Introduction
Coronavirus Disease 2019 (COVID-19) is an acute respiratory syndrome caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). In December 2019, this virus was transmitted seemingly from animals to humans in a seafood market in Wuhan, China and then spread to other parts of the world. The World Health Organization (WHO) Emergency Committee declared a global health emergency according to the rapidly increasing number of infected cases on 30 January 2020. The social distancing and self-quarantine to avoid the spread of virus highly affected peoples’ life styles, especially food habits and daily living tasks [1]. The two specific changes resulted from these measures were the need to stay at home (led to online education, working from home, restrictions of gatherings outdoor, and working out at home) and food storage due to purchasing limits. Leaving jobs and normal life due to self-quarantine can make people depressed and tired. Moreover, it is highly stressful to constantly hear or read about the COVID-19 news in the media. Stress may lead to overeating of sugar-rich fast foods which is defined as “food craving”. These kinds of food that are mostly rich in carbohydrates can reduce stress because they strengthen serotonin production by positively affecting the mood [2]. This effect of carbohydrate cravings is appropriate based on the glycemic index of foods, but is associated with increased risk of obesity and cardiovascular diseases, which have more complications than COVID-19. It is possible that this pandemic jeopardize maintaining a healthy and varied diet as well as regular physical activity. For example, limited access to the daily purchase of food may reduce the consumption of fresh foods, especially fruits, vegetables, and fish in favor of fully processed foods, such as canned foods, junk foods, snacks, and ready-to-eat cereals which are usually rich in fat, sugar, and salt. Furthermore, people’s psychological and emotional responses to the coronavirus outbreak may increase the risk of unhealthy food habits. Previous studies have proved how experiencing negative emotions can lead to overeating, the so-called “emotional eating” [3]. The general public may get tired of staying at home for long periods of time and turn to overeating as a means of escaping from monotony [4]. On the other hand, some people cannot eat properly due to physiological stress reactions. Eventually, it is possible that sedentary behaviors, change in smoking behavior, and sleep habits change lifestyle [1].
This pandemic disease is new; thus, scientists are currently trying to find effective vaccines medicines for this disease. The reduction of inflammation without endangering the patient’s normal immune response is one of the biggest challenges. In this regard, the science should focus on both medicine and nutrition. The proper food habits are highly important in this pandemic to prevent the presence of non-communicable diseases that can lead to more severe infections and to moderate the inflammatory status of patients. In fact, minimizing the importance of nutrition in COVID-19 patients can seriously affect their physical conditions. Development of healthy nutritional guidelines is a key strategy for healthcare providers and people [5]. Despite the best efforts of international organizations and other health-related communities to provide nutritional guidance during the COVID-19 pandemic, no nutritional guideline is available yet. 
This study aimed to investigate the current knowledge of the relationship between nutrition, immune system and COVID-19 in order to develop appropriate nutritional guidelines and programs during recovery from the disease. Since there was no evidence-based vaccine or treatment for COVID-19 at the time of this study, proper food choices and the use of appropriate healthy measures in food selection, preparation and storage are probably one of the approaches to manage this pandemic; hence, this study attempts to review the latest information about the nutrition during COVID-19. A search was conducted on information from several countries affected by the pandemic, and on published studies in scientific databases. The key study question is: What are nutritional recommendations for home quarantined people during COVID-19?
Nutrition recommendations by WHO
The WHO has recommended vitamins, minerals, dietary fiber, protein and antioxidants. Various appropriate foods can be obtained from them. Moreover, it was recommended to drink enough water and avoid excessive use of sugar, fat and salt. 
Recommendations for breastfeeding mothers with infection 
According to WHO, women with COVID-19 can breastfed. They should practice respiratory hygiene when feeding. They should wearing mask, if there are any symptoms. They should wash their hands before and after touching the baby. Furthermore, they should clean and disinfect the surfaces that they touch, regularly.
Nutrition recommendations by the united nations international children’s emergency fund 
According to the United Nations International Children’s Emergency Fund (UNICEF), people should eat fruits and vegetables more. They should choose dry or canned healthy food options if you do not have access to fresh products. Canned oily fish is rich in protein, omega-3 fatty acids and a variety of vitamins and minerals that are beneficial. People should create a set of healthy snacks, and reduce consumption of fully processed foods. They should make cooking and eating fun and meaningful, and spend more time for nutrition [6].
