1) Is it true that when excercising (cardio) that your body doesnt start burning fat till you have been doing it for at least 30min?
This is a really complex issue that people try to "boil down" to some kind of rule ... and it just doesn't work that way. The bottom line is NO .. this is not true. Any exercise you do burns calories, and ultimately burning more calories than you consume will burn fat. That's the bottom line.
That doesn't mean that during various stages of our exercise, and depending on the intensity, our bodies don't switch back and forth between the kinds of reserves they burn immediately. This article has a good explanation of the process at a very basic level:
When your body moves (or any cell in your body does any type of ”work”) it needs energy. The amount of energy needed is measured as a unit of heat—or a calorie. The fuel to produce this energy comes from several sources, mostly fat and carbs (glucose), and occasionally amino acids (protein). How and when fat is ‘”burned” (or metabolized to provide energy for the body) and how that affects body fat levels and weight is a very complex area of physiology research. There have been hundreds, maybe thousands, of studies exploring the utilization of fat for energy under a variety of different conditions. There is still much to be understood, but this is how we know it works so far:
Whether you are watching TV or running around a track, the fuel your body uses to give you the calories your cells need for energy comes from burning mostly fat and carbs. Your body nearly always burns a mix of both fat calories and carb calories. So normally, for every calorie burned, the fuels are around a 50/50 split of both fat and carbs.
How hard you are moving during exercise is one major determinant of which fuel your body will use. Carbs provide a faster energy source. So whenever you need to do something fast or produce force, carbs are the better fuel. Fats are favored during long, low-intensity activities. It’s not that you stop using one or the other fuel, it’s that the ratio of both shifts depending on your activity. In more scientific terms, you alternate between aerobic (more fat-burning) and anaerobic (more carb-burning) metabolism.
2) Is it true that Citrus fruits are the only food that can be eaten late at night and that will keep your metabolism going throughout the night?
Utter and complete hogwash.
You should eat balanced meals with a range of nutrients (including veggies, fruits, lean protein, and complex carbs) and within a reasonable calorie range (not too many, not too few). There's no such thing as "the only food that will keep your metabolism going" because factually, if you're metabolism isn't "going" then you're dead ... and what you eat won't matter.
If you are providing proper nutrition to your body throughout the day, then your metabolism will go just fine.
3) Does liquid meals (such as a protein shake) raise your metabolism just as much as the same quantity of solid food?
I'm not really sure what you mean by solid food "raising your metabolism"? Are you talking about the thermogenic effect of digestion (in other words, that eating and digestion causes your body temperature to rise, and therefore burn more calories)? If that's what you're talking about, then the truth is that eating is not going to "raise your metabolism" enough to make that much difference in your weight loss. Again, going back to what I said above about the citrus fruit, eating proper foods in reasonable quantities and making sure you get a nice balance of vitamins and minerals will sustain your metabolism and make it possible for you to exercise. If you get that through whole foods or a combination of whole foods and protein shakes, either way is fine.
I don't really recommend using protein shakes as food or meal substitutes for a lot of reasons, which I'll be happy to go into if you'd like me to. But as a supplement or an occasional meal replacement, there's no reason why protein drinks or protein powder aren't fine.
Hope that helps answer your questions some.
