1. Introduction
Magnesium (Mg), is the fourth most abundant mineral and second most abundant intracellular cation in the human body, acting as a cofactor for over 300 enzymatic reactions [
1,
2,
3]. The biological functions of Mg in the body include but are not limited to improved adenosine triphosphate (ATP) production, deoxyribonucleic acid (DNA)/ribonucleic acid (RNA) synthesis, insulin sensitivity, bone health, and cardiovascular health [
1,
2,
3,
4,
5]. Magnesium also plays an important role in vitamin D synthesis and metabolism in the body, thereby enhancing its function [
6]. The maintenance of an adequate Mg status is integral to overall health, and the predominant way to improve Mg status is through an increase of dietary Mg intake [
1]. Therefore, dietary Mg intake is an important consideration when evaluating an individual’s Mg status [
1,
2].
The Institute of Medicine Recommended Daily Allowance (RDA) for Mg for adults 31 to 70 years is 310 mg for women and 420 mg for men [
2]. Magnesium is found in high amounts in food sources such as fruits, vegetables, and whole grains [
2,
4]. Conversely, processed foods tend to have a lower Mg content [
2,
4]. Under normal physiological circumstances, approximately 50% of dietary Mg can be absorbed into the human body; however, the biggest factor that determines the percent of Mg absorbed is an individual’s Mg status [
7].
Individuals who are Mg deficient will have an increased intestinal absorption rate of Mg and a decreased urinary Mg excretion amount compared to individuals who are not deficient [
7]. The Mg loading test, the gold standard for measuring Mg status, assesses an individual’s Mg status through measuring urinary Mg excretion after Mg infusion [
8]. However, this infusion method is cumbersome and requires normal kidney and gastrointestinal functions for an accurate measure of Mg status [
2,
8,
9]. Additionally, Mg status can be negatively affected by high calcium (Ca) status, leading to an antagonistic effect of this mineral on intestinal Mg absorption [
10]. This is due to the a shared homeostatic regulating system involving Ca
2+ sensing receptor (CaSR), and the affinity of Mg ion transporters (TRPM7) for Ca [
10]. Consequently, a high serum Ca concentration reduces intestinal absorption rates for both Mg and Ca in humans, potentially exacerbating Mg insufficiency while causing further imbalance to the Ca:Mg intake ratio [
10]. High dietary fiber, phosphorus, oxalic acid, and phytic acid consumption can also result in lower Mg absorption and status [
11,
12,
13]. Furthermore, although commonly measured in a clinical setting, serum Mg concentrations are known to provide limited information on overall Mg status [
2]. Due to these limitations in Mg status measurement, accurately capturing dietary Mg intake to assess Mg adequacy is an important tool to provide a comprehensive representation of an individual’s overall Mg status [
7].
Food frequency questionnaires are convenient, low-cost tools that are widely used for assessing nutrient intakes in clinical research [
14,
15]. The current National Health and Nutrition Examination Survey (NHANES) Food Frequency Questionnaire (FFQ) is a semi-quantitative questionnaire that contains 139 questions designed to measure the intake of macro and micronutrients across the board [
16]. Although this questionnaire is comprehensive, it is lengthy and not specific to Mg intake [
16]. This may pose a heavy burden on individuals participating in the FFQ. Therefore, it may not be feasible and cost effective for clinical research studies. A validated, nutrient-specific FFQ may offer an efficient and cost-effective way to evaluate nutrient intakes for clinical research studies. While other FFQs have been developed and validated to assess different micronutrient intakes in various demographic populations such as calcium, vitamin D, and sodium, currently, no validated Mg-specific FFQ is available for this purpose [
15,
17,
18].
The “gold standard” for measuring a dietary nutrient intake is through the use of multiple days of food diaries [
19]. However, this method is labor intensive for both participants and researchers [
19]. Furthermore, this method can only capture dietary intake over several days, rather than capturing typical overall daily Mg intakes that are more representative of long term dietary consumption habits [
15,
19]. The purpose of this study was to develop and determine the validity of a newly developed, semi-quantitative Mg food frequency questionnaire (MgFFQ) in comparison to a 14-day food diary for use in clinical research to easily assess average daily dietary Mg intakes.
4. Discussion
4.1. Summary of Results/Major Findings
Maintaining an adequate Mg status is important for overall health due to Mg’s role as a cofactor for over 300 enzymatic reactions [
1,
2,
3]. The predominant way to improve Mg status is through dietary increases of Mg intake, though limited assessment tools exist to quickly and accurately measure dietary Mg intake to assess Mg adequacy [
2,
7]. Findings from our study overall showed that there was a strong positive agreement between the 33-item MgFFQ and the 14-day food diary in a sample of 135 adults of various sexes, ages, and BMI groups. This indicates that the MgFFQ is a valid method to quickly and accurately measure daily average Mg intake.
