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It’s 3:00 pm and that so-called salad you had for lunch is a distant memory. And come to think of it, all you had was a latte and a scone for breakfast. No wonder that piece of leftover birthday cake is calling your name. You know you want it, but at the same time, you don’t want to ruin your diet. But then again – you’ve lost 5 pounds already, so what’s one little piece of cake going to hurt?
Sound familiar? How often do you get the afternoon munchies? If you’re like me, it’s just about every day. Oh, who am I kidding? – it is every day. But why does this happen?
To answer this question, let’s take a step back in time – to your childhood. Were you a skinny, gangly kid or were you always a little on the “chubby” side? Your answer to this question can be very telling. According to biologists, each of us has a genetically predetermined body weight called our set point. Some of us are “programmed” to be thin and others are “programmed” to be full figured. Your body goes to great lengths to keep your weight within a certain set range. When you first start dieting to lose weight, the pounds come off fairly rapidly. Then after a week or so, your body wises up. You see, your body doesn’t care that you are trying to shed 10 pounds to have the perfect beach body or to be able to fit into a wedding dress that is two sizes smaller than your current size. All your body knows is that it’s not getting its caloric needs met and it enters “starvation mode.”
What does this mean for you? Just like your body knows the weight at which it feels most comfortable, it also knows the amount of energy (i.e., calories) it needs each day to perform all of its regular functions (e.g., keep your heart beating and your lungs pumping air in and out). This is called your basal metabolic rate (BMR). When people say that they have a fast or slow metabolism, this is what they are referring to. Your BMR is affected by a number of factors, including your body composition (percentage of body fat v. lean muscle), how much you exercise (both on that day and on a regular basis), whether or not you are currently restricting calories, and genetics. When your body senses that it is not getting enough calories to meet its needs, it will slow down your BMR – by as much as 45% to compensate for your caloric deficit. So when you feel like your body is fighting against you every time you go on a diet, you’re right. If you use extremely reduced calorie diets with little to no exercise, your BMR will slow way down to compensate for your caloric deficit. As a result, the scale won’t budging.
In addition to slowing down your BMR, dieting can also set you up for some pretty severe hunger pangs. After all, your body thinks that you’re starving, so it will be very vocal about demanding that you eat something. Right. Now.
Why? Although there are certainly numerous psychological reasons which we will discuss in forthcoming chapters, much of the starvation effect is biological in nature – and it all stems from your blood sugar – or lack thereof. While every diabetic knows the importance of their blood sugar level, most of us don’t give it a second thought. But maybe we should. You see, most of what we eat gets converted to glucose (blood sugar). That’s what provides us with the energy to get through our day. When we eat too much at one meal, our bodies use the excess glucose to top off our fat stores in case we need that energy in the future. But after a few hours, our blood sugar levels start to drop and our brain gets the signal from our liver that it’s time to eat again. This is when your stomach starts grumbling; you might also get light headed or experience mood swings. Those are all signals that your blood sugar level has gotten too low and you need to eat something.
And that low blood sugar explains why it’s 3:00 pm and you’re starving. If all you’ve eaten today is a tall Starbucks Caffe Latte (180 calories) and blueberry scone (460 calories) for breakfast and a Subway grilled chicken salad with lite Italian dressing (200 calories) for lunch, by 3:00 pm your gas tank is empty. As your body digests the Subway salad and turns it into blood sugar (glucose), your pancreas produces a hormone called insulin. Insulin regulates our blood sugar level, signals hunger and satiety, and converts any extra glucose into fat. Pretty soon your pancreas is cranking out the insulin, but there’s nothing left to convert to fat. So your body has to release some of its stored fat for you to use as energy. You’re probably cheering for all of those fat cells to release their stored fat, but sadly it’s not that easy. At this point, your liver is sending out a distress signal – it doesn’t want to release all of that fat that it just worked so hard to store. So it sends a message to your brain. A part of your brain called the lateral hypothalamus answers the call and begins secreting the hunger-triggering hormone orexin. Just in case you didn’t get the message, your stomach starts to produce ghrelin, which causes the pituitary gland to join the “I’m hungry” call to arms. And just to make sure you didn’t forget where you need to go to get food, ghrelin also stimulates your hippocampus, a brain area involved in learning and memory.
With all of those hormones signaling that Armageddon has come and you must eat right now to stave it off, it’s a wonder that any of us can lose weight. So what’s the secret? How do people lose weight when their body is so good at fighting against them? I’ll give you a hint: it’s not just willpower, but more on this in future blogs.
For now, let’s go back to our example. Suppose you need 1800 calories to maintain your current weight. At 3:00 pm you are well over halfway into your day and have only eaten 860 calories – less than half of your necessary caloric intake. No wonder you’re hungry! So you cave in and dive headfirst into that piece of birthday cake (300 calories). A number of things happen as you eat that cake. First, your stomach stretches to accommodate the meal. This is what causes you to feel full and is one of our first clues that we are satiated, or satisfied, and it’s time to stop eating. As the food moves from your stomach to your intestines, a hormone called Cholecystokinin (CCK) is released. CCK then signals the ventromedial hypothalamus that it's time to stop eating. Finally, as the insulin released by your pancreas begins to convert the excess blood sugar into fat to store for later, your fat cells release a hormone called leptin, which also signals your hypothalamus that you’re full. There’s obviously more to it than that – nutrient density, psychological factors, who you are eating with, etc., and we will get to all of that – but for now, just focus on the basic biology behind what happens when you eat.
Next week: How to Avoid the Afternoon Munchies
