When to eat lunch if I work out during lunch break?

Here is an interesting chart that may help you:

http://womensrunning.competitor.com/2012/08/girlfriends-guide-to-running/workout-nutrition-guide_5389

Gives you ideas of what to eat pre/post workout.

Calling SideSteel!!!

I don't believe it matters when you eat as far as "preventing calories from being converted to fat", that's a diet/calorie surplus issue, but I'll let SideSteel address that.

I personally don't eat before a workout, I find that it makes me nauseated during, which is no bueno. I usually plan my meals so that I eat something about two hours prior, and usually that's a snack or something around 200-300 calories. And nothing heavy. Then after I workout, I grab a protein shake and head home for something a little more substantial.

When is the best time to eat/workout to keep my body from converting my lunch calories to fat?

This does not matter at all.

Do not ever bother with trying to minimize acute fat storage. You simply cannot determine or control what happens in the short term and it's just one big exercise in mental *kitten* that won't get you anywhere other than frustrated.

Here is something to think about that will hopefully ease your mind:

You are constantly going through periods of fat oxidation and fat storage. When you eat food, fat oxidation is blunted and fat storage goes up. At some point between meals, fat oxidation increases and fat storage is blunted.

All that matters is the net difference between fat storage and fat oxidation. If you oxidize more than you store, you will lose fat over time. If you store more than you oxidize, you will gain fat over time.

You know what controls this? Energy balance.

Are you eating in a caloric deficit over time? Then don't worry about it.

As far as your nutrient timing goes, here are some factors that DO matter:
1) Performance in the gym.
2) Digestive issues/discomfort
3) Lifestyle/preference/satiety

So I'm going to do a strength training workout with a little bit of cardio bursts during my lunch break. The workout will be about 40 minutes and I expect to burn about 350 cals (using my HRM).

I'm using the gym at my office but now I'm wondering when I should eat lunch. If I eat lunch at 11 a.m. and give myself some time to digest and then workout at 1 p.m., is that a good idea?

Or should I workout at 11 a.m. and then eat around noon? I doubt that's a good idea because then the food in my belly would probably just get stored instead of burned away/

When is the best time to eat/workout to keep my body from converting my lunch calories to fat?

If you work out intensively that depletes your muscles of glycogen and damages the muscle fibres, your next meal (and sometimes subsequent meals) will be partly be used to refuel and repair, the rest will go to fuel your normal body processes and work until your next meal, only if you overeat is there any chance it will be stored as fat. Just split your calories up over as many meals/ snacks as it is possible to have, you don't have to label it lunch and it doesn't have to be a large meal. You should eat whenever you find gives you the most energy for that and the next workout and train whenever you can work the hardest - that varies from person to person. Personally I need to eat something quite soon after an intense workout to be able to train hard again a day or two later, and train much better in the late afternoon or evening than any time in the morning.

Eat whenever makes sense based on your schedule and energy levels.

I play floor hockey at lunch twice a week. What works for me is to have a fairly generous snack at about 11:00 and then eat a light lunch (at my desk) at about 1:30/2:00. If my snack includes some protein, I play better. E.g. almonds and yogurt or hard-boiled eggs. Another consideration is that both have to be quick or I'm working a long, long day to make up for all of my breaks. So left-overs and soup are my preferred lunches. More power to you! :O)