Dietary intake and stress fractures among elite male combat recruits

Military personnel represent a unique population exposed to intense physical and cognitive demands during both training and operational missions. Typically, military service commences with basic training (BT) which is characterized by intense physical training, emotional and mental stress [1]. It should be emphasized that such a challenging environment is enhanced during combat recruit training.

Individuals seeking to enhance physical performance through participation in arduous physical activity, particularly athletes and combat soldiers, must adhere to an appropriate and sufficient dietary intake [2, 3]. Inadequate energy intake can prolong recovery from illness and injury, depress immune function, and have a negative impact on physical performance in both training and operational activities [4, 5]. It has also been reported that inadequate energy intake, especially restrictive dietary patterns, may be associated with the development of stress fractures [4].

The U.S. Army has published regulations which define the nutritional responsibilities of the Surgeon General of the Army, the Navy, and the Air Force. These regulations, referred to as the Military Dietary Reference Intakes (MDRI), evaluate the effects of environmental factors on energy and nutrient requirements and outline nutrition education policy [5]. The MDRI is a quantitative estimate of the recommended dietary intake for healthy military populations based on US national standards [5]. The Nutritional Standards for Operational and Restricted Rations (NSOR) was established to take into account the higher energy expenditure in field exercises and other operational and logistic factors relevant for training [5]. As an example, studies that quantified energy expenditure during military operations report that Special Forces soldiers had up to 45% higher absolute energy expenditure compared to their non-combat counterparts [6, 7]. During prolonged training periods, if energy deficits occur, this may endanger the general health of the soldiers and reduce the muscle mass and bone strength needed for optimal performance. Of note, previous reports have found an association between insufficient dietary intake and increased risk for stress fractures among military recruits [8–10].

Bone overuse injuries, also referred to as stress reactions and stress fractures, are the most common overuse injuries among combat soldiers and are observed most frequently among young army recruits who undergo strenuous exercise during basic training [11]. The occurrence of severe cases of stress fracture has even reached rates as high as 64% in the Finnish army [12] and 31% in the Israeli Defense Forces (IDF) [13].

Stress fractures have been found to be related to several risk factors, both intrinsic and extrinsic [14], over most of which we have no control [13]. These include bone geometry parameters (studied thoroughly in the IDF), gender and hormonal factors, and genetic predisposition. Studies on bone density have been contradictory [14], and biochemical markers of bone turnover are also probably not related to stress fractures [15]. Calcium deficiency has been found deterrent to bone quality in animal models [16, 17] but studies on athletes and soldiers have been less conclusive. Calcium and vitamin D are probably important in women [18] and in Finnish males (who may be effected by the latitude) [19], but in general, there is not enough data on males. Lappe et al managed to reduce stress fracture incidence in female navy recruits by about 20% [9]. Smoking (present or history) has also been found to be related to stress fractures, particularly in the US [20], and is possibly related to risk taking behavioral patterns. However, this finding has not been reproduced consistently in other militaries [19, 21].

The purpose of this study was to evaluate nutritional intake in male combat recruits before induction and during a 4-month BT period. A secondary aim was to determine the possible association between specific nutrient intake, serum markers of nutrition and the occurrence of bone stress reactions.

Out of the seventy four recruits who completed all data collection during the 6-month training program, twelve recruits were diagnosed with stress fractures of the long bones (tibia and femur) by imaging during the 6-months. The results of the measured variables (i.e., anthropometry, nutritional consumption, and hematology) are presented for a total of 74 soldiers: 12 SF recruits vs. the 62 NSF recruits.

The aim of this study was to evaluate a possible relationship between nutritional intake before induction and during BT and long bone stress fracture occurrence among male combat recruits. We monitored 74 recruits through a 6-month period (4 months BT and 2 months advanced training) of intense physical and mental training. This period is also characterized by a major change in nutritional habits, partially resulting from eating in mess and rations provided in the field. One of the consequences of these changes in lifestyle and training regime was that 16% of the recruits developed stress fractures in their long bones, similar to previous reports on recruits performing this type of training.

Stress fracture susceptibility is multi-factorial. A main factor in the mechanism of long bone stress fractures is imbalance in osteoclasis and osteogenesis that are over-activated to enable the bone remodeling necessary for the bone strengthening [28–31]. In this milieu of excessive osteogenic needs, it is reasonable to assume that dietary deficiency of calcium and/or vitamin D could contribute to stress fractures, as has previously been implicated in rickets, osteomalacia, osteopenia, osteoporosis, fractures and other cases of excessive bone resorption [32].

