Food or eating habits that kills productivity?

Good question! Here's some high-level tips from me on maintaining a healthy diet:

1. Eat your fruits and veggies - Obvious but surprisingly hard for many engineers, especially if they eat out a lot
2. Drink mostly water - Avoid sugary drinks like soda and boba tea (very applicable for engineers, haha)
3. Don't eat within 2 hours of sleeping - Your metabolism is less active while you're sleeping, so it won't break down the food as much
4. Only eat until you're 80% full - Portion control is the best way to maintain a healthy weight
5. Eat slowly (20-30 minutes per meal) - Your fullness feeling is a lagging indicator. So if you eat slowly, you are more likely to reach a proper fullness as you give your stomach more time to process the food. This also allows you to savor the food more (and hopefully you're eating tasty food with good flavors)
6. Learn to cook - Restaurants don't care at all about your health, they just want your $$$. Even if you make an unhealthy meal like fried chicken (unhealthy food is often tasty, and it's fine to indulge every once in a while 😁), the version you make will almost certainly be far healthier than whatever KFC or Popeye's gives you

Check out this thread as well: "What to learn to cook? (as a software engineer)"

I would try to avoid eating processed food as much as possible. You can also get a food allergy test to see which foods don't work well with your body and avoid those foods.

I would avoid or minimize anything that makes you feel groggy or sleepy or stimulates your nervous system (i.e. sugar and caffeine).

Instead focus on small, balanced meals. Here are some guidelines:

1. Eat whole foods - think foods that come directly from nature (e.g. apples or fish) rather than made to resemble those (e.g. candy apples and fish crackers).
2. Avoid processed foods - most things you buy at the store are processed (e.g. bread or sliced meats or apple pies).
3. Drink more water - the recommendation is 9-13 cups a day. Drinking water also helps temper hunger.
4. Eat "balanced plate" meals - 1/4 plate lean (non-fatty) protein, 1/4 plate is clean carbs, 1/2 plate raw or lightly cooked green veggies and 1 tablespoon of healthy fats (olive oil, nuts, avocado). This is much easier when cooking for yourself. But when you have to eat out, try to match this formula.
5. Avoid high GI carbs - the glycemic index shows how quickly carbs convert into sugars. It's also an indicator of how quickly they are digested. High-GI foods digest quickly and give you a quick energy spike followed by a crash. Bread, white rice, potatoes, pasta and desserts all have this effect. But oatmeal, sweet potatoes, brown rice, quinoa and other grains fall lower on the GI scale. So if you want sustainable energy, opt for low-GI foods. They also happen to be the less processed ones.
6. Don't eat away your emotions - this isn't a dietary tip per se but many people fall victim to emotional eating. The most common is boredom. But it could also be frustration, fatigue or disappointment. When you want to reach for food, ask yourself if you're actually hungry. If not, get up and go for a walk instead. Getting some movement in is great for releasing emotions.