Written by Jennifer L. Gaudiani, MD, CEDS and originally featured on the Project Heal blog. 

There are so many medical complications that can occur in eating disorders and in the caloric restriction that can come with disordered eating. Let’s simplify our understanding and take a 30,000-foot view. To illustrate this concept, I’m going to use the concept of the “cave person brain.” From a not-very-neuroanatomical perspective, the “cave person brain” is the part of your brain that manages all the aspects of your body that you’re not consciously aware of. I call it the “cave person brain,” because it has kept us alive as a species through millennia of evolution. This first of two posts will review how the cave person brain affects certain vital signs.

When you don’t take in enough calories, your cave person brain recognizes this as a “famine situation” and kicks into gear to keep you alive. First, it slows your metabolism. What exactly is the famed metabolism? Essentially, it’s the amount of energy your body needs neither to lose nor gain weight…to keep all your organs working, your body temperature normal, and sustain daily needs. So when you restrict calories—whether it’s on a diet (they don’t work!), or on a fast or cleanse (unscientific! And they don’t work!), or in the service of an eating disorder—your cave person brain thinks, “Famine! Slow the metabolism so we get through it safely!”

 Your cave person brain slows your metabolism in several key ways, all intended to make each calorie count and reduce needless energy (caloric) expenditure. For one, your body temperature drops. Keep in mind that maintaining the body temperature at almost exactly 98.6 degrees F (37 C) is one of the key physiologic mandates of the body. Our enzymes work perfectly at 98.6 degrees, and the body works hard usually to keep our temperatures exactly there. However, when your metabolism slows from caloric restriction, the body finds energy efficiency in the same way you do in winter: it shuts off heat to non-essential areas. That is, just like you might shut off the heat in a little-used room to decrease your energy bill, your cave person brain clamps down on the circulation of your hands and feet, so that calorie-warmed blood isn’t “wasted” in keeping those non-life-saving appendages warm. Patients who restrict calories can thus end up with cold hands and feet all the time, with fingers and toes even becoming blueish in color, a condition known as acrocyanosis (literally “blue color pertaining to the end” of digits). As restriction becomes more extreme, you might develop fine hair on your face. This is called lanugo, and your cave person brain grows this to try and hold heat in, like a very fine pelt. In addition, you might feel cold all the time, as your furnace cools in the setting of lacking fuel. In extreme cases, the core body temperature actually falls. These are signs that your metabolism is getting really slow…that you’ve starved yourself to the point where you need very few calories a day just to maintain your body weight.

Second, your heart rate drops when you restrict calories. Your cave person brain literally doesn’t want to spend an extra calorie on an extra beat of your heart. Bradycardia refers to a heart rate less than 60 beats per minute. Something that can confuse patients and providers alike is that fine athletes also develop bradycardia, as the strong heart muscle becomes efficient with athletic training, and beats more slowly. Since many patients with eating disorders continue to exercise (either compulsively or excessively) despite calorie restriction, how does one tell whether a resting pulse of 45 is due to metabolic slowing from starvation, or from athletic efficiency? The key is what I like to call the “walk across the room test.” After checking a resting heart rate for a minute, walk across the room and back. A fine athlete’s heart will not so much as budge in rate with that minimal exertion. However, after extended caloric restriction and weight loss (regardless of current body weight), the restrictor’s muscle mass will have wasted away, possibly including cardiac mass. Even if they continue to push themselves to exercise daily, the exertion of walking across the room will cause the heart rate to increase by anywhere from 50% to 300%. That is, after walking across the room, their pulse might go from 45 to 70 (or 90 or 125). That cardioacceleration is the hallmark of a starved person’s heart. Metabolic slowing is achieved through increase of what’s called vagal tone, or firing of parasympathetic nervous system. This is what bears use when they slow their metabolisms to hibernate all winter. They drop their body temperature, slow their heart rate, and conserve calories during a period of prolonged caloric deprivation. PS: Bears sleep during hibernation. They do not work out, maintain an active job, do homework, or “get their steps in!”

No one wants a slow metabolism. It means that your body might need only 500-700 calories a day just to maintain weight. From my perspective, anything less than 1500 calories a day represents a deprivation diet, and your body will respond as described above. This explains why those who diet stop losing weight. Their metabolisms slow to equal their intake. It can explain, in certain cases, why caloric restriction stops causing weight loss in those with eating disorders, to the individual’s profound dismay. Their cave person brain defends that body weight fiercely, bravely saving a life. The key to jumpstarting the metabolism again, to warming up that body and resolving heart rate extremes is … eating again. Nutritional rehabilitation fixes all these problems, and often results in a hypermetabolic state! I like to remind my patients with anorexia nervosa that, amazingly, when they really eat enough (under the care of a dietitian), their metabolism can speed up so much that it’s like that of a 12-year-old boy playing three sports!

In part two, I’ll talk about some of the other ways that the cave person brain responds to the challenge of starvation. In the meantime, if any of this is happening to you, seek help. Be brave and get into good care. Your cave person brain will thank you.