Eating Questions
March 9, 2010 2:04 PM   Subscribe

I have two questions: 1) Why do people get hungry after only a few hours without eating when we can live for two weeks without food?, and 2) What are some foods that are nutritious, cheap, fast and easy to prepare?
posted by bobertdude to Food & Drink (12 answers total) 11 users marked this as a favorite
In answer to your first question, my off-the-cuff thought is because your body functions best at a stable blood sugar, and that's best maintained by a steady stream of nutrients from the digestive system.
posted by restless_nomad at 2:09 PM on March 9, 2010

For question 1, it may be because those people simply are so used to the feeling of being "full" that their body overreacts to the slightest hunger, and causes them to feel hungry. It's not that much different than people who feel cold at 70 degrees because they've spent their entire lives indoors with the thermostat set at 75. The body can adapt to difficult situations. At the same time, it gets weak when it doesn't get exposed to a variety of them.

Here's a Fitness Spotlight article that claims intermittent fasting will retrain your body to only become hungry when it needs to.
posted by meowzilla at 2:20 PM on March 9, 2010 [1 favorite]

My first thought is off the cuff and an analogy to breathing. Apparently people can go without air for about three minutes (consciously only for 1 or 2) before brain damage is caused, but cells function optimally when receiving fresh oxygen far more often.

Again, off the cuff, before agrarian society took hold hunter gatherers would sometimes have to fast. The body had to be able to survive without food, and eating regularly during good times helped create a fat store to be drawn on in lean times. If our ancestors routinely fasted during times of plenty, they would not have the adequate health and nutrient stores to survive lean times. Short answer is just because we can survive those two weeks in an emergency, does not mean we could do so routinely.
posted by bunnycup at 2:20 PM on March 9, 2010

People get hungry because they are living. Living burns calories. You can survive for two weeks but you won't really live. Remember the first season of survivor when they started starving? They could hardly even interact.
posted by srboisvert at 2:34 PM on March 9, 2010

The question was about "a few hours", not literally starving for two weeks. I think all of us have had days where we forgot to eat for 10 or so hours, this isn't starvation.
posted by meowzilla at 2:37 PM on March 9, 2010

1. Because there is a huge gap between having enough sugar/fat/protein to maintain your body's normal habits and patterns ... and starving to death. Almost two weeks' worth of a gap.
2. Lentils. You can make them tastier or less tasty but they were one of the sort of "perfect foods" if you don't eat [or can't store] meat.
posted by jessamyn at 3:17 PM on March 9, 2010

I'm not a scientist so here is a rough answer. It's something to do with the way the body accesses glucose. After the glucose in the blood is used up the body starts to access other reserves. It starts to eat itself. It can eat itself for a long time before you die but it uses up the sugar in the blood quickly. That's how we lose weight.

"After a meal, glucose floods into the blood from the gut and the high levels stimulate beta cells to secrete insulin, which stimulates liver, fat, and muscle cells to take up glucose (using their glucose transporters) and metabolize the glucose or store it as glycogen or convert it to long-term stores as lipids. As you go longer after a meal your blood glucose levels fall, and you feel hungry. If you ignore this feeling for a while it goes away, because your alpha cells respond to this drop in glucose level, and secrete glucagon, which stimulates muscle, liver, and eventually fat cells to break their glycogen stores back into glucose and release it into the blood to be used all over the body."

See e.g.

Google also "homeostasis"
posted by y6t5r4e3w2q1 at 3:17 PM on March 9, 2010 [1 favorite]

The first question is addressed as a kind of side note in Gary Taubes's Good Calories, Bad Calories. You feel hungry for two reasons: one, when your blood sugar levels dip (i.e. roughly some hours after you last ate), which is related to the second reason, which is when your body isn't yet using its stored fat reserves for fuel. Your body will burn glucose for fuel primarily if you're eating moderate to high carbohydrates, and this has become the usual state for most people that aren't explicitly doing some kind of low-carb diet. Your body can also burn fat -- both stored and dietary -- for fuel, and this is called ketosis. Protein contributes to blood glucose but not enough to stave off ketosis in most people. If you're not in ketosis -- meaning you have been eating moderate to high carbohydrates -- and abruptly stop eating, for a day or two (the time varies depending on a lot of factors) your body will try to break down the protein in your muscles to reach its glucose needs. This isn't a great way to get energy, and you'll continue to feel hungry (and usually pretty weak, too).

After a day or two, though, if you don't eat anything to jack up your insulin levels, you will be solidly in ketosis. These aren't entirely binary fuel systems -- glucose v. ketosis -- because there's usually some of each going on at any time, but the binary idea is close enough for most people, in that it doesn't take a ton of glucose to keep your body from turning to stored fat for its primary source of fuel. This is all simplified, but the short of it is that once your body can actually draw on its fat reserves after a few days, you quit feeling quite as hungry. And that is exactly what starvation studies mentioned in the book found: after a couple of days, your body starts using those fat reserves and you actually don't feel as hungry as you did during the first couple days. So you have a 'paradox' where you have even less food in you then you did days before, but you're less hungry; it's because your body is scrambling for energy for a couple days without any great sources, then a few days later it's getting its caloric needs met from stored body fat. As far as your body was concerned, it was getting no calories so you felt hungry, then it was getting an okay source of calories so you don't feel as hungry.

To answer your question: you get hungry even though you could survive two weeks because the hunger is the symptom of a transitional phase between the body switching from burning one thing (glucose) for fuel to another (stored fat) -- it hasn't yet gotten into the stuff that will give it weeks of survival. Or to put it another way, you feel hunger depending on what you eat.

