Cardio and what kind?

As long as your diet puts you in a calorie deficit, you can lose fat without doing any cardio [1]. But that does not mean cardio is useless. Doing cardio a few times per week can help you lose more fat by increasing your energy expenditure. The next obvious question is, what type of cardio is best? Should you do HIIT or moderate-intensity cardio?
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When it comes to losing fat, there is no 1 type of cardio that is inherently more effective than the other. A 2017 meta-analysis gathered data from 31 studies to analyze whether high-intensity cardio provides a fat loss benefit over moderate-intensity cardio (while calorie-burn is matched). No significant difference in fat loss between HIIT and moderate-intensity cardio was found [2].
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So if you want to increase fat loss via cardio, you have flexibility as to what type of cardio you should do. If you are short on time, high-intensity cardio is a great way to burn calories in a short period of time [3]. But if you already train with weights several times per week, moderate-intensity cardio (up to 3x/week) is less demanding on your body and may interfere less with your recovery from lifting weights.
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For athletes, it is also important to consider that the type of cardio you do impacts the type of adaptation your body goes through. When you perform mostly high-intensity cardio, you train your ability to exert maximum effort and have a high-force output. When you do low- to moderate-intensity cardio, you mostly get endurance adaptations. Based on your training goals you can pick a cardio modality.
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References:
1. https://www.ncbi.nlm.nih.gov/pubmed/10838463
2. http://onlinelibrary.wiley.com/doi/10.1111/obr.12536/full
3. https://www.ncbi.nlm.nih.gov/pubmed/18362686
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