Why can't I just take a pill instead of eating meals? I don't want to worry about getting enough protein, carbohydrates, fruits & veggies, and healthy fats; I just want to take a pill each day and get my 'daily nutritional needs' over with.
It is impossible to replace a healthy diet with a cure all (and prevent-all) pill. Reasons why I don't think you'll ever be able to simply take a pill and call it lunch:
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1) Size. Pills are too tiny to contain all we need. Think of the average sized pill: it weighs about 1 gram. Now think of the amount of protein that a 150-pound male needs in a day (~70 grams). That means you'd need 70 pills just for your PROTEIN for the day.
2) Hydration. Food is full of water (especially fruits and veggies). It is recommended that we drink 2 liters of water a day in addition to food. Most people struggle in getting these 2 liters. If we then take food-water out of the equation, these people are going to be parched!
3) Absorbable. Many foods are digested and absorbed differently in the body, so cramming them all into one neat-little package will make digestion difficult. Carbohydrates, for instance, begin to be broken down in the mouth... so taking them in a swallow-able pill means this digestion step will be skipped. Also, some foods need fat for absorption, others need loads of water... and what about those foods that compete with each other? Like the phytates in green vegetables and iron; they would have to be contained in separate pills and taken at different times.
4) Fiber. Sort of along the same line as the hydration, you need fiber to get those bowels moving! And not just for number twos... our good gut bacteria depend on fiber to feed them. We also need fiber to help other nutrients be absorbed and removed (e.g. excess cholesterol leaves the body this way).
5) Bulk. We need to have mass amounts of food flowing through our digestive system at all times. Even when our stomach is empty, the intestines are full. That's the way the body is happiest because when the stomach and the intestines are nearing depletion, then we get hunger pangs like no other. Stretch receptors in the stomach tell the intestines that food is coming and stretch receptors in the rectum tell us it's time to use the potty. In addition, the actual chewing and saliva interaction with food in our mouth activates all kinds of other processes (in order to prep for the food's arrival) further down the digestion pathway.
6) Variety. We need variety in our diet to prevent food allergies and/or intolerances, and to make sure we are getting enough nutrients (including those we don't quite know about yet (see #7 for more)).
7) Mental/Emotional/Culture. Our brains enjoy the act of eating. How would we change as a culture, society, or species if we no longer fussed over food?
8) Synergy? The crazy, complicated, and confusing thing about food is that we still don't really know how it works. For instance: eating a whole orange is much better than just drinking the juice. The juice is missing fiber, water, and special compounds like flavonoids, which---even though they don't belong to one food group---do wonders for the body. But it is the synergy between the orange's sugars, vitamins, minerals, flavonoids, waters, fibers, enzymes, that is best for the body, and this healing and health promoting combination is what we can't quite figure out.
Note: there are many more reasons why we won't be dining on pills (instead of real food) anytime soon... those above are the ones that make the most sense to me.