However, we're assuming that feeling hungry hours after one eats is the only state of things. It's not. This requires another explanation...

There are many studies noted in the book that the idea that hunger is "mental" is complete crap, and that instead if your body is not getting enough energy you'll be hungry, and if it is, you won't be. If your insulin and blood glucose levels prevent your body from using stored body fat, you will feel hungry until it does start using that body fat. There were studies on both humans and rats, iirc, but of course the most ingenious studies were on the rats, since you can show very easily that normal rats will not become obese, but if you control for calories and feed some of them things that spike their blood sugar, their body can no longer utilize stored body fat and they do become obese whereas the control group doesn't. Similarly, if you breed rats with insulin issues, they'll be fat forever, no matter how many calories they do or don't eat. There were also studies done that showed that people did not feel hungry even if they ate as little as 600 calories a day on a low-carbohydrate diet (similar studies were done at higher calorie gradients as well); the body stays in ketosis because its insulin levels are never spiked, and your body doesn't care whether it's burning dietary fat or stored fat -- it just gets energy from the two sources simultaneously. It's getting its energy needs met, so no hunger is felt.

There are several pages in the books about these studies, including a study that showed that people eating 5000 calories a day would still feel hungry if they were eating moderate to high carbohydrates -- they also gained a lot of weight because their body is storing the extra unused energy AND the same mechanism cuts off the body's access to it as fuel. Another study had people eat 10,000 calories -- yes, ten thousand, and no, that's not some confusion between a calorie and a kilocalorie, it's 10k calories in the sense we all normally think of calories -- on a low-carb diet and found that while some people even lost a little weight, none of them could seem to gain weight. This is because in the absence of the elevated insulin and blood glucose levels, the body cannot store new fat -- for the sake of simplification, it literally lacks the molecule that would bind it down, which is a product of the process that burns glucose -- and the fat will circulate until it's used in some way, even if your body just gives off the energy as excess heat or waste. (Which is why the whole "laws of thermodynamics" argument about all calories being equal isn't a real argument here; calories aren't magically disappearing, they're being used whereas they weren't able to be used before. Hormones make a much bigger difference than calories.) You don't feel hunger because you have plenty of energy circulating around looking for something to do and your body can readily draw upon it.

So the second answer is you feel hunger after a few hours only depending on what you eat. Even when in ketosis your body will tend to demand some outside food, but it takes a lot, lot longer to feel hungry. When I eat something with a lot of carbohydrates, I usually feel hungry in three or four hours -- which is one reason I rarely do that. If I eat something that's purely sugar and starch, I'll feel hungry in an hour or less. I normally eat low carb though, and it usually takes anywhere from eight to even twenty four hours before I'm hungry again -- it depends on how much I ate and how many carbs were still in it, and also how active I am that particular day.

If you're still curious about the mechanics of this -- why, for example, elevated insulin and blood glucose levels keep the body from accessing stored fat, and why it can't store fat in the absence of those things -- then I recommend you read the book for details I didn't get into; it goes deeper into the molecular biology of it than I have the patience to write out for an AskMeFi question, and it's a book well worth reading anyway. I hope I've simplified it enough to be understandable without being misleading, but refer to the book if you have any questions.
posted by Nattie at 3:36 PM on March 9, 2010 [32 favorites]

1) Insulin causes hunger. Eat something that is sweet, has sugar, or metabolizes into sugar (carbohydrates) and your body will produce insulin. Insulin will in turn make you hungry.

Here's a few page preview of this section of the book Good Calories, Bad Calories by Gary Taubes which goes into tons more details on hunger.

2) Eggs. Very quick and versatile to cook at home. If you want them on the go, hard boil them and off you go.
posted by Nerro at 4:04 PM on March 9, 2010

In addition to the answers already given, I can vouche for the fact that your body does learn to be hungry at certain times. If you habitually eat right after you wake up, for example, you will probably not need an alarm clock to wake up at that time every day -- your body will do it for you to make sure it gets fed. If you eat a big, carby dinner one evening, you may be extra-hungry the next. (It is easy to see how this can lead to a cycle of overeating.) Fortunately I have found that it only takes a few days for your body to change its habits to ones more amenable to your lifestyle.

As to why -- as others have said, glucose. Your body burns it preferentially to other fuels. It can burn other fuels, but the sugar is most efficient, and more readily mobilized (and less easily exhausted) than ketones from fat breakdown when you need to run for your life from a hungry predator. Your body can store a few days' worth of glycogen (in the muscle tissue) which can be quickly converted to glucose, but your body seems to consider that "emergency reserves" and does not like to dip into it except in times of no dietary carbs.
posted by kindall at 5:19 PM on March 9, 2010

My favorite nutritious, cheap, fast and easy to prepare food are canned sardines.

Omega-3 fatty acids on the go and great protein and relatively low mess.

They come in various sauces too - tomato, garlic, etc.

All you need is a fork or sometimes I bring bread.

They are underrated in my opinion.
posted by simpleton at 6:20 PM on March 9, 2010

Can of tuna, add some pepper and lemon juice.

Toast with peanut butter.

Grilled cheese or french toast.

Eating a sandwich at the moment, two slices of bread, dijon mustard, sliced meat, and some lettuce. Not sure where the tomatoes went to, roommate might have swiped them for his lunch.
posted by hungrysquirrels at 7:44 PM on March 9, 2010

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