Slightly elevated blood sugar (120-160mg/dL, what you'd get from an ordinary amount of junk food) is imperceptible, even with training. Moderately elevated blood sugar (160-200mg/dL, what you'd get from a thanksgiving dinner) causes slight lethargy. Highly elevated blood sugar (200-250mg/dL, what you'd get from eating a bag of cookies quickly) causes full lethargy, nausea, and excess urination leading to dehydration. All of these can be produced psychosomatically, so a test kit is required. The interesting symptoms don't start until your blood sugar gets higher than normal biochemistry will allow it to go, and stays there. However, frequent or chronic high blood sugar causes slight damage to every organ, causing increased risk for a long list of things including heart disease.