Imagine surfacing from a dive and breathing crisp cold air with a panorama of snow-capped mountains in the distance. The shore is empty, the ocean around you quiet, devoid of drunken summer pleasure cruisers. You have just made a deep dive to a depth you used to only feel comfortable attempting in the summer. Or you have swum several kilometers to an undiscovered dive site. Perhaps you are lost in contemplation of eagles soaring overhead, the crisp winter visibility below, or the nirvana one might attain trying to heat the whole ocean with a singularity-sized metabolic furnace.
The winter freediver is best described as an adventurer. This species of breath hold diver is willing to suffer the bite of cold water on his or her hands and feet—but only to a certain point. This adventurer uses every available strategy to prolong and enjoy cold water diving, not merely to endure it. With this approach, deep training, long distance swims, night dives, and lengthy recreational freediving sessions are all possible.
Kimmo Lahtinen freedives in the lakes of Finland all year round. The water temperature is a constant 4 degrees Celsius. "It is a huge amount of work to make a hole in the ice," he says. But it is their only option for winter diving.
Juneau, Alaska is lucky to have a milder North Pacific climate. Ward Ward, a Deeperblue denizen living in Juneau, dives throughout the year and especially relishes winter diving. "For those of us freediving in the northernmost latitudes, water temperature is a moot point—here I am, there’s the water, why am I still standing here? The phrase ‘I am, therefore I dive’ comes to mind."
Eric Fattah, former constant ballast world record holder and inventor of the Hypothermic Diving System, revels in the challenges of winter freediving in Vancouver. His reward for studying the effect of diet on freediving and thermogenesis, equipment configurations, training techniques and supplementation is evident in the deep dives he has made in the darkest winter months. Most people moan about the break in constant ballast training. "I’ve always wanted the option to train deep all-year-round," he said, after a recent training session. "Now it’s a reality."
If winter to you means trading in your Speedo for a 3mm suit, then you may not understand the addiction of winter freediving. However, if you scrape ice from your car windows in the morning on the way to the dive site or suit up in the freezing rain, I hope this article will give you that extra edge for enjoying the treasures that await the winter freediver.
"Shiver Your Weight Off" TM
Most people have an aversion to shivering. Why is this? Shivering stimulates the body, raising metabolic heat production up to seven times the basal metabolic rate. This increases blood flow to the skin and muscles and also produces sufficient anxiety to encourage you to move to warm shelter. It is a clear and unpleasant signal: "Get warm, you idiot." Unfortunately for freedivers, shivering in cold water usually comes with this negative psychological impact, enough to ruin the entire diving session for most.
Winter diving is cold. Part of enjoying the experience is to change the feeling associated with shivering from a negative to a neutral one (or a positive feeling, if you are in to S&M). Instead of retreating into the stress of getting cold and letting anxiety take over, it is important to view shivering as a meaningful signal from the body and nothing more. "Ah, this is an early warning—if I don’t change anything, I will continue to lose heat and fall into hypothermia."
As a person’s core body temperature drops, at some critical body temperature, shivering will begin. That critical temperature is often called the ‘critical shivering temperature.’ It has been shown that frequent exposure to cold water can bring an habituation and a reduction in the critical shivering temperature. This does not mean that your body stays warm, but demonstrates that shivering is largely influenced by perception of cold.  By mid-winter, a freediver diving at least once a week might experience this habituation. If he or she learns to make the experience a neutral one, then shivering gradually becomes a non-issue, except as a tool for gauging body temperature.
In the overall approach to enjoying winter freediving, habituation plays a minor role. There are so many other ways to fight the cold, providing you use them. The greatest potential danger of winter freediving is overestimating your ability to cope with the elements. Coping with shivering is one thing, but ignoring it is stupid. When shivering and numbness result in a loss of motor control, that is when accidents happen.
