The human body strives to maintain a core temperature of around 37°C (98.6°F).
In environments where the temperature ranges between 28°C and 31°C (82-89°F), your body is able to comfortably maintain its core temperature even when standing as naked as the day you were born. But without adequate protection, your body does begin to lose heat to the surrounding environment when the ambient temperature drops below 28°C (82°F) [1]. Preventing this heat loss isn’t just crucial for the prevention of hypothermia, it’s crucial in preventing the unnecessary energy expenditure that would be required to make the heat to offset it.
Your body relies on this internal heat production, which it achieves through the metabolism of food or the utilisation of stored energy resources. In remote locations, acquiring this energy isn't straightforward – there’s no grabbing groceries from your local supermarket. On prolonged expeditions, your food supply may be limited to what you can carry. Squandering energy due to unnecessary heat loss either means lugging around more food—adding weight—or facing hunger. The energy output from persistent heat loss to the environment has to be replaced, which your body largely does through adaptive thermogenic responses like shivering. When the energy reserves required to do this become exhausted there will be a decompensation of these adaptive processes, and the hypothermic process can accelerate unchecked.
So how much energy does it take to heat yourself? Well, a decrease in ambient temperature of just 6°C between the relatively mild temperatures of 28°C and 22°C has been shown to increase heat production by 7% (an additional 457kJ in heat production across a 24-hour period) [2]. This is equivalent to one 250mL can of regular-variety Coca-Cola each day, just to offset the additional heat losses occurring with what is a fairly mild change in temperature.
At colder temperatures this is even more pronounced – another study found that staying at 7.5°C for 12-24 hours increased heat production by 30-50% [3]. At sub-zero temperatures this can be expected to have an even more significant effect. The increase in energy expenditure that comes from cold exposure (as well as it’s suppressive effect on insulin secretion) has even led some leading lights in the Canadian scientific scene to suggest sticking people out in the cold as a putative therapy for obesity and type II diabetes – which all sounds a bit Spartan [4]. The important point here is that keeping warm is therefore crucial in the attempt to conserve rations and energy.
Note that in early hypothermia you can be energy-deplete without being hypoglycaemic. Initially there is a catecholamine-induced glycogenolysis which causes a paradoxical hyperglycaemia. However, as energy stores get depleted with time, hypothermia can induce a profound hypoglycaemia - especially once the catecholamine surge becomes depleted or wears off during re-warming [5]. The inverse is also true, as hypoglycaemia will impair thermogenesis and further exacerbate hypothermia, creating a vicious cycle. Heat production in cold environments is predominantly a result of carbohydrate and lipid catabolism. This is about 33-78% reliant on carbohydrates and 14-60% reliant on lipids [6], though as glycogen reserves get depleted, muscles do seem to make a switch to preferential lipid peroxidation, with carbohydrate metabolism falling 2.4-fold and lipid metabolism doubling [3].
Cold weather environments facilitate heat loss through various avenues: radiation, convection, conduction, evaporation – all are formidable enemies that demand mitigation efforts, one of which is the replenishment of these energy stores. The question is, with what?
In one study, Naval Special Warfare (NSW) SEAL Qualification Training (SQT) students were found to have a 1,420 kcal/d deficit during alpine training [7]. Now, a large part of this deficit will of course be exercise-related, though the study authors also measured the deficit occurring in a non-alpine environment and found only a 1044 kcal/d deficit, so there may be some contribution from cold weather (though it may also be that it’s just harder work operating in a mountainous environment). It’s difficult to draw any real conclusions from this as it relates to cold exposure, given there are some very large reference ranges and we don’t know what the difference in physical activities between days were. Either way, the special operations community have been actively using food as medicine to tackle this issue, and utilising specialist diets as a means of offsetting the energy deficit and negative protein balance that occurs as a result of cold exposure. Of all the macronutrients, it’s been found that high fat diets in the field had the greatest effect on minimising the loss of body mass and maintaining net-protein balance, at least when studied on Norwegian soldiers conducting a 4-day 51km Arctic ski march [8]. High fat diets are also more energy dense (9kcal/g) than carbohydrates (4kcal/g) and therefore most efficient when it comes to carriage and storage, so their dietary supplementation in cold weather environments was recommended [9]. It has been the authors experience however, as it has been for others [10], that high-fat foods are often not particularly palatable in a high-altitude environment where appetite suppression is a factor.
