This very large (n = 26,680), randomized, comprehensive assessment of SM caffeine consumption found 87% of SMs consumed caffeinated products, with an average estimated consumption of 243 mg/day for consumers. Among SMs who regularly consumed caffeinated products, men consumed 29% more caffeine than women (251 vs 195 mg/day), but when caffeine consumption was adjusted for body weight, consumption was similar in men and women (2.93 vs 2.85 mg/day/kg, respectively, p=0.12), as reported previously in the general population [1, 15]. Coffee was the beverage most frequently consumed (by 68% of the population), followed by soda (42%), tea (29%), energy drinks (29%) and gums/candies/medications (4%). Total caffeine intake from coffee was 69%, energy drinks 17%, tea 8%, soda 6%, and gums/medications <1%. Consuming any caffeinated product was independently associated with female gender, older age, White race/ethnicity, higher BMI, less resistance training, current or former tobacco use, higher alcohol intake, and higher enlisted or officer rank. Higher energy drink prevalence was associated with male gender, younger age, less formal education, White race/ethnicity, higher BMI, more resistance training, current or former tobacco use, higher alcohol consumption, lower enlisted rank (compared to officers), and service in the Army or Marine Corps (compared to the Air Force) or in the Air Force (compared to the Navy).
It is well documented that the civilian and military populations are generally aware of the effects of caffeine on human cognitive and physical performance. Surveys of SMs and college students found they consume caffeine-containing products for several reasons related to the performance benefits of caffeine [19, 28, 29]. Furthermore, SMs assigned to units in Afghanistan and likely to be engaged in combat consumed higher levels of caffeine than SMs at their home bases. Caffeine use by these SMs was higher among those reporting difficulty remaining awake during guard duty, poor sleeping conditions, and sleep disruptions during nighttime operations [18]. In addition, a survey of active duty Army aviators found they consumed more caffeine than their peers in non-aviation units, especially to enhance performance degraded due to insufficient sleep and very disruptive work schedules [19].
US Department of Defense laboratories and their international collaborators have conducted multiple studies designed to simulate military operations demonstrating the cognitive and physical benefits of caffeine consumption by military personnel [30–32]. The Department of Defense recognizes the ability of caffeine to enhance cognitive performance and provides it in rations, when necessary, with appropriate labeling to inform SMs of the presence and effects of caffeine [33].
Caffeine Prevalence and Daily Consumption
Previous studies have been conducted on caffeine prevalence and daily consumption among Air Force [22], Army [20], and Navy/Marine Corps [21] personnel. All of these studies [20–22] used a slightly different questionnaire but the same definitions for caffeine sources. The Air Force [22] and Army [20] studies used a convenience sampling technique involving volunteers in face-to-face administrations at installations across the US and overseas, and the Navy and Marine Corps study [21] identified a random sample and asked for volunteers by postal letter and e-mail. The present study was quite similar to the Navy/Marine Corps study [21] in that a random sample of SMs were studied, but the questionnaire differed from that of previous studies [20–22]. Those studies listed not only generic sources of caffeine (e.g., coffee, tea, soft drinks), as in the present study, but specific products (e.g., Dr. Pepper soda, Monster energy drink, No Doz Gum) as well. Given these differences in study design, Table 4 compares caffeine use prevalence and daily consumption among the military services in the current and past studies. Prevalence estimates for any caffeinated product (≥ 1/week) were similar across all studies. With regard to individual caffeinated products, the previous Army and Navy/Marine Corps studies [20, 21] found the highest consumption prevalence for coffee, but Air Force personnel were unique in that cola was the most ingested product, with coffee ranking second [22]. The current study found that in all services, coffee was the product consumed most often. Daily caffeine consumption estimates were similar for Air Force personnel in the current and past [22] investigations, but estimates for Army, Navy, and Marine Corps personnel were 38% lower, 21% higher, and 16% higher, respectively [20, 21]. Differences in estimation of caffeine consumption from individual products in past studies [20–22] versus estimates from generic types (coffee, tea, soda) in the current study likely accounted for these differences. Most past studies [20, 21] and the current one agree in that SMs in all services consumed the most total caffeine (mg/day) from coffee, with energy drinks ranking second.