Recommendations for breastfeeding mothers 
The nutritional balance of the immune system is important at any age. During childhood, breastfeeding can protect against infections and respiratory diseases, because breast milk (containing antibodies, enzymes, and hormones) has health benefits. Some nutrients, such as omega-3 polyunsaturated fatty acids and probiotics have been linked to anti-inflammatory reactions and more resistance against upper respiratory tract infections. Breastfeeding mothers should pay attention to the following: Breast milk for 6 to 24-month children is a good nutrition. If a mother is unable to breastfeed due to COVID-19 infection or other complications, she should be supported to breastfeed in an alternative way. They should practice respiratory hygiene during feeding while wearing a mask. They should wash their hands before and after touching the baby, and clean and disinfect surfaces, routinely. 
Recommendation for the use of packaged foods
Any unnecessary packaging of foods should be removed and put in a waste bin. Packaging such as cans can be cleaned with disinfectants before opening or storing them. Wash your hands with soap and water for at least 20 seconds or rub your hands with an alcoholic disinfectant. Unpackaged products, such as fruits and vegetables, should be thoroughly washed under running water.
According to the European Food Safety Authority, there is no need to disinfect food packaging until following precautions are taken: 
Keep a safe distance (1-2 m) from others at the time of buying;
If you are infected, order food online or ask for help from family or friends, if possible;
Do not touch the food on the shelf unless you are shopping;
Limit going to the supermarkets by planning the type of meals; 
Cover your mouth and nose with a tissue or using sleeve when coughing or sneezing, and then wash your hands;
Avoid touching your face, nose and mouth after washing hands.
Recommendations for providing food security at various levels 
There are recommendations for the prevention and management of COVID-19 at various levels.
Individual level
Try to eat balanced meals, avoid irregular snacks.
Choose foods rich in vitamins A, C, E, B6 and B12, zinc and iron such as citrus fruits, dark green leafy vegetables, nuts and dairy products. 
Have a healthy lifestyle with exercise (at home), regular sleep, and meditation. 
Avoid smoking, alcohol and drugs. 
Do not spread misconceptions about food, diet, and COVID-19.
Community level 
The awareness about the consequences of hoarding and panic buying should be increased.
Malnourished people in the community, especially the elderly and patients with chronic diseases should be identified and supported. 
An organized and reliable supporting system should be established to ensure the availability and affordability of basic food items to all members of the community [6].
National level 
It is required to define, fund, and distribute food baskets to realize the people’s health needs and ensure the use of local agricultural products, and minimize dependency on food imports.
Resources should be mobilized to fund the purchase and supply of foods. 
Tax abolition for staple foods and commodities should be on the agenda.
Agricultural and food industries should be supported. 
Food prices and markets should be monitored and inspected, properly. 
High transparency is very important to build trust and support.
Global level 
It can be useful to ensure a continuous flow of world trade, avoid any trade limitations in order to prevent worsening local conditions that communities have already faced. It can also be useful to have sustainable food and feed, as well as agricultural production resources. Furthermore, import tariffs and other restrictions on food products should be reduced [6].
Recommendations for nutrition of obese people 
The high rate of obesity and diabetes in a small number of individuals may, at least, reflect the health differences observed during the COVID-19 pandemic in these groups [7]. It has been reported that these individuals do not have easy access to healthy food choices and nutrition education may be due to the rising poverty rate and declining access to high-quality health care [7]. Those who are not normally able to prepare food should have easy access to healthy and fresh foods in order to fight the disease. Studies have shown that eating healthy foods, even in the presence of obesity, has a rapid anti-inflammatory effects [8]. Changes in these policies increase the effectiveness of vaccines, as vaccines have been shown to be less influential in obese people and have long-term advantages in preventing diseases, including COVID-19 [9]. 
It is expected that most of the patients with COVID-19 to be recovered in most at-risk populations, although can have long-term indirect consequences. The potential effects of COVID-19 on the nervous system function as well as the resulting long-term potential lung damage are important, because it has been shown that peripheral inflammatory events can cause a severe and persistent neural inflammatory response in vulnerable individuals.
Overall, bad lifestyle such as unhealthy diets affect the COVID-19 sensitivity and the recovery rate from it. Unhealthy diets may even damage those who have recovered from the COVID-19. Therefore, it is recommended that people avoid foods high in saturated fat and sugar, and consume foods with large amounts of fiber, whole grains, unsaturated fats, and antioxidants to strengthen the immune system [8]. 
Nutrition recommendations for the prevention of COVID-19 disease
The human immune system plays an important role in the prevention of respiratory diseases, including COVID-19. Underlying diseases such as diabetes and lung disease, heart disease, and malnutrition intensify the disease. In infected people, nutrition can determine the severity of clinical complications. Zhang and Leo explained that dietary supplements containing vitamins (e.g. A, B, C, and D), minerals (e.g. selenium, zinc, and iron), and omega-3 fatty acids have been considered as a treatment option for COVID-19 patients and a preventive treatment against lung infection [10]. 