Strong agreements between the MgFFQ and the 14-day food diary were found in subsets of both women and men. The FFQ also appeared to be a valid tool for assessing daily average Mg intakes across all age groups, ranging from young adults ages 18 to 29 years, to older adults ages 60 to 69 years. The 40 to 49 years group, 50 to 59 years group, and 60 to 69 years group were small subsets of 8, 15, and 5 individuals, respectively, therefore requiring further study to generalize the validity of the MgFFQ in these specific populations.
Strong agreements between the MgFFQ and the 14-day food diary were also found across normal weight, overweight, and obese BMI groups. The agreement between the MgFFQ and the 14-day food diary in the underweight BMI group did not reach significance. However, this BMI group was a small subset of 6 individuals, therefore requiring further study to determine if a significant agreement exists in a larger sample of this specific population.
Overall, the strong positive agreement between the MgFFQ and the 14-day food diary in all participants, as well as in many of the subsets based on sex, age, and BMI groups support the use of the MgFFQ as a valid measurement tool that can be used to accurately assess average Mg intake in various demographic populations. Due to the low cost and quickness of administration of the MgFFQ, this tool can be used in both research and clinical settings, as an efficient and comprehensive representation of an individual’s Mg status. Compared to other methods of assessing Mg status, including cumbersome and expensive serum Mg measurements, and other FFQs such as the lengthy 139-item Nutrition Examination Survey (NHANES) Food Frequency Questionnaire (FFQ), the MgFFQ is a low-cost, quick to administer assessment tool that is specific to Mg intake [
2].
4.2. Intake Ranges of Food Frequency Questionnaire Compared to Recommended Dietary Allowance
Median intake of Mg as measured by the MgFFQ for all participants (246.12 (178.27) mg/day) was approximately 40 mg/day lower than the intake of Mg measured by the 14-day food diary (280.80 (139.15) mg/day). This was true across both sexes. This difference in measured Mg intakes between the MgFFQ and the 14-day food diary may be due to the inherent structure of the MgFFQ, which contains 33 commonly consumed foods that are considered good sources of Mg in the diet. The MgFFQ does not capture minimal amounts of Mg that may be obtained from other food items, which are able to be captured by a 14-day food diary. This can potentially explain the small inconsistency of approximately 40 mg/day of measured Mg intake between these two measures.
Despite the small difference in measured average daily Mg intake from the MgFFQ compared to the 14-day food diary, average intakes of Mg as quantified by both measures were less than the RDA for sex and age on average for adult women and men in this study. The RDA for Mg for adults 31 to 70 years is 310 mg for women and 420 mg for men [
2]. In our study, women consumed on average only 78% of their RDA for Mg, and men consumed on average only 59% of their RDA for Mg. This is consistent with previous reports that that in the United states, approximately 60% of adults do not reach their RDA for Mg intake [
2,
4].
Overall, poor Mg status in the United States is likely due to poor eating habits and low diet quality [
2,
21,
22]. Magnesium is found in greatest amounts in foods that are minimally processed, such as nuts and legumes, green leafy vegetables, fruits, meat, and fish, which are often poorly consumed in the Westernized culture due to the high availability and consumption of ultra-processed, calorie-dense, nutrient-poor food choices [
2,
21]. Since poor Mg status has been associated with increased risk of different chronic diseases such as type 2 diabetes mellitus, a fast and efficient method to assess Mg intakes is prudent, in order to quickly identify and correct poor intake and reduce the risk of Mg deficiency [
2].
4.3. Ca:Mg Ratio
In our study, the overall Ca:Mg ratio was 3.39 (2.11), with women having a higher Ca:Mg ratio compared to men. This may be related to the slightly higher percent of women, 18.7%, reporting Ca supplement use of higher amounts per day, compared to only 13.6% of men reporting supplemental Ca use per day. Among age groups, the 50 to 59 years group had the highest Ca:Mg ratio, and the 40 to 49 years group had the lowest Ca:Mg ratio. Both the 40 to 49 years group, and the 50 to 59 years group were small subsets of 4 and 6 individuals, respectively, and no individuals in the 60 to 69 years group and 70 to 75 years group had Ca:Mg ratio measurements, therefore requiring further study to generalize Ca:Mg ratios in these specific populations. Among BMI groups, the highest Ca:Mg ratio was found in individuals who were underweight, and the lowest Ca:Mg ratio was found in individuals who were overweight. The underweight group was a small subset of only 2 individuals, therefore requiring further study to generalize these findings.
