Serum 25-hydroxyvitamin D fluctuations in military personnel during 6-month summer operational deployments in Afghanistan

Changes in physical performance, body composition and physical training during military operations: systematic review and meta-analysis

Systematic review and meta-analysis applying PRISMA guidelines with a PICOS format was constructed to provide an overview of changes in physical performance, body composition and physical training in soldiers during prolonged (≥ 3 months) military operations. Twenty-four studies out of the screened 4431 records filled the inclusion criteria. A small decrease in endurance performance was the most consistent finding (Hedge's g [g] - 0.21, 95% CI - 0.01 to - 0.41) while small overall increases in maximal strength of the lower (g 0.33, 95% CI 0.16-0.50) and upper body (g 0.33, 95% CI 0.19-0.46) were observed. In addition, small increases in strength endurance (push-up, g 0.34, 95% CI 0.15-0.52; sit-up g 0.26, 95% CI 0.07-0.44) were observed. The overall changes in body composition were trivial. Heterogeneity in the outcome variables varied mainly between low to moderate. Large inter-individual variations were observed in physical training volume, including decrements especially in endurance training frequency and volume. A reduction in total training load was often associated with negative changes in body composition and physical performance according to the principle of training specificity. Individuals with higher initial fitness level were more susceptible to decrements in their physical performance during operation.

Nutrition, Immune Function, and Infectious Disease in Military Personnel: A Narrative Review

Consuming a diet that meets energy demands and provides essential nutrients promotes a healthy immune system, while both under- and over-nutrition have been associated with immune dysfunction. Military personnel comprise a unique population who frequently endure multi-stressor environments, predisposing them to immune decrements. Additionally, 49% and 22% of active duty U.S. military personnel are classified as overweight and obese, respectively. A literature search on PubMed was conducted to identify studies, reports, review papers, and references within those sources relevant to the topic area. Military personnel experiencing either under- or over-nutrition can suffer from degraded health, readiness, and performance. Insufficient intake of nutrients during military operations increases infection risk and negatively impacts infection recovery. Energy, protein, iron, zinc, and vitamins C and D are nutritional areas of concern that may impact immune competence in a multi-stressor environment. Over-nutrition can promote accretion of excess body fat and obesity, which contributes to a chronic inflammatory state that coincides with immune impairments. Prioritizing efforts to optimize nutrient intake is one approach for reducing disease burden and improving readiness. This review discusses nutritional concerns concomitant to multi-stressor environments that impact immune function, and the relevance of obesity to infectious disease risk in the military population.

Vitamin D status in active duty Navy military personnel: a systematic review

OBJECTIVES

Active duty Navy military personnel are prone to vitamin D deficiency due to an occupational environment detrimental to sunlight exposure. The main objective of this systematic review is to provide a worldwide overview of vitamin D status in this population.

METHODS

The Condition, Context, Population (CoCoPop) mnemonic was used to define the inclusion criteria (vitamin D status; all contexts; active duty Navy military personnel). Studies with recruits or veterans were excluded. Scopus, Web of Science and PubMed/Medline databases were searched from inception to 30 June 2022. Joanna Briggs Institute and Downs & Black checklists were used for quality assessment and data were synthesised in narrative and tabular formats.

RESULTS

Thirteen studies published between 1975 and 2022 and conducted in northern hemisphere Navies, including mainly young and male service members, were included. The prevalence of vitamin D deficiency was globally reported as significant. Nine studies included a total of 305 male submariners who performed 30-92 days submarine patrol and reported the effect of sunlight deprivation in the decrease of vitamin D levels.

CONCLUSIONS

This new systematic review underlines the high prevalence of vitamin D deficiency in the Navy, especially in submariners, and the need to implement measures to prevent vitamin D deficiency. Serum 25(OH)D data available and the heterogeneity of the studies limited a pooled analysis. Most studies included only submariners, which may limit generalisability to all active duty Navy military personnel. Further research on this topic should be promoted.

PROSPERO REGISTRATION NUMBER

CRD42022287057.

