body composition such as fat and muscle mass. It also does not factor in the biological, physical, economic, psychological, and social aspects that contribute to weight status. Therefore, it is important to ensure that health professionals use a holistic approach to measuring health and do not rely on weight and the BMI alone.

There is a significant focus on excessive weight and chronic pain, but it is essential to highlight that being underweight is also associated with chronic pain. The electronic Persistent Pain Outcome Collaboration (ePPOC), an Australian initiative, synthesises a standard set of data from participating chronic pain services in Australia and New Zealand. In 2020, the ePPOC reported that 2% of the 20,000 patients seeking pain management were underweight, 32% were overweight, and 39% were obese [23]. Undernutrition may also occur amongst adolescents with chronic pain and eating disorders, especially those who experience gastrointestinal issues, anxiety, and a greater functional disability [24,25]. Importantly, malnutrition and frailty are common contributors and are consequences of chronic pain. Malnutrition occurs when, over time, a person consumes too many or too few nutrients to meet their nutritional needs. This can cause adverse effects on the body, how it functions, and lead to poor health outcomes, such as a reduced life expectancy and quality of life [26]. Malnutrition may occur by not eating enough, not eating the right foods, or being unable to absorb nutrients. In Australia, up to 50% of older adults (>65 years) are malnourished or at an increased risk of malnutrition [27]. A moderate-high risk of malnutrition has also been reported by other studies conducted in different countries [28–30]. The evidence suggests that those experiencing hip and knee pain are at an increased risk of sarcopenia (a form of malnutrition where the loss of muscle mass occurs) and falls [31–33]. Pain can also impact the sensory pleasure related to food which may lead to a decrease in satiety and an increased risk of malnutrition [34]. Frailty is characterised by a decline in physical, mental, and multisystem functions and can be described as a multidimensional state of depleted physiological and psychosocial conditions [35]. A serious consequence of frailty is the increased risk of disability and death from minor external stresses, such as a mild infection or facing a stressful event [36,37]. For older adults in particular, frailty is a severe consequence related to malnutrition and chronic pain [30,38], which can predict future adverse health outcomes, such as falls and physical disability, as well as hospitalization and even mortality [36]. A recent systematic review pooled the findings from 12 cross-sectional and 12 longitudinal studies in a meta-analysis and found that older people (>60 years) with chronic pain were almost two times more likely to develop frailty after an average follow up of 5.8 years, compared to those without chronic pain [39].

The western lifestyle and diet are contributing factors impacting the global development of overnutrition, or excess weight (overweight and obesity) [40]. At the population level, overweight and obesity may explain the rising trends in chronic pain amongst middle-aged adults [41]. An Australian longitudinal study of an elderly cohort reported a relationship between fat mass, the BMI, and pain [42]. This trend has also been confirmed in the clinical populations. For example, it was found that over 25% of chronic pain patients had a comorbidity of obesity [43], much higher than the general population in Sweden [44]. The prevalence was even higher in Australia. Up to 45% of patients from a tertiary pain clinic were classed as obese [45]. Undernutrition, on the other hand, is most common among older patients [30,38,46] as well as patients with orofacial pain [47] or functional gastrointestinal disorders [48,49]. Large clinical cohort studies identified that obese patients had, in general, a worse pain profile than the normal weight patients, for example, a higher pain intensity, an increased pain interference, and more constant pain [43,50].

Recent evidence has acknowledged the importance of nutritional factors affecting specific pain conditions. For example, studies exploring fibromyalgia have demonstrated that overweight or obese patients experienced more pain, impaired function, had higher levels of depression, and medication use than patients who were normal weight [51,52]. Underweight, overweight, and obesity coexist with chronic pain due to the nutritionrelated underlying mechanisms. There is an interrelationship between the nutritional status, chronic pain pathophysiology, and eating behaviors. Diet profoundly impacts the body and has a complex relationship with the pain experience [4,13]. Dietary intervention (i.e., diet patterns and eating behaviors) has also been identified as one of the integrative treatments to alleviate chronic pain [4,53]. According to the existing evidence, common chronic pain conditions have been associated with nutritional factors, such as osteoarthritis [54], rheumatic arthritis [55], fibromyalgia [56], back pain [57], irritable bowel syndrome (IBS) [49], pelvic pain (e.g., endometriosis) [58], diabetic neuropathy [59], migraine headache [60], post-herpetic neuralgia [61], and carpal tunnel syndrome [62]. A summary of common pain locations related to over- and undernutrition are shown in Figure 1. Multiple site pain conditions and spreading pain conditions, such as myofascial pain syndrome and fibromyalgia are not illustrated in the figure. Based on the IASP classification of chronic pain [63], these pain conditions may not always belong to one category (nociceptive, nociplastic, or neuropathic pain), depending on the grading of the predominant central sensitization [64,65].

**Figure 1.** Pain Sites Related to Poor Nutrition. IBS: Irritable Bowel Syndrome; CTS: Carpal Tunnel Syndrome.

Poor nutrition not only impacts chronic pain pathophysiology, but also impacts other health outcome measures. For instance, compared with non-obese patients, obese patients with chronic pain had more physical limitations [66,67], a lower psychological wellbeing [68], more sleep disturbances [69,70], a poor health-related quality of life (HRQoL) [71], and a function dependence [72,73]. Multiple physical and/or mental diseases also frequently coexist with chronic pain, such as type 2 diabetes, cardiovascular disease or

metabolic syndrome, anxiety (or post-traumatic stress syndrome), and depression [21,74]. These conditions can be modified using nutrition-related treatments.