Table 4
Comparison of studies on prevalence and amount of caffeine consumption in military servicesa
Measure | Study | Air Force | Army | Marine Corps | Navy |
Prevalence (%)b | Previousc | 84 | 82 | 86 | 88 |
Current | 86 | 87 | 85 | 89 |
Daily Consumption (mg/day) | Previousc | 212 | 347 | 232 | 217 |
Current | 215 | 252 | 270 | 264 |
aCaffeine consumers only |
bCaffeine prevalence is defined as use ≥1 time/week. |
cPrevious studies include those of the Army [20], Air Force [22], and Navy/Marine Corps [21]. |
Several population-based estimates of caffeine consumption in adult Americans based on very large population samples using state-of-the-art dietary intake procedures are available. NHANES caffeine intake [1, 13, 15] was calculated based on 24-hour dietary recalls in 2001–2012. Estimated caffeine use prevalence in adults (>19 years) was 89% for men and 89% for women [1]. Caffeine consumption estimates for consumers of caffeine varied from 189 to 211 mg/day for men and 149 to 161 mg/day for women [1, 13, 15]. The Kantar Worldwide Beverage Consumption Panel obtained data on US consumers from an online, 7-day beverage consumption record and found ~90% of individuals ≥ 18 years of age consumed caffeinated beverages, with average caffeine consumption equal to about 200 mg/day among caffeine consumers (males and females were not separated) [2]. Prevalence values in these population-based studies were similar to the 87% observed in SMs (≥1 week), while the average consumption in SMs of 251 and 195 mg/day for males and females, respectively, was somewhat higher than in the civilian population.
At least three other surveys [20–22] of the individual branches of service have observed caffeine-intake levels similar to those reported here and higher than those in the civilian population. The extensive and unique demands of military service may be a factor that explains the difference in caffeine intake in military versus civilian personnel. Differences in the methods and the demographic characteristics of the samples used in civilian studies and the current investigation must also be considered when interpreting these differences. For example, active duty SMs are younger, fully employed, and sleep somewhat less than the general population [16].
The estimated average daily caffeine consumption of military personnel who are regular caffeine consumers was well below the levels that are widely recognized as safe: 400 mg/day for men and 300 mg/day for women of reproductive age [5–7]. Nonetheless, the present study found that caffeine consumption of ~15% of men and 8% of women exceeded 400 mg/day, and that of 15% of women exceeded 300 mg/day. These proportions are very similar to those found in past military studies [21, 22]. Some individuals may be able to consume higher amounts of caffeine without adverse effects, although this cannot be determined from the current data. A genetic polymorphism allows some individuals to metabolize (N3-demethylation) caffeine in the liver more rapidly than others, and another polymorphism may be associated with higher caffeine tolerance and consumption [34–36].
Energy drink use prevalence (≥ 1 time/week) was 29% in the present study and varied from 21–39% in the military studies [20–22, 37–39]. Various studies of energy drink consumption among US college students found that 39% reported consuming in the past week [40], 36% within the past 2 weeks [41] and 36% within the past year [28]. Data from several NHANES cycles indicated that prevalence of daily consumption of energy drinks among adults has increased from 2003 to 2016 [10]. With regard to caffeine consumption, the current study found that 17% of the total caffeine was consumed from energy drinks. Data from NHANES suggested only 1–2% of total caffeine consumed by Americans was from energy drinks [1, 13], but a study of a convenience sample of geographically dispersed college students in the US found 22% of their total caffeine consumption was from energy drinks [28]. In summary, the prevalence of energy drink consumption by SMs, and the proportion of total caffeine consumption from energy drinks by SMs, are similar to those of college students— despite the generally older age of SMs—and much higher than those of the general US population.
