Food security is one of the most important and effective factors in a country's economic development, and no one can claim to have a healthy and dynamic society without it . According to the FAO definition, food security exists when all people, at all times, have physical and economical access to sufficient, safe, and nutritious food that meets their dietary needs and food preferences for an active and healthy life. Thus, food security is based on four key elements including food availability, access to food, utilization, and stability . In terms of food security, in addition to the fact that households must have access to adequate and healthy food 24 hours a day, they must also maintain diversity and balance in order to meet all of their macronutrients and micronutrients requirements. The purpose of food security is not to satisfy apparent hunger, but to meet all nutritional demands that humans require for proper organ and cell function . To achieve this goal, households should have good knowledge about food units and be able to properly spend the portion of their income allocated to food purchases. Our findings revealed that, when compared to other food groups, the protein group accounts for a larger part of overall average household food expenditure, accounting for about 39.9% of food expenditure. However, the protein group had the lowest mean purchase of food units per person in comparison to other food groups and this amount was lower than the daily recommended values in a balanced diet, which is 2–3 servings per day. Meats and plant protein sources have high nutritional value and provide protein requirements as well as certain micronutrients such as iron, zinc, vitamin A, and vitamin B12. As a result, the cost of protein has a direct relationship with the household's diet quality and food security . The majority of the body's iron and vitamin B12 requirements are met by consuming red meat and other animal-derived proteins. A decrease in these micronutrients consumption can lead to a wide variety of health problems for individuals, including anemia and vulnerability to infections . As well, the reduction in zinc and vitamin A intake might have an impact on several metabolic processes in the body. According to various studies, vitamin A plays an important role in the visual and the immune systems, and its deficiency causes anemia via immobilizing iron in the reticuloendothelial system, reducing hematopoiesis, and increasing susceptibility to infections [27, 28]. Likewise, zinc, as a cofactor of different enzymes, plays a critical role in the body's metabolic processes such as RNA and DNA synthesis and its deficiency can interrupt protein synthesis, gene expression, cell division, skeletal growth, gonad development, appetite, and immunity [29, 30].
In some countries, particular food groups, such as protein and dairy, are more expensive than others, depriving low-income households of these foods. Additionally, the population in these regions has lower social and economic levels, which limits their nutritional options even more. In this situation, the best solution is to improve households' knowledge of food units and alternative food groups so that they may manage their limited income to purchase suitable food alternatives at a lower cost.
Dairy, protein, and fruits were the most expensive food groups in comparison to others in terms of cost/unit, while the starch group had the lowest mean cost per unit. Nevertheless, the starch group accounted for approximately one-fifth of the household's food expenditure (17.7%), demonstrating the significance of starches and carbohydrate resources in the household food basket. It seems that when the price of food groups per unit decreases, the purchase and consumption of them increases. Different investigations have demonstrated that bread, cereals, and rice are the most important source of energy in the Iranian diet . Since the most of flour used to make bread in Iran is white flour, many of the minerals and essential elements found in wheat bran are removed. Due to government subsidies, the price of bread in Iranian society is cheaper than other carbohydrate sources, hence most people with low SES and income consume it as their main food, resulting in nutritional deficiencies and many chronic diseases in the long-term. Several investigations have reported that consumption of a high carbohydrate diet is associated with the increased risk of mortality [32, 33]. However, carbohydrate consumption is influenced by a variety of factors, including geographical, social, cultural, and economic properties, so that the percentage of energy derived from carbohydrates is higher in low-income societies than those in high-income societies .
Our findings indicated that the mean percentage cost for the miscellaneous group from total household food expenditure was lower than the mean cost percentage of the vegetable (13.2%), close to the fruit (13%), and much higher than dairy (3.8%) groups. The miscellaneous group, including fats and sweets, decreases the quality of the diet due to the lack of essential nutrients. Since a high consumption of fats and sweets increases the risk of cardiovascular disease and diabetes, it seems that raising the price of fats and sweets will reduce the incidence of these diseases.
