Calories are energy. More calories equals more energy. Calorie dense foods tend to be kinda bad for you, but not always. “Empty calories” means calories that don’t have any nutritional value, like alcohol and trans fats. The reason things like Splenda don’t have “calories” isn’t because they don’t have energy. They do. It’s because your body can’t digest them, thus can’t take any of that energy. Therefore, metabolically speaking, it has no calories.

Carbs are carbohydrates, meaning sugars and starches. This is the kind of food your body directly uses as energy. It can’t be broken down into proteins (because it’s not made of proteins) and it can’t be used to digest fat soluble vitamins (because it’s not fat), it can only be used to provide energy. Your body will either use it as energy or turn it into fat. It is one of four major groups of things your body can digest, carbs, fats, proteins, and alcohols. (You need vitamins too, but you don’t really digest them, so they don’t have calories.)

Glucose is a simple sugar. Unlike starches, your body doesn’t need to break it down to use it. Your cells can directly use it. Great for if you need an energy boost during vigorous exercise.

Gluten is a protein mostly found in grains. Your body will break it down into amino acids and use it to make the proteins it needs.

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A lot of nutritional advice is over complicated. The best advice I’ve ever heard comes Dr Steven Novella:

“Eat a varied diet, mostly plants.”

Another thing is that if you’re talking about weight loss (and only weight loss), the only thing that matters is calories in to calories out. If you are on a calorie deficit (disregarding severe medical conditions), you will lose weight. If you eat at a calorie surplus (again, disregarding medical conditions, like the inability to digest food), you will gain weight. You can lose weight eating nothing but chocolate cake and Cheetos. You will be wildly unhealthy and feel like shit, but you will weigh less.

For the most part, you don’t need to worry about eating the right things, as long as you aren’t eating the wrong things. (Fast food and snacks are generally bad for you in excess.)