It sounds like the question of a 4-year-old making his first forays into understanding life, biology, and the nature of the universe: “Mom, why do we eat?” On some level, there’s an answer that’s both basic and true – the kind with which we’d respond to little Junior: “We eat because we’re hungry.” If it – and we – were only that simple…. Beyond the hormonal cues that tell us our bodies need feeding exists an intricate constellation of reasons. They stem from the gamut of higher order thinking, social reference and self-organization that characterize the human mind . And just when you thought the biological picture was at least an easy call, it’s important to note the environmental inputs that can skew our hormonal responses and convince us we need to raid the refrigerator at midnight . (Yes, analyzing human instinct can feel like nailing Jello to the wall.) So, what are the various reasons we eat? What influenced your food intake today? Truth be told, sometimes we’re not even conscious  of our simplest motivations. I thought I’d take up the multifaceted question in a series of articles. We spend a lot of time talking about what we eat/should eat , and the why can lead us down some interesting roads in terms of both research and experience. But let’s start today with the fundamental biology behind our food intake.
However much we misuse or misapply the concept of “hunger” to justify eating what and when we want, genuine hunger itself is a basic physiological instinct and response to a host of physical cues  in the body itself. If we ever allow ourselves to make it to the actual point of a truly empty stomach, we can feel when our stomach  is empty with the characteristic gnawing. We may disturb those around us with the cavernous rumblings (and then make gestures assigning blame toward the person next to us). Hunger pangs feel like our stomach is doing somersaults or turning in on itself because it’s actually a contraction.
In truth, most of us could live several weeks (or even longer ) without food, but our bodies have their own “idiot light” signals reminding us subtly or not so subtly that it’s time to refuel. Hormonally speaking, the body experiences a rise in ghrelin, the central hunger-stimulating hormone  produced in the gastrointestinal tract that reaches the hypothalamus to spur the body’s hunger related mechanisms and messaging (i.e. Sparky should eat soon). Our sensory perceptiveness, for example, is kicked up, particularly our sense of smell  because of how certain receptors activate the olfactory circuit . The result? Food seems more appealing.
Among the symptoms we most commonly associate with hunger is a drop in blood sugar  in addition to the related energy slump  and the physical (and emotional – a.k.a. “hangry”) agitation we can experience when we’re running on empty. Of course, this is where we begin to separate the sugar burners from the flexible fat burners . If you’re not fat-adapted and rely on glucose, you can feel the initial cues toward hunger (and that dragging sensation) within as little as 3-4 hours. Those who are fat-adapted  (whose bodies can flex efficiently  between glucose and fat for fuel) can generally go considerably longer without these cues.
Yet, hunger isn’t just hunger. The hormonal “hunger” cues can vary considerably in their timing and extent based on what kind of diet we eat as well as other physical inputs. The body, for one, senses the depletion of nutrients in the bloodstream, and this gauging of nutrient stores  can influence hunger itself. It’s yet another reason people who eat a poor diet high in empty carbs and low in essential nutrients are prone to reaching for more. Research shows, for example, that a higher protein diet can increase the levels of a hunger-dampening hormone  known as peptide YY. Likewise, the overall nutrient-density of our diets  appears to impact our experience of hunger. Study subjects  who switched to a more nutrient dense diet reported feeling hungry less often, experiencing fewer and milder hunger symptoms and even sensing hunger from different locations in their bodies.
Aside from diet influences, exposure to blue light  at an evening meal time increased hunger (as well as insulin resistance). Likewise, even a single night of inadequate sleep  has been shown to elevate ghrelin levels in healthy subjects. Not surprisingly, this translates to the next day’s choices in significant ways. Subjects who were sleep deprived chose bigger portions  for meals and snacks.
All this begs the question: what is our relationship to hunger? Do we know what it even feels like ? (Many people surprisingly don’t.) At what point or with what “symptoms” do we assume we’re hungry? How much consciousness do we bring to our physical sensations and cues? Have we given thought to how our food choices influence these sensations? These kinds of questions are why I’m such a fan of self-experimentation , not to mention fasting .
The Primal Blueprint  isn’t a prescription but a guide that invites greater awareness around what does and doesn’t work  for your health. It’s a challenge to let go of what we’re told we should eat or should want and instead consider the personal effects as well as relative costs and benefits of each choice. When we allow ourselves to fast, for example, we not only see what it can do for our mental alertness  or weight loss but also how our body feels at different stages of temporary deprivation and how it responds to certain foods  or the lack thereof. This sounds amazingly elementary, but it can be a revelation to some people that not every odd feeling in the stomach is a hunger pang. I’ve had any number of folks tell me after going Primal that the sensations they attributed to hunger were actually signs of food allergies or other digestive issues . Our culture pushes the concept of “more” so often that we forget how much we can learn from the context of less.
I’m not about deprivation for deprivation’s sake, but we do well to know what basic hunger is. This isn’t at all to minimize or romanticize the chronic, wasting hunger with which much of this world’s population lives. However, to be able to differentiate the extraneous messages and influences about our eating and health, it’s helpful to get in touch with our bodies’ basic cues . It isn’t something you can simply read about. Grok  and his kin undoubtedly felt the range of their bodies’ functioning and learned about physical limits as a result. In a modern environment, we live with inputs grossly manipulative of our physiological instincts. Coming back to basic sensations – what it feels like to be genuinely hungry, what it feels like to lift to exhaustion , what it feels like to get in touch with circadian rhythm without artificial light sources – can be an illuminating first step in realigning our health with our bodies’ natural and evolutionarily set expectations.
Thanks for reading today, everyone. I’ll pick up the series again next week, but let me know your feedback on the biology behind why we eat.