If you look at the nutrient label on the packaging of any food item, you would see the groups: calories, protein, carbohydrates, fat, sodium, and occasionally other items such as minerals. We are all familiar with calories being the amount of energy within the food. “Calorie counting” is a common practice for people looking to watch their weight, as consuming calories faster than you can metabolize them can lead to weight gain. However, without calories your body wouldn’t have energy to survive.
Calories in your diet come from fat, proteins, organic acids, and especially carbohydrates. Carbohydrate is an umbrella term for all types of sugar, starch, cellulose, and even dietary fiber. The sugars in most algae though, are not digestible by most humans. The sugars in most algae are known to be β(1→4) linkages in glucan polysaccharides. Most of the human population lacks the ability to digest these types of sugars as we are adapted to eat alpha(1→4) linkages in glucan polysaccharides (i.e. sucrose), and therefore, we don’t get the energy associated with these calories from most seaweed sugars.
There is one human population in Japan, however, that can digest these sugars. Apparently these Japanese have become hosts to common gut bacterium (Bacteroides plebeius) that exhibits polysaccharide-degrading enzymes. This is likely due to many generations of seaweed consumption and adaptation.
Just for fun, let’s see how much seaweed you would need to consume to get enough energy to survive. Assuming you are not Japanese, the only calories will be protein derived. The average person needs 2000 calories a day to maintain. You get about 4 calories per gram of protein. Now let’s use dulse as our reference seaweed. Dulse has 3.5% protein content. That means you would need to eat at least 31.49 pounds of dulse to satisfy your caloric needs. Now this is only in reference to calories, almost no single food item has all the nutrients the body needs for survival, so please don’t try this diet at home.