If you are still afraid of shivering, then I’ll let you in on a little secret, it’s called Laminar’s Secret Weight Loss Program. Depending on your physiology and diet, shivering remains one of the best workouts around. I know at least two people who lost over twenty pounds over a winter of freediving. Mind you, cold exposure is catabolic and could reduce muscle mass as well, so it is not a proven technique! However, many winter explorers will agree that despite eating mountains of fatty foods and protein-rich meals they were still unable to maintain their weight during prolonged cold exposure. 
The point of this article is that you do not need to torture yourself. The intelligent approach to winter freediving involves generating your own heat, retaining it for as long as possible and then, once you are cold, reheating as effectively as possible. You might even avoid shivering all together.
PART ONE: INTERNAL HEAT PRODUCTION
If you have been freediving in cold water (< 12°C), you may have noticed, with frustration, that some days you start shivering almost immediately, while on other days, it takes much longer, if you even shiver at all.
What is the reason for this difference in cold tolerance? And why do some people seem to have a much greater ability to withstand cold water immersion?
Body composition, especially body fat percentage, can make a big difference in cold tolerance. Just look at the Beluga whale with its great tracts of blubber. But then how do we explain a slim person’s ability to outlast a person with more body fat? The partial answer lies in diet and heat production pathways.
The process of heat generation within the body is called thermogenesis. There are two primary methods by which your body can generate heat:
- Shivering thermogenesis
- Non-shivering thermogenesis
Shivering thermogenesis uses glycogen stored in muscles as the fuel — it may also catabolize muscle protein if insufficient glycogen is available. Non-shivering thermogenesis produces heat without shivering. Brown adipose tissue (also called brown fat), has the ability to burn free fatty acids and oxygen to produce heat, without shivering.
Therefore, in order to allow your body to generate heat to its full ability, your muscles should be loaded with glycogen, and there must be free fatty acids in your blood. Since both your muscle glycogen and your free fatty acid level depend on your diet, it follows that your diet will have a profound impact on your cold tolerance. Kuroshima et al. found that "an integrating effect of cold and high fat diet could improve cold tolerance much more than cold acclimation itself, possibly through enhanced non-shivering thermogenesis caused by metabolic modifications such as increased lipid use." 
The Metabolic Furnace
The human body requires a certain level of activity to stay alive. This process, called the basal metabolism, produces heat. The functioning of the major organs, heart, liver, kidneys, and lungs are responsible for 75% of the body’s total basal heat production.
Many freedivers tend to fast on the day of diving, especially before deep training. The resulting reduction of the metabolic rate decreases oxygen consumption, CO2 production and core temperature. This may be great for summer diving or in warm water, but it is foolish in the winter. Insufficient food intake over several days can ruin your winter freediving experience and poses the potential danger of increased susceptibility to hypothermia. Something unexpected can always happen. A tear in your suit, a sudden storm or rip current, or a boat accident could prolong or increase the intensity of cold exposure. Having full stores of fats, carbohydrates and proteins should be considered an added safety measure for winter diving.
For those freedivers looking to do deep dives, you may be tempted to stick with the intuitive argument that fasting is the only way to achieve good results. We have made deep and long dives here in Vancouver, in the middle of winter, after gorging ourselves on food between sessions. This has not been proven in a study, only in our diving experience. Digestion generates heat and keeps the supply of nutrients topped up. We are not sure if our dives are unaffected because digestion is inhibited to some extent or if the dive reflex after an hour in cold water is so strong that it diving ability is not affected. Whatever the reason, we only seem to benefit from eating before deep diving.
On the topic of Food
What you eat in the days leading up to your freediving session is crucial. Since the art of staying warm is a complex process, and not terribly well-understood, it is best to cover all the bases.
Fats (lipids): Haman et al. found that " lipids produced as much heat as all other fuels combined." . However, one must be very careful as to what kind of fat to eat. ‘Bad’ fats have been associated with every possible disease. Healthy fats come mainly from uncooked foods such as nuts, seeds, olives and avocados. Avoid fried foods, trans-fatty acids and other mutant fats formed from cooking.