On the other hand, the mean cost per unit for the dairy, vegetable, fruit, and fat groups was 1496 ± 481, 696 ± 42, 1074 ± 74, and 253 ± 17 Tomans, respectively. This significant difference indicates the effect of food price on a household's food choices, so that the most expensive mean cost/unit, which is related to the dairy group, had the lowest mean number of daily purchasing units per household (1.7 ± 0.47). The dairy group had the lowest percentage of the total amount cost on food with 3.8%, while the protein group with 39.9% had the highest total cost. With the deterioration of household economic situation, the amount spent on the most expensive food group (dairy group) declines, depriving the poor part of society of dairy resources with high nutritional value. Dairy is an important source of nutrients such as protein, calcium, and B vitamins, particularly riboflavin . According to The American Heart Association/American College of Cardiology guidelines, adults should intake 2–3 servings of dairy per day [35, 36]. Dairy products have important compounds including lactose, various bioactive peptides and fatty acids such as caseins, whey proteins, milk polar lipids (MPL), α-linolenic acid (ALA), conjugated linoleic acids (CLA), palmitic acid, and some important micronutrients like calcium, phosphorous, magnesium, riboflavin, and vitamin D. As well, they have different physiological functions including anti-carcinogenic, anti-inflammatory, anti-oxidative, anti-adipogenic, anti-hypertensive, anti-hyperglycemia, and anti-osteoporosis [37–40]. However, all of the health-promoting advantages of dairy on people may be lost or decreased if dairy products are decreased or eliminated from households’ food baskets due to high prices and low purchasing ability.
In recent years, with the cessation of dairy subsidies in Iran, the amount of milk consumption has decreased even more [41, 42]. It may be inferred that inappropriate food and nutrition policies, as well as a failing to pay attention to improving nutritional knowledge and culture, have resulted in a decrease in dairy intake as the main source of human calcium among lower-income households.
Besides, following the dairy and protein groups, the fruit group had a relatively high cost/unit (1074 ± 74 Tomans) and only accounted for 13% of the cost of the household food basket, demonstrating that a higher price could affect food group intake. With a decrease in fruit consumption, a substantial part of households' intake of vitamins and fiber would decrease, possibly leading to malnutrition among vulnerable groups such as children and pregnant women in the future .
When the price of foods with high nutritional value increases, the tendency to consume foods with lower nutritional value rises, resulting in a poorer-quality diet that just satisfies calorie requirements and causes abdominal satiety without meeting cellular needs. Long-term persistence of this situation may lead in several chronic diseases such as obesity, metabolic syndrome, diabetes, and so on. However, by improving household nutritional knowledge through proper education and substituting cheaper foods with high nutritional value in the household food basket with expensive ones, the incidence of chronic diseases associated to malnutrition and its side effects may be prevented.
In the present study, increasing the SES of the households significantly raised the daily mean total food expenditure, suggesting that higher SES improves food security. The same results were not observed for the SES and the percentage of food cost from total income, so that by increasing the SES, the percentage of food cost from total income significantly decreased. The main explanation for the inconsistency of these parameters is the significant difference in household income between low and high SES, which causes people in higher SES to spend a smaller percentage of their income on food. In this way, households with lower income, which are also at lower SES, spend three-quarters (75.2%) of their income on food. Spending a large portion of the income on food, reduces the cost of other essential life necessities such as health care, education, and culture in these households causing the children of these families to be in a lower SES in the future.
Also, there was a direct and significant relationship between SES, education, and income levels with the amounts of energy, protein, carbohydrate, and fiber intake. Numerous studies have shown that SES may affect food security, so that low-income households experiencing the most food insecurity, leading problems in growth, health, and nutrition [44, 45]. Similarly, Eşturk and Oren reported that enhancing the education level of heads of households (university graduates) raises the chances of food security in their families by 5.6 times . On the other hand, Davis et al., revealed that the homemaker's educational level had no significant effects on household food expenditures, but his/her higher nutritional knowledge can improve the efficiency of food purchasing activities .
It seems that when the family members' educational level increases, they will have much more job opportunities and, as a consequence, will also earn more money. Also, SES and educational level might have an impact on healthy eating habits. Thus, in our investigation, the relationship between education and fat intake was not significant. People with a higher level of education have better nutritional knowledge and skills, and they can better manage their limited income in terms of nutrition so that their families do not experience food insecurity.
Similarly, SES levels can affect the development of healthy eating patterns, so that when family members, particularly heads of households, have a higher social and cultural level, their food choices alter, and they buy and consume less fat. Nevertheless, some factors, such as a community's eating habits and culture, can affect the formation of food patterns. In addition, the economic situation, the local culinary culture and the geographical location of the residence can all affect eating habits [47, 48].
Different studies have revealed that households do not properly use available food sources due to a lack of knowledge on how to choose, cook, and store food. Besides, misconceptions and poor eating habits can threaten the intake of adequate nutrients at the household level. This highlights the necessity of ongoing nutrition education in improving eating habits and promoting a healthy food culture [49, 50].