In this group of recruits, we found considerable dietary deficiency. First, despite the high energy needs during this period of training, the recruits consumed only 70% of the energy recommendations of the NSOR, with the NSF group reporting an 8.4% decrease in their BT total energy intake compared with the pre-induction total energy intake. This low intake may be explained by the presence of fundamental stressors in the military environment, such as periodic food restrictions, sleep deprivation, mental burden, and constant physical evaluations. These findings are in accordance with previous studies pointing to the fact that military personnel normally consume insufficient energy, whether or not they are provided with an adequate amount of food [33]. In this study, the deficient energy intake was not associated with a weight loss but rather an increase of body weight during BT by 1.5%. This is also in line with previous studies, specifically that in this training program the gained weight was in lean body mass and not in fat [34].

We are concerned that our participants did not meet MDRI requirements. These deficiencies were observed for nearly every nutrient evaluated in the FFQ. The highest deficiencies were for vitamin D and calcium in the SF group, both around 60% of the MDRI before induction and also during BT. Of note, among the NSF group, vitamin D intake was the second most deficient variable, reported to be consumed at a level of 78.7% from MDRI before induction and at even a lower level of 59.6% during BT. In our study, the SF recruits reported 41.0% less initial calcium and vitamin D intakes on induction day than the MDRI recommendations. Although vitamin D3 (cholecalciferol) is either formed in the skin after exposure to sunlight or obtained from nutritional sources, especially fatty fish [32], most IDF soldiers use sunscreen and wear long-sleeved clothing during military training. This may limit vitamin D3 synthesis, and therefore, the importance of balanced nutritional intake, especially of vitamin D and calcium, should be emphasized, even though we did not actually find low serum levels of vitamin D.

Release of PTH is controlled by the level of calcium in the blood, with low blood calcium levels causing an increase in PTH. The main purpose of this hormone is calcium homeostasis. It is therefore not surprising that in these healthy young recruits, we did not find any pathological PTH or calcium values. A slight trend towards higher levels of PTH in the 4-month BT may represent a lack of dietary calcium. However PTH differences between SF and NSF or between induction values and 4 or 6 month values were not significant.

Furthermore, we found that the SF group compared to NSF had insufficient intake of zinc and magnesium (93.8% to 80.5% and 98.0% to 82.9% of the MDRI, respectively) throughout BT. In addition to calcium, minerals and trace elements such as zinc and magnesium are involved in skeletal growth and are required for normal bone metabolism. An adequate intake of these dietary components is therefore necessary to assure optimal bone quality and prevention of bone loss [35].

It is also evident that during BT, SF soldiers developed iron deficiency and anemia symptoms associated with 39% low transferrin saturation (< 16%), 36.4% ferritin deficiency (< 20 ng/ml), and 37.9% hemoglobin deficiency (< 14 g/dl). Notably, similar findings were observed in previous studies involving elite Israeli male athletes [36, 37], and in female combatants [38]. Moreover, it is important to note that iron and ferritin levels are a part of an innovative prediction model for stress fractures in young female recruits during basic training, which managed to correctly predict stress fracture occurrence in 76.5% of a sample population [39].

The study has several limitations. Assessing food consumption based on a person's memory is always problematic. This is more so with recruits in a very intense physical and mental training schedule. We also did not monitor personal initiatives in taking nutritional supplements. Previous surveys have demonstrated this to be negligible, with recruits showing minimal interest in calcium and vitamin D. Another problem is the lack of finding of low vitamin D levels, despite the dietary deficiency. We also did not measure serum zinc levels, however, following these results it would seem beneficial to measure these levels for future research on recruits.

The main conclusion from this study is that, contrary to previous beliefs, male infantry recruits in the IDF are nutritionally deficient, specifically in calcium and vitamin D, and those who were more deficient developed more stress fractures. This directly arouses the debate on supplying supplements, following Lappe et al. in the US Navy female recruits [9]. But it is doubtful whether such an intervention is justified for a 20% decrease in stress fracture incidence in the IDF, and further research would be necessary to prove the efficacy in IDF male combat recruits. Another issue is related to the fact that there was dietary deficiency before induction, making intervention by the military at the most appropriated time more complicated. Based on our findings it might be plausible to perform nutritional screening (e.g., questionnaire) of elite combat recruits on induction and possibly assess the deficient subjects for serum levels. We could then treat those with low levels.

It should therefore be emphasized that while engaging in strenuous physical training, proper nutrient intake may act as a long-term protector against bone resorption and stress fracture development, and is recommended for maintaining healthy bones [40].