Eating before meditation is only inadvisable if what you're eating is high in carbohydrates. Eating a high-carb meal will spike insulin levels and destabilize blood-sugar. This is also known as "carb crash."
This will make you sleepy, foggy, and unfocused; qualities of mind unsuitable for meditation.
If you alter the macronutrient ratio of your meals such that it is high in healthy fats, moderate in protein, and low in carbs, you won't have the aforementioned issues with eating and meditating.