The Effect of Sunlight Exposure on Vitamin D Status in Countries of Low and High Latitudes: A Systematic Literature Review

PURPOSE OF THE REVIEW

To conduct a systematic review of studies to investigate the association between climate and vitamin D in humans.

RECENT FINDINGS

There is growing interest in the association between the environment and vitamin D, but robust methods to understand this relationship are lacking. Studies focus mainly on seasonality and latitude. Research quantifying sunlight exposure necessary for adequate vitamin D synthesis in people with darker skin color and those people living at low latitudes are scarce, as are studies in urban populations that may have limited opportunity for sunlight exposure. There are gaps regarding values and timing of exposure to UV radiation required for adequate vitamin D synthesis considering skin color, geography, climate, and local irradiation. Nature-based solutions (NbS) that can mitigate climate change will become increasingly important for preventing hypovitaminosis D. For example, tree-shaded spaces might encourage more participation in outside activities and thereby favor vitamin D synthesis by the skin.

No protective benefits of low dose acute L-glutamine supplementation on small intestinal permeability, epithelial injury and bacterial translocation biomarkers in response to subclinical exertional-heat stress: A randomized cross-over trial

Exertional heat stress disrupts gastrointestinal permeability and, through subsequent bacterial translocation, can result in potentially fatal exertional heat stroke. Glutamine supplementation is a potential countermeasure although previously validated doses are not universally well tolerated. Ten males completed two 80-minute subclinical exertional heat stress tests (EHSTs) following either glutamine (0.3 g kg FFM^-1) or placebo supplementation. Small intestinal permeability was assessed using the lactulose/rhamnose dual sugar absorption test and small intestinal epithelial injury using Intestinal Fatty-Acid Binding Protein (I-FABP). Bacterial translocation was assessed using the total 16S bacterial DNA and Bacteroides/total 16S DNA ratio. The glutamine bolus was well tolerated, with no participants reporting symptoms of gastrointestinal intolerance. Small intestinal permeability was not influenced by glutamine supplementation (p = 0.06) although a medium effect size favoring the placebo trial was observed (d = 0.73). Both small intestinal epithelial injury (p < 0.01) and Bacteroides/total 16S DNA (p = 0.04) increased following exertional heat stress, but were uninfluenced by glutamine supplementation. Low-dose acute oral glutamine supplementation does not protect gastrointestinal injury, permeability, or bacterial translocation in response to subclinical exertional heat stress.

Does vitamin D supplementation prevent SARS-CoV-2 infection in military personnel? Review of the evidence

For most individuals residing in Northwestern Europe, maintaining replete vitamin D status throughout the year is unlikely without vitamin D supplementation and deficiency remains common. Military studies have investigated the association with vitamin D status, and subsequent supplementation, with the risk of stress fractures particularly during recruit training. The expression of nuclear vitamin D receptors and vitamin D metabolic enzymes in immune cells additionally provides a rationale for the potential role of vitamin D in maintaining immune homeostasis. One particular area of interest has been in the prevention of acute respiratory tract infections (ARTIs). The aims of this review were to consider the evidence of vitamin D supplementation in military populations in the prevention of ARTIs, including SARS-CoV-2 infection and consequent COVID-19 illness. The occupational/organisational importance of reducing transmission of SARS-CoV-2, especially where infected young adults may be asymptomatic, presymptomatic or paucisymptomatic, is also discussed.

Influence of aerobic fitness on gastrointestinal barrier integrity and microbial translocation following a fixed-intensity military exertional heat stress test

PURPOSE

Exertional-heat stress adversely disrupts gastrointestinal (GI) barrier integrity, whereby subsequent microbial translocation (MT) can result in potentially serious health consequences. To date, the influence of aerobic fitness on GI barrier integrity and MT following exertional-heat stress is poorly characterised.