The use of foods with lack of vitamins such as vitamins C, A and D and the weakening of the immune system increase the risk of COVID-19. Vitamin A has a role in the proliferation of T lymphocytes and the production of immune-responsive cytokines and normal killer cells. Vitamins D and E regulate the immune system, and fiber modifies intestinal microbiota by affecting the immune system, positively. Zinc and selenium reduce the symptoms of cold. Essential fatty acids control inflammation, infections, and the production of hormones and antibodies [11, 12, 13].
Evidence from animal models have shown a direct relationship of diets and vitamin A, E, and D deficiency with the immune response to respiratory infections caused by COVID-19. The vitamin-deficient diets can also reduce the effectiveness of vaccines for inactivated bovine coronavirus and make them more susceptible to infectious diseases. Consequently, studies have highlighted the significance of consuming vitamin-rich foods, especially during a pandemic [13, 14] (Table 1).


In this regard, the Centers for Disease Control and Prevention, WHO, and the Food and Agriculture Organization have issued recommendations for food safety [15]. 
Fruits and vegetables are also good sources of water, antioxidants, and fiber, all of which are involved in controlling high blood pressure, diabetes, and weight gain [16]. Fruits such as oranges, tangerines, lemons, kiwis and vegetables with vitamin C such as cabbage, cauliflower, turnips, green peppers and bell peppers, parsley, onions, watercress, tomatoes are good food sources of vitamin C. Daily consumption of one orange or two tangerines provides the daily vitamin C needed by the body. 
Dark green vegetables such as spinach, beet leaves, dark lettuce, squash and carrots are also good sources of vitamin A. In general, in order to prevent diseases and strengthen the immune system, daily consumption of 3 servings of vegetables (except starchy vegetables) and at least 2 servings of fruits is recommended. 
The elderly, children under 5 years old, pregnant women and patients taking corticosteroids are at higher risk and should follow these tips to prevent COVID-19:
Daily intake of vegetables or salad with lemon juice or fresh orange juice;
Daily intake of carrots and squash;
Eating raw onions due to having vitamin C;
Avoiding sausages and other fast foods and high-fat foods 
Including protein sources such as legumes or eggs in the diet;
Eating food sources containing iron and zinc such as legumes (as a suitable alternative to meat), milk, dairy and green leafy vegetables, and nuts 
Avoiding half-cooked foods (e.g. soft-boiled eggs and a number of kebabs) 
Avoiding consumption of food and liquids in places that are not clean; 
Eating watery foods such as soups and stews with fresh lemon juice and hot liquids, if have cold symptoms;
Daily intake of wheat germ, mung bean and clover containing vitamin C. 
Eating fruits containing seasonal antioxidants such as pomegranate, red oranges, grapefruit and etc.
It has been suggested to drink water and being hydrated; however, there is no guidance on the right amount of water intake. Evidence has already confirmed the direct relation between water supply status and health. Water is essential for cellular homeostasis, kidney function, body temperature control, mood regulation, cognitive function, gastrointestinal function, and headache prevention [17]. Of course, it is important to pay attention to the balance and variety in a daily dieter regimen and be ensured of adequate intake of micronutrients and protein [18].
The ways to increase mental and physical health during COVID-19 pandemic 
There are some methods that can help improve people’s mental and physical health during the COVID-19 pandemic such as exercise, online chat with friends and relatives, regular and adequate sleep, and relaxation. 
Conclusion
The COVID-19 pandemic has widely affected health, economy, and livelihood of people, causing sudden changes in their lifestyles due to social distancing and staying at home, which have irreparable psychological consequences. Improving public health during this pandemic requires to have knowledge of medical and biological concepts, as well all concepts related to lifestyle and social and behavioral acts, including eating habits. To strengthen the immune system and fight diseases such as COVID-19, it requires to modify the nutrition and have healthy diet rich in nutrients.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research.

Funding
This study was supported by Hematology and Oncology Research Center, Tabriz University of Medical Sciences and University of Tabriz. 

Authors' contributions
All authors equally contributed to preparing this article. 

Conflicts of interest
The authors declared no conflict of interest.

