Discussion
The current study explored the relationship of dietary sulfur amino acids intake and severity and frequency of MPs in adults. According to the results, there was not any significant relationship between sulfur amino acids consumption and severity and frequency of MPs; whereas higher intake of protein was related to the higher feeling of pain interceded by calorie intake. In addition, almost 71% of adults felt low to moderate pain. In the present study, there was no significant relationship between gender and the severity of musculoskeletal pain in the adult population over 18 years. Neither Palmer and Goodson did not see any differences in review article, nor did Roberts et al (20,21). But there was a positive relation in frequency of pain among women so that 56% of women felt pain more than twice a week. It has been considered that about 60% of the participants were young and age has a significant positive relationship with the severity and frequency of pain in individuals (22). Increasing pain with getting older can effect on progressive loss of neuromuscular function, decreased muscle mass, cartilage, bone mass and skeletal and muscular degenerative injuries (23). Recently, more attention has been paid to the effect of sex hormones on pain induction. Microglia (the remnant of the components of the immune system in the central nervous system) play a vital role in inducing pain in the body differently in men and women (24). As we know, in the central hypothalamus Arcuate Nucleus produce beta-endorphin, a potent analgesic for relieving of chronic pain in the body, is strongly influenced by estrogen. Estrogen receptors influence in synthesis and secretion of the methionine-enkephalin (delta-epithelial ligand receptor) and beta-endorphin secretion. This is why women feel less pain during pregnancy (25).
In the present study, there was a strong correlation between weight, waist circumference, waist height and body mass index with the severity and frequency of musculoskeletal pain in women. Also, in men, there was no significant relationship between any of the anthropometric indicators with pain intensity and frequency. It should be noted that the number of female participants in this study was higher and this could be one of the possible reasons for existence of significant pain with anthropometric figures in women. Also, the percentage of body fat mass is higher in women, which can be a factor in shortening the height between discs and increasing the severity of pain among them (26). Weight loss and healthy eating pattern may be helpful to manage obesity, reduce inflammation and disability of pain (27). In addition, endogenous enkephalins have been shown to increase food intake in cats, thereby increasing obesity and pain (28). One study have also shown that the expression of catecholamine-stimulating lipolytic enzymes and the hormone-sensitive lipase was inhibited in adipose tissue and total body weight and three glyceride level were increased in the liver of mice with a high cysteine ​​diet after 12-weeks. In general, excessive consumption of cysteine causes obesity and undesirable metabolic phenotype in mice. Cysteine plasma and obesity in humans may be causal (29).
There was mostly high sulfur amino acids among participants even more than RDA determinate (14 mg/kg/day) (30). In line with this study, in a systematic review Elma et al. found that in most patients with pain, the protein intake was higher than the allowable intake but the severity of pain was not significantly related to dietary intake (9). Although protein is a major component of maintaining skeletal muscle health, most sulfur-rich sources of amino acids are found in animal proteins, especially red meat and poultry which can be lead to inflammation (31). In addition, the consumption of proteins, especially from animal origin, more than 0.8 grams per kilogram of body weight has no positive effects on human health. Animal proteins in the body produce acidic precursors due to incomplete oxidation and plant proteins produce alkaline precursors in the body. An imbalance between acidic and alkaline precursors has been shown to disrupt a chronic net dietary acid load, which may have adverse consequences on bone health (32). Also Hackney et al. found that excessive consumption of sulfur amino acids was associated with increased bone resorption. Dietary methionine can decrease blood pH and increases skeletal pain. Consistent with the present study in the study of Freyberg et al. which was conducted to find the relationship between dietary sulfur amino acids and rheumatoid arthritis, no sulfur amino acid deficiency was observed in people with arthritis (33). In a cohort study with 546 people with rheumatoid arthritis, no significant correlation was observed between the type of protein consumed and the risk of rheumatoid arthritis (34,35). There was just a positive relationship between carbohydrate intake and the group that feel most pain.
In addition the internal pain relief system in central nerves system need essential fatty acids, especially Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA)(36,37). Whereas the consumption of omega-3 fats (high in fatty fish) was very low in the population of this study. In addition consumption of salt was positively correlated with pain.
These two ​​are precursors of S-adenosyl methionine (SAM), hydrogen sulfide, taurine and glutathione which can are protective(38,39). Oral cysteine ​​alone has been shown to have low absorption capacity from the gastrointestinal tract (40).
In addition taking high dose of oral methionine may increase the level of homocysteine and could be harmful for persons. Bioavailability of cysteine ​​is a limiting step in glutathione production. One study showed that oral cysteine ​​alone has been absorbed in a low capacity from the gastrointestinal tract. In order to increase the level of glutathione, whey protein and almond could be useful (40).
Homocysteine ​​is one of the mediators of methionine metabolism in the body. This substance has many roles in transporting in cells. In muscle disorders, for examples, the role of homocysteine as an aggravator of key processes such as oxidative stress, nitric oxide (NO) signaling, endoplasmic reticulum stress, methylation and gene expression, alteration in cellular signaling, especially G proteins, is prominent (41).
Among painkillers drugs no relationship between was seen acetaminophen and SAAa. Glutathione detoxify drugs such as acetaminophen by the sulfur element (42,43). In line with this study, an intervention study was conducted in 2012. They selected 11 people over the age of 69 who had arthritis. Participants received 3 grams of acetaminophen per day for 14 days. They found that despite the excretion of small amounts of paracetamol as conjugated sulfate in elderly patients, the amount of sulfur required to detoxify 3g of paracetamol per day was equal to 20% of the sulfur supplied by 1.06 g/kg body weight of dietary protein. Also, after 14 days, the body’s total protein stores remained unchanged. However, there was an increase in the oxidation of sulfur amino acids and a decrease in the body’s antioxidant capacity. The safety/risk ratio of this drug to compensate for deficiencies may be greatly need via new dietary changes and guidelines (44). Hyperhomocysteinemia can be a risk factor for pathology of many diseases. Homocysteine ​​can damage the vagus nerve and the superior cervical ganglion in the central nervous system and disrupt both the sympathetic and parasympathetic systems involved in skeletal muscle, which can lead to dysfunction of the skeletal muscle and spinal cord in the long term (45). In addition, the body’s autonomic system and adrenal glands need methionine to produce norepinephrine from epinephrine in process of methylation (46). In chronic pain, norepinephrine increases inflammation in the body by producing pro-inflammatory cytokines. Due to the fact that the consumption of sulfur amino acids in this study was high, norepinephrine may have increased chronic pain by producing more inflammatory cytokines (47).
Conclusion
In conclusion, the present study didn’t show a positive and relative association between the dietary sulfur amino acids and severity and frequency of pain. Although, targeting a decrease in dietary animal protein intake might be a useful strategy for reducing pain. However, further studies are recommended to improve understanding the relationship and managing pain.
Strengths and Limitations: According to the searched data bank, this study was the one which had focused the role of dietary SAAs and existence of pain in individuals. However, the population of this study was mostly among adults group (18-35 years) and assessing to older people in quarantine period of covid-19 was difficult for us and this may influence the final result. Further research is recommended to concrete our result.