Carbohydrates: Carbohydrates are required to replenish muscle glycogen in muscle (the main fuel for shivering thermogenesis) and catalyze lipids for thermogenesis. Since shivering has the potential to fully deplete glycogen stores, preventing the utilization of lipids to produce heat, it is important to start off with full reserves. If you run out of glycogen, you’re screwed. 
Protein: Some protein is used as fuel with glycogen as the helpful catalyst, supplying energy for about 10% of total thermogenesis.  Freediving in cold water can be a catabolic experience for the body’s muscle tissue if the body lacks available protein and fats. As in rigorous training, sufficient protein intake before and after diving can offset muscle catabolism. 
"It’s All Over When the Neoprene Man Shivers"
Once I start to shiver violently, not small tremors, but body rippling waves of involuntary muscle contraction, then diving is over for the day unless I warm up again.
In this state, it is difficult to relax muscles or to release the tension in the diaphragm for a full breath. Carbon dioxide production and oxygen consumption can double or triple normal levels. Not only is intense shivering bad for performance freediving, shivering itself is an inefficient heat production mechanism and can actually increase heat loss by 25%. While the body initially reduces blood flow to the skin to conserve heat, the onset of shivering causes warm blood to be shunted from the body core to the skin and muscles, further reducing core temperature. Vasodilation of skin and muscle tissue results in heat loss. 
To understand this mechanism, compare an insulated thermos of boiling water to a metal pot of boiling water with no lid—which do you think cools faster? The more insulated the body core is from the cold environment, the more slowly it will cool. This principle is what makes the freediving wetsuit so effective and why when swimming in cool water without one you feel "accustomed" to the cold after the initial shock: "Come on in, it’s all right once you get used to it." Stewart explains that the "vasoconstrictor mechanism is so effective that when skin is exposed to cold and the vessels constrict, the core temperature RISES about 0.5 [degrees Celsius]." 
Shivering ruins the insulation of the warm body core by directing warm oxygenated blood to the skin and muscles, thus dissipating valuable heat. On the other hand, it gives the body the opportunity to do something about being cold. Otherwise, we would sit immobile and incapacitated like other poikilotherms (an organism unable to regulate its own heat), such as snakes and frogs, and plummet into hypothermia.
For this reason, I disagree with people who pour hot water down their suits to warm up, or even before they get in the water. The hot water counters vasoconstriction, dilates the blood vessels of the skin and promotes blood circulation from the core, making the body loose heat even faster.
Surface Swims and Jumping Jacks
Voluntary physical activity, assuming sufficient nutrition, can raise heat production over fifteen times the basal metabolic rate.  For instance, swimming between a short series of dives can extend total diving time by at least another hour, especially when combined with eating snacks.
Let’s dispel another common freediving myth: The best diving comes with the least amount of physical activity beforehand. To qualify that, if you do aerobic exercise before diving, it is likely that your heart rate will be elevated and freediving may be more difficult. But you don’t have to swim a marathon to stay warm. A swim is not enough to switch the body into full aerobic mode, but sufficient to provide enough heat to last another hour. We frequently make deep dives immediately after a long surface swim.
The heat benefit of surface swims or some other activity is too great to ignore. As always, proper nutrition is a necessity and be aware that prolonged physical exercise can, of course, increase vasodilation and subsequent heat loss.
 Rintamaki, Hannu., "Adaptation to Cold." Laboratory of Physiology, Oulu Regional Institute of Occupational Health., Oulu Finland.
 Giesbrecht, Gordon. Video documentary The Cold Embrace written, directed and produced by Andrew Gregg. Executive producer: Gordon Henderson of 90th Parallel Productions.
 Kuroshima et al, Can J Physiol Pharmacol 1977 Aug;55(4):943-50
 Haman F et al. J Appl Physiol 2002 Jul;93(1):77-84
 Stewart, Charles. "Regulation of Heat Gain and Loss." Charles Stewart and Associates: 1994.
 Haman F et al.
 Faigin, Rob. Natural Hormone Enhancement. Extique: August, 2000.
For links to references, please contact Peter Scott by email.