The old nutrition mantra "Calories in, calories out" is losing ground everyday. On the flipside, "You are what you eat" is gaining support, but probably not how you think.
When you eat good fats from fish or avocado, your body doesn't store a little bit of that away as fat, right next to the pizza and wings from last weekend. Everything you eat goes through your digestive tract. Your mouth and stomach mash food up and let a tablespoon at a time into your small intestine. The mashed food mixes with enzymes that break down the food to its smallest nutrients. These small nutrients can cross the border from your gut into your blood - they travel to your liver, where they are packaged up to be sent out to your other organs, stored away for a rainy day, or transformed into something else.
There are some nutrients, such as fiber, that we can't break down. It does great things in your stomach and intestines, though. When you eat fiber, it makes you feel fuller longer because it slows your stomach down, and food is let into your small intestine much slower. When that fiber-food combo gets to your small intestine, the fiber binds on to some of that nutrient, not letting it be digested. It carries it to the end of your intestines, the colon or large intestine.
I highly suggest watching the Magic School Bus episode below to understand digestion. It's a fun way to learn and provides a visual aspect to something that is otherwise hard to picture.
Have you heard of "good bacteria"? It's a common term used in this age of probiotics, or "good bacteria" supplements in the form of drinks, yogurt, or capsules. Those good bacteria reside primarily in your colon, and there are ALOT of them. In fact, if you take all of their DNA, it would be 100x larger than all of our human DNA.
So what are they doing there? I mentioned before we can't break down fiber...but they can! When they break fiber down, they generate special molecules that provide us with good energy, that keep our colon healthy, and sometimes ones that give us a bit of gas!
Learn more about probiotics and prebiotics (food the bacteria eats) in my blog post from July.
The thing is, that the types of food you eat can impact what TYPE of bacteria live in your colon. If you eat alot of processed foods, fast foods, refined grains (sweets & desserts, white carbs), and high (bad) saturated fat foods, you will decrease the variety, or diversity, of bacteria in your colon. And the whole process will shift to harvesting more energy from the food you digest, meaning you absorb more energy (calories), and your stool (poop) has less calories. Researchers have compared stool from overweight and normal weight people, and often find that overweight people have less calories in their stool.
If the effect of your diet stopped in your colon, it would be interesting, but not worth a blog post I don't think. Your body's immune system is very complex, and actually plays a big role in your gut. If you think about it, your gut is a closed off system, isolating the "outside" the enters through your mouth and eventually exits, from the rest of your body. The lining of your intestines is "selectively permeable" - it only lets certain things pass through, like the nutrients you want to absorb.
But if your bacteria shifts from eating a processed diet, those different bacteria that dominate can release signal molecules that trigger your immune system. Your immune system responds with it's number one defense: inflammation. Research is pretty solid on the fact that obesity is characterized by a constant state of low-level inflammation. This is how we think people develop diabetes and hypertension and other problems from being overweight.
On the flip side, if you eat more fiber (whole grains, fruits, vegetables) that provides the good food your bacteria need, and everything will right itself.