Only two essential fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid). The biological effects of the ω-3 and ω-6 fatty acids are mediated by their mutual interactions. Closely related, these fatty acids act as competing substrates for the same enzymes. The biological effects of the ω-3 and ω-6 fatty acids are largely mediated by essential fatty acid interactions. The proportion of omega-3 to omega-6 fatty acids in a diet may have metabolic consequences. Unlike omega-3 fatty acids and omega-6 fatty acids, omega-9 fatty acids are not classed as essential fatty acids because they can be created by the human body from monounsaturated and saturated fatty acids, and are therefore not essential in the diet.