What foods cause fatigue?

Oxidized PUFA & Suga. Polyunsaturated fatty acids (pufa) can contribute to fatigue. Pufa's interfere in normal mitochondrial energy production by inhibiting the ability of your normal fat metabolism. Avoid fried foods and vegetable oils as these contain unhealthy oxidized pufa sugars and excess carbs cause fluctuations in blood sugar which can also cause fatigue.
High glycemic index. Basically anything "white" that contains simple sugars or is broken down into simple sugars (think of baked potatoes, the white part) can make your blood sugar go ballistic. You will initially feel energetic as blood sugar rises, but the fatigue as hypoglycemia (low blood sugar) occurs is intense. Think of turkey (containing tryptophan). It's really probably the mashed potatoes and dessert.