Agave nectar and brown sugar are both sweeteners with strong caramel notes in their flavor profiles. To find out more about their other similarities as well as some differences, check out the SPICEography Showdown below:
How does agave nectar differ from brown sugar?
Agave nectar and brown sugar are both plant-based sweeteners, but they come from two different kinds of plants. Agave nectar comes from certain types of agave succulents. Agave is the same kind of plant used to make tequila. The refined sugar used to make brown sugar may come from the sugarcane plant or sugar beets.
Agave nectar and brown sugar are very different in terms of consistency and appearance. Agave nectar is a viscous amber-colored liquid with a consistency similar to that of honey. Brown sugar consists of fine sugar crystals coated with molasses, which makes them slightly sticky.
Brown Sugar 
Agave Nectar
Both of these sweeteners have different health effects. Agave nectar contains both fructose and glucose, but it contains more fructose than it does glucose. Between 60 and 90 percent of the carbohydrate in agave nectar is fructose. Fructose has a lower glycemic index rating than glucose, which means that it does not get absorbed into the bloodstream as quickly. Slower absorption means that it does not cause the severe spikes in blood sugar that you would see with glucose. Those spikes can contribute to your likelihood of developing type 2 diabetes.
Because of the high fructose content, agave nectar has a lower glycemic index rating than other sweeteners, including brown sugar. While this low glycemic index rating is often used as a selling point for agave syrup, it actually points to serious health concerns about the sweetener. Fructose is processed in the liver, which puts out triglycerides that can wind up clogging arteries and causing fatty liver.
Brown sugar is chemically identical to refined white sugar. It consists of white sugar with a tiny amount of molasses. Refined white sugar is a 50-50 mix of fructose and glucose. It has a higher glycemic index rating but less of the fructose that winds up as triglycerides.
Agave nectar is much sweeter than brown sugar. Some reports say it is almost twice as sweet.
Can you use agave nectar as a substitute for brown sugar and vice versa?
Agave nectar can stand in for brown sugar in preparations where the sweetness and caramel flavor of the brown sugar is what you want. If the dish depends on the mouthfeel of the brown sugar crystals, agave nectar will not be ideal.
The dark or medium types of agave nectar will have a similar molasses flavor as brown sugar. Lighter agave nectars won’t be ideal brown sugar substitutes. Remember that agave nectar is sweeter than brown sugar, so you should start by using about 2/3 of the amount that your recipe requires for brown sugar and increasing to taste if necessary. You will also need to decrease the liquids in the recipe.
Brown sugar can work as a substitute for dark or medium agave nectar if you add some water to it. Note that this syrup won’t give you the same viscosity that you would get from agave nectar, so it won’t work in some recipes, but it can provide essential sweetness and moisture in many. Brown sugar won’t be an ideal substitute for the lighter types of agave nectar.
When should you use agave nectar, and when should you use brown sugar?
Agave nectar is best used in beverages and applications where you need a versatile liquid sweetener. Use brown sugar when you need sugar crystals for sprinkling or want a sweetener with a moderate amount of fructose.