Final answer:
Essential nutrients, including nine essential amino acids and essential fatty acids, must be obtained through the diet because they cannot be synthesized by the human body in sufficient amounts.
Step-by-step explanation:
The substances that humans must include in their diet because they cannot be synthesized in sufficient amounts by the body are known as essential nutrients. Among these, certain amino acids, known as essential amino acids, cannot be produced by the body and thus must be obtained through dietary sources. Humans can synthesize all but nine of the standard amino acids, which needs to be supplied by foods such as meat, eggs, and dairy products that are rich in protein.
In addition to amino acids, essential fatty acids are also required in the diet as the body cannot synthesize them. Sulfur is a component of these fatty acids and is also required for the synthesis of many proteins. Dietary proteins are critical not just for their role in building and repairing tissues but also because they can be converted to energy by the body when needed.