I don’t think anyone will be surprised to see “Not Eating” + “Doing Exercise” is the fastest way to lose weight. However the noteworthy comparison is the Efficiency of weight loss. This can be used to limit the pain needed to lose weight.
There are plenty of websites about fasting for multiple days, look into those or the Electrolyte loss + Refeeding Syndrome can kill you.
Fasting with exercise wins, this is easier said than done if you’ve fasted for a few days. I’ve been able to run ~1 mile and walk 2-3 miles during a 5 day fast. This is approx 300+ calories.
Something interesting here. The extra exercise is gives you the same weight loss per day as if you never fasted, however the absolute when fasting is higher. Its easy to dismiss this as obvious, but if you ever found yourself buying an extra item because it was Buy One Get One Half Off and you ended up spending 50% more money than you expected. My point being: If you are going to be suffering during a fasting day anyway or its urgent to lose weight, you could add an extra 300+ calories in exercise.
This also highlights how insignificant some exercise is. 1 hour of causal walking burns ~300 calories max. It is quite time intensive to burn calories this way. Meanwhile if you could run 6 or 13 miles, you could be burning 800 or 1700 calories in a day. These stacked with a caloric deficit can get decently high in weight loss per day.
If you can handle fasting, it clearly shows to be the best. I will do this a few times a year for ~2-3 days to lose weight but not get into ketosis. Being obsessed with Efficiency, I admittedly will do sometimes a 1 mile run then take the kids ~2 miles round trip to the park.
More regularly, I’ll eat 1 large-ish meal and 0-1 smaller meal/snacks through the day. Then I’ll do some cardio, often 6 miles. 1500 calories eaten, ~800 calories burned. 1800 Caloric deficit! Lose a half pound in 1 day!
This is semi-reasonable to do this and similar all week. Swapping 6 mile runs for biking to let muscles rest won’t burn as many calories, but it will give you a break and still burn extra.
I consider losing weight a temporary condition. I don’t plan to live like this forever. I plan to lose the weight and stay some-what around that weight.
If I gain 5-15lbs in a year, I can lose this weight in weeks. This can beat some of the mental difficulties with weight loss. We are often inspired for the first ~2 months of a new lifestyle before the excitement is gone.
Apply this law of nature and sprint at weight loss for 2 months before taking a break and eating normal. When you are excited to take on another round of weight loss, you are ready to begin.