Characteristics Associated with Caffeine Consumption
In the univariate analysis, there was little difference in the prevalence of consumption of caffeinated products and coffee by men and women. In the multivariate analysis, however, women had greater odds of consumption than men. This was primarily due to the influence of alcohol consumption in the statistical models, although smoking and smokeless tobacco also had minor effects. Caffeine consumption increased as alcohol intake increased, or if individuals were tobacco users; men were more likely to be higher alcohol consumers or tobacco users. Alcohol or tobacco use accounted for a larger proportion of the odds ratio for the effect of sex on caffeine consumption in men than in women. Because of this, the odds of consuming caffeine became lower in men than in women. Dividing the higher odds of caffeine intake in women by the lower odds of caffeine intake in men resulted in the larger odds ratio for women for any caffeinated product and coffee. If alcohol consumption, smoking, and smokeless tobacco use were not included in models 1 and 2 (Table 3) the odds ratios (95% confidence intervals) for women (compared to men) were 1.02 (0.92–1.14) and 1.02 (0.94–1.11), respectively.
In agreement with the current study, others [1, 12, 13, 20, 21] have reported that men consumed more caffeine than women. Nonetheless, this study and others [12, 21, 22] found that when caffeine consumption was determined on a per kg body weight basis, men and women consumed similar amounts. Although coffee was the major source of caffeine for both men and women, female SMs consumed more caffeine from tea while male SMs consumed more caffeine from soda and energy drinks. Acute caffeine consumption modestly affects moods such as vigor and fatigue as well as hemodynamic measures (e.g. blood pressure, cardiac output) in men and women [42–44], although cardiovascular effects are more likely to be observed at higher doses. Both men and women report consuming caffeinated products to provide behavioral benefits such as increased alertness [19, 28, 29].
Investigations involving representative civilian [1, 2, 12, 13, 27] and military [21, 22] samples reported that overall use and/or amount of caffeine consumption increased with age, although prevalence and/or caffeine amounts decline at the highest age groups (generally >65 years) [1, 2, 12, 13, 27]. Also in general agreement with past military studies [20–22], the current study found that coffee consumption accounted for most of the caffeine ingested in all age groups, but younger (<40 years) individuals consumed over twice as much caffeine from energy drinks as older (≥40 years) individuals (46 vs 22 mg/day, p<0.01) and were almost twice as likely to consume energy drinks (33 vs 17%, p<0.01). Energy drinks were introduced into the American market in 1997 [45], and their advertising was targeted to teenagers and individuals in 18- to 34-year-olds [46]. This advertising may have influenced energy drink consumption in the younger age groups in the current study.
Other civilian [13, 15] and military [20–22] studies have reported that compared to Whites, Blacks have a lower prevalence of caffeine use and a lower total caffeine consumption, accounted for largely by less coffee consumption [20–22, 47]. There are race/ethnic differences in dietary intake [48, 49], and some of these differences appear to be partly accounted for by educational level and income [49, 50]. In the current study, differences between Black and White SMs in caffeine and coffee prevalence remained after controlling for formal educational level, rank (a surrogate for income), and other factors, in agreement with past military studies [21, 22]. The reasons for the race/ethnic differences are likely complex and may be different in the military compared to the general population.
In agreement with other investigations [20, 21, 28], the current study found no systematic association between weekly aerobic exercise duration and caffeine use prevalence. One study of Air Force personnel [22] found prevalence decreased with increased aerobic activity duration; in the current study, when Air Force personnel were considered separately, this relationship was not duplicated (data not shown). For resistance training, both univariate and multivariable analysis showed the lowest caffeine use prevalence in the group exercising the most with little difference among the other groups, in general agreement with most other military studies [21, 22]. One study which separated Army personnel into those who performed weight training and those who did not found that trainers had higher overall use prevalence [20], also in agreement with the current study. Previous military studies have shown that dietary supplement use was strongly associated with increasing resistance training duration [21, 23]. Many dietary supplements contain caffeine, and the caffeine content of some of these can be very high [51]. Accurately determining the caffeine content of dietary supplements is difficult because manufactures are not required to list the amount of caffeine on their supplement facts labels, amounts are usually not available on company websites, and if the ingredients are proprietary, the manufacturer is not required to list caffeine at all [51]. It is possible that SMs involved in large amounts of resistance training consumed less caffeine from beverages to avoid adverse effects resulting from high dosages of caffeine in their dietary supplements. Overall, the current data and previous investigations suggest little relationship between aerobic exercise duration and caffeine use prevalence, but for resistance training there appears to be a bimodal relationship such that those performing the least or the most training have lower use prevalence than those performing moderate amounts of training.
