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

In my last blog post, we started to consider how one’s brain responds to starvation, aka caloric restriction. In case you didn’t catch that one, I consider our cave person brain to be the part of our brain that keeps our body running on a day-to-day basis, operating separately from our thinking brain. And as a Cliff’s Notes version to that blog, by way of introducing today’s topic, is when your cave person brain doesn’t get adequate nutrition, it shifts into calorie conservation mode, reducing your metabolic rate (the calories needed neither to lose nor gain weight). These blogs on the cave person brain go into further details about how exactly your body achieves its goal of slowing metabolism. Remember: humans only exist today because our ancestors developed multiple mechanisms to survive famine. When food resources were scarce, humans became extraordinarily efficient at surviving and functioning. I’m always struck by the magnificence of our bodies’ innate functioning…vastly more responsive to our environments and scientifically and beautifully set up to keep us going than our own coarse attempts to over-control food and exercise.

As far as your brain is concerned, starvation includes deliberate caloric restriction in the service of disordered eating or an eating disorder, fasts or cleanses done in the name of health (pro tip: fasts and cleanses are the opposite of healthful), or even a diet. It’s a good moment to put in my expert physician note that diets don’t work. The data all point to 95 percent of weight lost through dieting being regained. Because our brains are so superb at reacting to famine (aka restriction of calories) by reducing our metabolisms in many ways, all restriction does is make us highly cranky, less able to make sound decisions, feel deprived and think constantly about food, and slow our metabolism. So, when a dieting person restricts long enough, nature kicks in to demand lots of food—preferably calorie-dense food that satisfies the cave person brain’s needs—and this consumption on top of a slowed metabolism leads most commonly to regaining even more weight than was lost. Thus what dieting most often causes in the long term is weight gain.

Back to how our cave person brains slow our metabolism and react to caloric restriction. In my last post, I talked about body temperature changes and slowed heart rate. Another way the body accomplishes slower metabolism is through slowed gastrointestinal function. When your cave person brain senses caloric restriction, it no longer wants to spend an extra calorie on normal wriggles of the stomach, small intestine, and colon. Pooping every day isn’t considered essential to life (compared with keeping the heart beating, for instance…), so many patients who restrict find that they become constipated. In addition, with stomach wriggles (called peristalsis) diminished, food might sit in a patient’s stomach for far longer, undigested, than it otherwise would in a well-nourished person with a faster metabolism. This condition, called gastroparesis (which can also be caused by many other things), causes bloating, early fullness where just a few bites might make someone feel full, and nausea. While a gastric emptying study can diagnose gastroparesis, these symptoms may occur even before a study shows any abnormality. Thus, someone who is restricting calories and adamantly telling themselves they are “fine” because their bloodwork is normal, but who has symptoms of constipation or gastroparesis, clearly isn’t fine. That slowed GI function is a sign of a body suffering due to inadequate nutrition. Usually, GI function will speed back up over time to normal, with consistent, adequate nutrition and body weight restoration if the person was underweight. But this can occur in people of all sizes and shapes and the unifying factor is caloric restriction.

Caloric restriction that lasts a long time, often associated with substantial weight loss, can cause the cave person brain to shut down sex hormones. To the cave person brain, malnutrition plus high stress hormones (cortisol, which increases due to the stress of starvation) means, “This body isn’t safe to participate in procreation.” To achieve this, the part of the brain called the hypothalamus basically recedes back in time to pre-adolescent levels of hormone production, which substantially reduces estrogen and testosterone levels. For many females, menstrual periods stop (although some continue to get their period despite prolonged malnutrition), and vaginal tissue can become dry and fragile, increasing the risk of bladder infections and of painful intercourse. For males, sex drive diminishes and erectile dysfunction occurs. For both sexes, bone density can drop fast, increasing the risk of fractures. These hormones nearly always come back on line when an individual has consistently rehabilitated nutritionally, usually with restoration of healthy body weight.

Finally, caloric restriction can cause the cave person brain to become severely anxious, even paranoid. Now, it’s true that for some people, restricting can numb their emotions. However, even when this effect occurs, it can be offset by a more “animal anxiety” from the cave person brain. Any of you who have pets know that a dog or cat who’s had surgery at the vet, or been through severe weather, may end up freaked out for a couple days…under the bed, eyes huge, fur up, claws out. Their animal brain knows that they’re in a vulnerable situation, and they’d better be extra vigilant about potential causes of harm. Similarly, when you’re restricting calories, your cave person brain understands its animal body to be vulnerable, not in a safe place to weather another challenge. Thus, the risk radar goes up and my patients feel like they are terrified at the prospect of making a big decision, or a meal plan increase, or even just the arrival of lunch. It’s a disproportionately paranoid, self-protective instinct that can cause a lot of suffering both to patient and family/friends. I remind patients, after bringing this up, “This may not be you, and it may not even be your eating disorder. It may be your starved brain. So let’s nourish you and see how this plays out. You have my compassion in the meantime.” Invariably, my patients tell me that this animal anxiety does really lessen with consistently adequate nutrition.

To finish the discussion of the cave person brain and metabolic changes with caloric restriction, I have to include a discussion of people’s widely varying responses to malnutrition. There are people who drop weight fast when they restrict (keeping in mind that restricting in the service of a mental illness is vitally different from dieting), and there are people whose body weight simply does not change despite intense caloric restriction. There are people who’ve never had a moment of GI trouble and those who are crippled by GI issues, despite similar intakes and body sizes. Some never lose their menstrual period, while others have to wait years at a good weight to regain theirs. What explains this is our individual, genetically-determined variability in how our body responds to malnutrition. To use a more familiar example, a young man who only drank alcohol heavily for five years might develop severe liver damage, while a 90-year-old who drank his whole life might attribute his longevity “to the whiskey.” Similarly, every person will respond differently to the stress of caloric restriction. However your body responds, remember that your cave person brain is wise and working hard to keep you alive all the time. Give it compassion, rest, and plenty of nutrition.