Now, this is all very well and good, and it does tell as a lot about the relationship between diet and performance in cold weather environments – but not necessarily what the optimal macronutrient intake is when imminently succumbing to the cold. It’s been argued that defending glycogen reserves for as long as possible through simple carbohydrate supplementation is probably the best strategy, as they can be rapidly metabolised and distributed to tissues [11,12]. Snacking on small regular portions of chocolate, confectionary, and gels may be the easiest to do as they are energy dense and can be eaten in smaller more frequent portions [10], but they can be harder to manipulate with cold hands or bulky gloves. We will definitely go as far as making the sweeping statement that that caloric intake of some sort will provide the supplementary substrates necessary to reinforce endogenous stores and allow shivering thermogenesis to continue for as long as possible [13]. This is important for both hypothermia prevention and the treatment of early hypothermia (e.g. HT1), but whether there would be sufficient splanchnic perfusion to allow functional gastrointestinal food absorption beyond that point is another question entirely – a question for which I can find no real answers. You could argue that the diversion of blood away from the vasoconstricted peripheries might increase visceral blood flow, but you could equally argue that the increase in sympathetic tone might slow gastrointestinal motility and divert blood away from it. There are numerous rodent studies and studies of patients on cardiopulmonary bypass that for a variety of reasons are not necessarily generalisable to the hypothermic patient in the field. You could also argue that if they’re hypothermic enough to have impaired gut perfusion there’s probably some impaired brain perfusion (e.g. in HT3 or beyond) and they may be in no state to be taking on oral food and fluids – in which case, you’re giving your calories in the form of IV dextrose (ideally warmed to 42°C). This is partially possible in some remote clinic environments – its been shown that sticking a 500mL bag of fluid in the microwave for 100 seconds at 400 Watts or 50 seconds at 800 Watts will achieve fluid temperatures of about 36°C [14]. There are two issues with this – firstly, the study only looked at non-dextrose crystalloids, presumably because the authors were worried that if they put glucose-containing solutions into the microwave they might pull out a muffin, and secondly, microwaves are in rather scant supply on the mountainside.
In summary:
Exhaustion and exposure go hand in hand. Keeping warm means you will minimise your energy expenditure and conserve your rations, but conversely maximising energy intake will also assist you in keeping warm. Remember when you see hypothermia, think about hypoglycaemia and vice versa. While hyperglycaemia is often present, this does not reflect the presence of adequate energy stores, but instead a hypercatabolic state due to energy depletion, and hypoglycaemia may still occur upon rewarming. One therapeutic modality that should always be considered in the prevention and treatment of hypothermia is the restoration of energy reserves, because without it, we will always lose the battle against the second law of thermodynamics.
Credit:
RN1, DR1
References:
[1] Davis PR, Byers M. Accidental hypothermia. J R Army Med Corps. 2005;151(4):223-33.
[2] Dauncey MJ. Influence of mild cold on 24 h energy expenditure, resting metabolism and diet-induced thermogenesis. Br J Nutr. 1981;45(2):257-67.
[3] Haman F, Mantha OL, Cheung SS, et al. Oxidative fuel selection and shivering thermogenesis during a 12- and 24-h cold-survival simulation. J Appl Physiol. 2016;120(6):640-8.
[4] Ivanova YM, Blondin DP. Examining the benefits of cold exposure as a therapeutic strategy for obesity and type 2 diabetes. J Appl Physiol. 2021;130(5):1448-59.
[5] Strapazzon G, Nardin M, Zanon P. Respiratory failure and spontaneous hypoglycaemia during noninvasive rewarming from 24.7C (76.5F) core body temperature after prolonged avalanche burial. Ann Emerg Med. 2012;60(2):193-6.
[6] Haman F, Legault SR, Weber JM. Fuel selection during intense shivering in humans: EMG pattern reflects carbohydrate oxidation. J Physiol. 2004;556(1):305-13.
[7] Beals K, Perlsweig KA, Haubenstriker JE, et al. Energy deficiency during cold weather mountain training in NSW SEAL Qualification Students. Int J Sport Nutr Exerc Metab. 2019;29(3):315-21.
[8] Margolis LM, Murphy NE, Martini S, et al. Effects of supplementary energy on protein balance during 4-d Arctic military training. Med Sci Sports Exerc. 2016;48(8):1604-12.
[9] Margolis LM, Pasiakos SM. Performance nutrition for cold-weather military operations. Int J Circumpolar Health. 2023;82(1):2192392.
[10] Viscor G, Corominas J, Carceller A. Nutrition and hydration for high-altitude alpinism: a narrative review. Int J Environ Res Public Health. 2023;20(4):3186.
[11] Haman F, Peronnet F, Kenny GP, et al. Effect of cold exposure on fuel utilisation in humans: plasma glucose, muscle glycogen, and lipids. J Appl Physiol. 2002;93(1):77-84.
[12] Tikuisis P, Eyolfson DA, Xu X, Giesbrecht GG. Shivering endurance and fatigue during cold water immersion in humans. Eur J Appl Physiol. 2002;87:50-8.
[13] Haman F, Blondin DP, Imbeault MA, Maneshi A. Metabolic requirements of shivering humans. Front Biosci (Schol Ed). 2010;2(3):1155-68.
[14] Lindhoff GA, MacG Palmer JH. An assessment of the thermal safety of microwave warming of crystalloid fluids. Anaesthesia. 2000;55(3):251-4.