Current or former tobacco use (smoking or smokeless tobacco) was associated with a higher use and higher intake of caffeine, especially for coffee and energy drinks, in both univariate and multivariable analyses. Although associations with smokeless tobacco have not been previously investigated, associations between caffeine prevalence and smoking have repeatedly been reported in both military [20, 22] and civilian populations [14, 15, 52–58]. Smoking accelerates caffeine metabolism and reduces its half-life [59, 60] suggesting that smokers consume more caffeine to achieve stimulatory effects. In addition, both caffeine and smoking increase dopaminergic activity in different brain regions, and the two substances may be used concurrently to potentiate stimulation [61]. Studies of monozygotic and dizygotic twins suggest that both genetic and environmental factors may be involved in the relationship between tobacco use and caffeine [62–65] with genetic factors becoming more important as individuals age [64]. There is also specific genetic evidence for a casual effect of smoking on caffeine intake. Specific single nucleotide polymorphisms (SNPs rs16969968/rs1051730) on the nicotinic receptor subunit gene (CHRNA5) increase daily cigarette consumption among smokers. As the number of these SNPs increases, so does coffee consumption [58].
Another lifestyle factor strongly associated with caffeine prevalence and consumption was alcohol intake. In both univariate and multivariable analyses, use prevalence of caffeinated products of all types increased in a dose-response manner as alcohol consumption increased. The amount of caffeine consumed from coffee and energy drinks increased as alcohol intake increased. Similar relationships have been found in other studies for coffee [14, 21, 56, 66], energy drinks [21, 67–69], and overall caffeine use [15, 21, 69]. Studies of monozygotic and dizygotic twins suggested that there was a common genetic factor underlying this association, but environmental influences still seemed to contribute to the variance in caffeine consumption [63, 64, 70]. A recent study based on variations in SNPs support that the genes underlying the use of both coffee and alcohol were heritable [71]; however, two-sample Mendelian randomization suggested there was no causal association between coffee consumption and alcohol consumption [71, 72]. Psychosocial factors may play a role in this association since studies have consistently shown that higher levels of sensation–seeking behaviors are associated with both higher caffeine and alcohol use [73–75].
In the current study, SMs who reported less daily sleep consumed more caffeine for all sources, in agreement with past military [21, 22, 76] and civilian [77] studies. Military personnel sleep less than civilian populations [16, 17] and averaged 6.3 hours in current study, less than the recommended ≥7 hours/night [78]. Military training and operations can occur at any time of the day, can extend continuously for many days, and can involve substantial loss of sleep. Caffeine can increase alertness due to its ability to block central adenosine receptors [79]; when ingested in sufficient dosages, it can reduce sleep duration [80], and it improves cognitive performance, especially vigilance [81–83].
Strengths and Limitations
A major strength of this study was recruitment of a very large, stratified random sample of SMs who answered a standard set of questions on their consumption of specific caffeinated products. With a few exceptions, the data largely confirm results of past investigations of caffeine prevalence and consumption involving smaller studies of separate military services, using convenience samples [20–22]. Nonetheless, there are several limitations to the current analyses, most of which relate to difficulty in estimating daily caffeine consumption. First, all data were self-reported, and the usual shortcomings associated with this method, including recall bias, social desirability, errors in self-observation, and inadequate recall, apply [84, 85]. These biases could account for errors in reporting serving sizes and how many times per week SMs used caffeinated products and, as a consequence, errors in estimating caffeine consumption. Second, caffeine data for this study were obtained from beverages and gums/medications; we purposely did not assess caffeine intake from food sources as beverages account for 98% of caffeine consumption [1]. Third, caffeine contents of products were estimated based on standardized values of each type of caffeinated product. Specific products can differ in caffeine content [26, 86–88]. Finally, a large number of statistical tests examining relationships between caffeine prevalence and consumption and the demographic, lifestyle, and military characteristics were conducted, thus increasing the probability of Type 1 errors.