In all sex, age, and BMI groups, the measured Ca:Mg ratio values were higher than the recommended Ca:Mg ratio of 2:1 [
20]. It is important to note that the median intakes for Ca and Mg for all participants did not meet RDA, likely contributing to the poor Ca:Mg ratios found. This is reflective of the typical Western diet, which favors calorie-dense foods with low micronutrients content [
21]. An imbalance of Ca:Mg ratio is associated with higher total mortality and increased risk of cardiovascular disease mortality based in previous studies [
10,
23]. Moreover, a high Ca:Mg ratio along with low Mg intake may exacerbate Mg deficiency. Previous evidence indicates that a high dietary calcium intake independently decreases intestinal absorption rates for both Ca and Mg, increases urinary excretion of Mg, and increases competition for the absorption of intestinal Mg due to a shared homeostatic regulating systems between Ca and Mg [
10,
24,
25]. Poor Mg status has been previously linked to increased low-grade inflammatory stress, which is one of the risk factors for chronic diseases such as cardiovascular disease, type 2 diabetes mellitus, and certain types of cancer [
2,
23,
26]. Therefore, there is a need to lower the dietary Ca:Mg ratio in the U.S. population.
4.4. Strength and Limitations
Nutrient-specific FFQs, such as the MgFFQ that was validated in this study, provide a quick and efficient means of measuring average intake of a nutrient such as Mg over an extended period of time. Compared to the 14-day food diary, a labor intensive method to collect Mg or other nutrient intake information, the MgFFQ is a quick method to capture typical dietary intake over several days or an extended period of time, therefore acting as a valuable tool for both clinicians and researchers to determine an individual’s typical dietary intake [
19]. Additional strengths of this study include recruitment from a large sample of healthy adults with diverse sexes, ages, races/ethnicities, and body weight statuses.
Major limitations of this study include that the majority of participants were younger adults aged 18 to 39 years, due to most recruitment methods being conducted on a university campus. Additional research is needed to determine the validity of the MgFFQ in children under 18 years who were not examined in the present study, as well as in older adults ages 40 to 69 years, who were represented only with a limited number of participants in the present study. More so, while racial and sex variety did exist within the study sample, participants in these groups were uneven, potentially indicating better generalizability in females, and Caucasian and Asian participants, the largest gender and ethnic groups represented in this study sample. Finally, in all genders, ages, and ethnicities, additional research is needed to determine the correlation of the MgFFQ with weekday versus weekend day food consumption, as well as within specific food groups such as dairy and grains, since only a collective MgFFQ was assessed against a combined weekday and weekend 14-day food diary in this study.
Limitations of the MgFFQ inherently include its reliance on the memory of the participant for accuracy of information reported, as opposed to food diaries which are recorded in present time and therefore do not rely heavily on memory. While the labor intensity of the 14-day food diary may be suggested to be a limitation in this study due to the potential result of poor recording by the participants, this was accounted for by researchers reviewing all food diaries for accuracy and detail of recording prior to analysis. Specific food choices on the MgFFQ may also be considered a limitation in this study. The MgFFQ contained only major commonly consumed foods that are good sources of Mg, it did not fully represent all food sources that can provide Mg, including foods with trace amounts of Mg, as well as different cuisines of various cultures that provide Mg, that may contribute overall to measurement biases and limit the MgFFQ generalizability. Also contributing to this measurement bias, diet intake of alcohol consumption was not captured with the MgFFQ, though certain types of alcohol, such as beer, do contain a considerable amount of Mg [
27]. Lastly, while the MgFFQ was considered a valid measurement to quantify average reported Mg intake per day in adults, no duplicate portion study or assessement of Mg intake and biochemical indicators of serum Mg statuses were examined or established in the present study, in order to determine if average Mg intake was associated with overall Mg status in the study sample.
5. Conclusions
Overall, the MgFFQ was considered a valid measure of daily Mg intake in adult men and women of varying ethnicities, ages, and weight statuses due to its strong agreement with Mg intake measured by the 14-day food diary. Future studies should expand the MgFFQ to include water, alcohol, and dietary supplements as potential sources of magnesium, as well as expand recruitment efforts to capture participants with a wider age range, gender distribution, and various ethnicities in order to further validate generalizability of the MgFFQ for use in these different populations. To assess Mg adequacy, future studies should also measure biochemical indicators of Mg status. Serum ionized or total Mg values can be assessed with Mg intake measured by the MgFFQ to determine overall sufficiency. Overall, the strong positive agreement between the MgFFQ and the 14-day food diaries validates that the MgFFQ serves as an accurate and quick method to estimate average daily Mg intake.