References

1. Muscogiuri G, Barrea L, Aprano S, Framondi L, Di Matteo R, Laudisio D, et al. Sleep quality in obesity: Does adherence to the mediterranean diet matter? Nutrients. 2020; 12(5):1364. [DOI:10.3390/nu12051364] [PMID] [PMCID]
2. Ma Y, Ratnasabapathy R, Gardiner J. Carbohydrate Craving-not everything is sweet. Current Opinion in Clinical Nutrition and Metabolic Care. 2017; 20(4):261-5. [DOI:10.1097/MCO.0000000000000374] [PMID] [PMCID]
3. Evers C, Dingemans A, Junghans AF, Boevé A. Feeling bad or feeling good, does emotion affect your consumption of food? A meta-analysis of the experimental evidence. Neuroscience & Biobehavioral Reviews. 2018; 92:195-208. [DOI:10.1016/j.neubiorev.2018.05.028] [PMID]
4. Havermans RC, Vancleef L, Kalamatianos A, Nederkoorn C. Eating and inflicting pain out of boredom. Appetite. 2015; 85:52-7. [DOI:10.1016/j.appet.2014.11.007] [PMID]
5. Fernández-Quintela A, Milton-Laskibar I, Trepiana J, Gómez-Zorita S, Kajarabille N, Léniz A, et al. Key aspects in nutritional management of COVID-19 patients. Journal of Clinical Medicine. 2020; 9(8):2589. [DOI:10.3390/jcm9082589] [PMID] [PMCID]
6. Naja F, Hamadeh R. Nutrition amid the COVID-19 pandemic: A multi-level framework for action. European Journal of Clinical Nutrition. 2020; 74(8):1117-21. [DOI:10.1038/s41430-020-0634-3] [PMID] [PMCID]
7. Dharmasena S, Bessler DA, Capps Jr O. Food environment in the United States as a complex economic system. Food Policy. 2016; 61:163-75. [DOI:10.1016/j.foodpol.2016.03.003]
8. Connaughton RM, McMorrow AM, McGillicuddy FC, Lithander FE, Roche HM. Impact of anti-inflammatory nutrients on obesity-associated metabolic-inflammation from childhood through to adulthood. Proceedings of the Nutrition Society. 2016; 75(2):115-24. [DOI:10.1017/S0029665116000070] [PMID]
9. Green WD, Beck MA. Obesity impairs the adaptive immune response to influenza virus. Annals of the American Thoracic Society. 2017; 14(S 5):S406-9. [DOI:10.1513/ AnnalsATS.201706-447AW] [PMID] [PMCID]
10. Zhang L, Liu Y. Potential interventions for novel coronavirus in China: A systematic review. Journal of Medical Virology. 2020; 92(5):479-90. [DOI:10.1002/jmv.25707] [PMID] [PMCID]
11. Gombart AF, Pierre A, Maggini S. A review of micronutrients and the immune system–working in harmony to reduce the risk of infection. Nutrients. 2020; 12(1):236. [DOI:10.3390/nu12010236] [PMID] [PMCID]
12. Hachimura S, Totsuka M, Hosono A. Immunomodulation by food: Impact on gut immunity and immune cell function. Bioscience, Biotechnology, and Biochemistry. 2018; 82(4):584-99. [DOI:10.1080/09168451.2018.1433017] [PMID]
13. Bearden A, Van Winden K, Frederick T, Kono N, Operskalski E, Pandian R, et al. Low maternal vitamin D is associated with increased risk of congenital and peri/postnatal transmission of Cytomegalovirus in women with HIV. PloS One. 2020; 15(2):e0228900. [DOI:10.1371/journal.pone.0228900] [PMID] [PMCID]
14. Nonnecke BJ, McGill JL, Ridpath JF, Sacco RE, Lippolis JD, Reinhardt TA. Acute phase response elicited by experimental bovine diarrhea virus (BVDV) infection is associated with decreased vitamin D and E status of vitamin-replete preruminant calves. Journal of Dairy Science. 2014; 97(9):5566-79. [DOI:10.3168/jds.2014-8293] [PMID]
15. Ribeiro KD, Garcia LR, Dametto JF, Assunção DG, Maciel BL. COVID-19 and nutrition: The need for initiatives to promote healthy eating and prevent obesity in childhood. Childhood Obesity. 2020; 16(4):235-7. [DOI:10.1089/chi.2020.0121] [PMID]
16. de Faria Coelho-Ravagnani C, Corgosinho FC, Sanches FL, Prado CM, Laviano A, Mota JF. Dietary recommendations during the COVID-19 pandemic. Kompass Nutrition and Dietetics. 2021; 1(1):3-7. [DOI:10.1159/000513449]
17. El-Sharkawy AM, Sahota O, Lobo DN. Acute and chronic effects of hydration status on health. Nutrition Reviews. 2015; 73(S 2):97-109. [DOI:10.1093/nutrit/nuv038] [PMID]
18. Nutrition Improvement Office of the Ministry of Health, 2020. https://nut.behdasht.gov.ir/