METHOD

Ten untrained (UT; VO_2max = 45 ± 3 ml·kg^-1·min^-1) and ten highly trained (HT; VO_2max = 64 ± 4 ml·kg^-1·min^-1) males completed an ecologically valid (military) 80-min fixed-intensity exertional-heat stress test (EHST). Venous blood was drawn immediately pre- and post-EHST. GI barrier integrity was assessed using the serum dual-sugar absorption test (DSAT) and plasma Intestinal Fatty-Acid Binding Protein (I-FABP). MT was assessed using plasma Bacteroides/total 16S DNA.

RESULTS

UT experienced greater thermoregulatory, cardiovascular and perceptual strain (p < 0.05) than HT during the EHST. Serum DSAT responses were similar between the two groups (p = 0.59), although Δ I-FABP was greater (p = 0.04) in the UT (1.14 ± 1.36 ng·ml^-1) versus HT (0.20 ± 0.29 ng·ml^-1) group. Bacteroides/Total 16S DNA ratio was unchanged (Δ; -0.04 ± 0.18) following the EHST in the HT group, but increased (Δ; 0.19 ± 0.25) in the UT group (p = 0.05). Weekly aerobic training hours had a weak, negative correlation with Δ I-FABP and Bacteroides/total 16S DNA responses.

CONCLUSION

When exercising at the same absolute workload, UT individuals are more susceptible to small intestinal epithelial injury and MT than HT individuals. These responses appear partially attributable to greater thermoregulatory, cardiovascular, and perceptual strain.

Low Vitamin D States Observed in U.S. Marines and Navy Sailors with Early Multi-Symptom Illness

Research has implicated immune system inflammation as an underlying etiology of multi-symptom illnesses, and vitamin D has been shown to have a significant role in immune system function. In this retrospective review performed on the medical charts of service members who presented with signs and symptoms of multi-symptom illnesses, we focused on serum 25(OH)D₃ levels and looked for associations of vitamin D status (deficient, insufficient, and normal) with age (20–31 years versus 31–56 years) and deployment status (war zones versus other). Two groups (U.S. Marines and Navy Sailors) were sampled and both showed high incidences of below normal vitamin D levels. However, with the Marines, age-related differences in serum levels (p = 0.009) were found only among those who deployed to Iraq/Afghanistan in comparison to those in non-combat locations. The comparison within the Navy sample showed that mobilized sailors had lower 25(OH)D₃ levels than the group that did not deploy (p = 0.04). In addition, 100% of the sailors who deployed had below normal levels versus only 33% in the cadre group. The data suggest that personnel returning from a war zone with signs of early multi-symptom illness should be checked for low vitamin D status.

Dietary Assessment Methods in Military and Veteran Populations: A Scoping Review

Optimal dietary intake is important for the health and physical performance of military personnel. For military veterans, the complex nature of transition into civilian life and sub-optimal dietary intake is a leading contributor to the increased burden of disease. A scoping review was undertaken to determine what is known about the assessment and reporting of dietary intakes within both military and veteran populations. In addition, this review determines if studies reporting on the dietary intake of military personnel or veterans include comparisons with dietary guidelines. Six databases were searched to identify papers published from the database inception to April 2019. Observational and intervention studies were searched to identify if they assessed and reported whole dietary intake data, reported data exclusively for a military or veteran population, and included only healthy populations. A total of 89 studies were included. The majority of studies used one dietary assessment method (n = 76, 85%) with fewer using multiple methods (n = 13, 15%). The most frequent methodology used was food frequency questionnaires (FFQ) (n = 40, 45%) followed by 24-hour recalls (n = 8, 9%) and food records (n = 8, 9%). The main dietary outcomes reported were macronutrients: carbohydrate, protein, fat, and alcohol (n = 66, 74%) with total energy intake reported in n = 59 (66%). Fifty four (61%) studies reported a comparison with country-specific dietary guidelines and 14 (16%) reported a comparison with the country-specific military guidelines. In conclusion, dietary intake in military settings is most commonly assessed via FFQs and 24-hour recalls. Dietary intake reporting is mainly focused around intakes of energy and macronutrients. Most studies compare against dietary guidelines, however, comparison to specific military dietary guidelines is minimal.