I wanted to start this article with a statistic, with the percentage of army professionals that are malnourished. I searched and searched, but there isn't any quantification of this problem.
Does this mean that maybe there is no problem then? I suspect the lack of statistics stems by the fact that these problems are difficult to quantify, and is not an indicator that everything is going well.
Why would I say that? Well, firstly because it's particularly difficult to get data on weight, metabolism, food consumption, and other relevant metrics from military populations. The priorities lay elsewhere, and the equipment and time dedicated to tracking nutrition status are enemies of efficiency. In addition, not all branches and deployment locations were made equal, so generic equations won't cover this either. However, there is a number of studies addressing specific nutrients that have been found low in soldiers.
Vitamin D
Soldiers spend a significant amount of time indoors. Sometimes, it's the nature of the training - a simulation of combat situations inside a facility. Some other times, it's the weather conditions, especially in specific deployment areas with extreme heat. And most of the time, it's the computer systems that need to be monitored or the administrative tasks that have to be performed. Vitamin D cannot be synthesized without exposure to the sun, and balancing that out with diet is very, very, very hard.
Iron
Iron is in high demand within the body when rigorous physical exercise is performed, because it transfers oxygen to our muscles. This makes it hard to obtain enough iron, especially if one has low iron intake in the first place, and/or constant losses.
Especially in the case of army, iron-rich food can be hard to find in specific locations. Poultry, seafood, and meat, which contain the most easily-absorbed iron, do not make convenient food items: refrigeration is needed, and their availability is limited. On top of that, the requirement to maintain a healthy body weight can limit the food one eats in general, further decreasing one's iron intake from other sources.
Losses of iron can occur in combat, where there's an increased risk of injury. But this type of blood loss isn't the only one to worry about: female soldiers are especially vulnerable to iron deficiency, because of they are menstruating. Monthly.
Calcium
Important for muscle contraction and lost through sweat, calcium is another element our body uses extensively when physical demands are high. If we can't get it through nutrition, our body will start ransacking the storage areas in our bones and teeth. In that case, bone density decreases and bone fractures can occur from falls and bumps that wouldn't hurt a healthy person.
One can get calcium from fresh fruits, vegetables, and dairy products - things hard to find and maintain when deployed. And there also the limited exposure to sunlight: Vitamin D is necessary for calcium absorption. Without it, soldiers are unlikely to manage to obtain enough calcium from their diets.
Vitamin B12
Gastrointestinal disorders are not uncommon among military personnel. Both the training and the missions can be stressful, and they usually include alterations to the diet, harsh environments, infections, and medications. All these elements can seriously disturb the intestines, which in turn makes is difficult to absorb enough vitamin B12 from the diet. Factoring in the challenge of obtaining and maintaining animal-based food products, we can confidently say that things are not looking well for soldiers regarding their vitamin B12 intake.
Omega-3 Fatty Acids
Another food that's difficult to be included in military diets is fatty fish. Salmon, mackerel, and tuna contain the essential for the human body omega-3 fatty acids, but they are often unavailable to soldiers deployed to remote or hostile environments. Which is an issue, since they are extremely relevant to soldiers. Specifically, stress and trauma can increase the needs of soldiers in omega-3 fatty acids, which have anti-inflammatory properties.
Magnesium
Magnesium demands increase proportionally to the physical demands, which is the reason why soldiers are at high risk for a magnesium deficiency. Soldiers also suffer excessive losses of magnesium through sweat, especially if deployed at a location where high temperatures are frequent. There's also the widespread abuse of antibiotics, to keep infections at bay, and diuretics, to reduce body weight: these medications that can affect magnesium absorption. Army food can't meet these requirements - mostly found in vegetables and whole grains, magnesium is not regularly in the diet of army personnel.
The good news is that there are plenty emerging fields of nutrition science and food technology to address these needs. Especially precision nutrition is expected to be at the forefront of tackling these issues and winning, eventually, the battle against nutrient deficiencies.