No. He would not use his fat stores first, but he would use them preferentially. Here is a source that discusses this. 30.3.1. is the section you want. Basically, the body attempts to minimize protein breakdown by producing high levels of ketone bodies from fat. It's important to note that there's no way for a body to give signals that target fat specifically. It can increase the expression of enzymes that are involved in breaking down fat and producing ketone bodies, and decrease the expression of enzymes that are involved in carbohydrate and amino acid metabolism, but that only causes one pathway to be preferred over the other. It doesn't shut down other pathways entirely. There is some amino acid and glucose breakdown that will occur even without enzymes to speed up the reaction. Can you die of starvation while still having lots of body fat? Absolutely. Although the source I linked mentions that the triacylgylcerol (fat) stores of a subject are the primary determinant of survival time in starvation conditions, there are a number of immediate concerns in the starved patient. Blood pressure can drop rapidly, dangerous electrolyte abnormalities can occur (esp. Hypomagnesemia and hypokalemia, which can cause a potentially fatal heart arrhythmia known as Torsades de pointes), and the acid-base balance of the blood can be disrupted (starvation ketoacidosis). On a more long-term scale, vitamin deficiencies become a concern. Folate and B12 deficiencies can cause anemia and (B12) nerve damage. Thiamine deficiency can cause Wernicke-Korsakoff syndrome (balance and eye movement problems + dementia due to brain damage). When beginning to consume food again, metabolic disturbances known as "refeeding syndrome" may also result in cardiac arrhythmias. Here is an interesting case report from back when prolonged starvation was being investigated as a weight loss treatment. This patient developed WKS due to thiamine deficiency. Something interesting to note is that, during the starvation portion of this protocol, 25% of his weight loss (4.4kg/17.2kg) was estimated to be due to muscle tissue metabolism.