(Notice that you simply made an assertion in your first comment, without any logic or argument, but we’ll leave that aside for now)

Ok, let’s take your argument, and put it in logical form.

1. Hypothesis: Complex carbs cause weight gain.

2. Argument: I avoided complex carbs and lost weight.

3. Conclusion: I have therefore proved that complex carbs cause weight gain.

Ok, here’s a counter argument.

1. Hypothesis: Complex carbs do not cause weight gain.

2. Argument: 18 years ago, when I dropped 30 pounds (and have maintained the loss since), complex carbs were a main part of my diet

3. Conclusions: I have therefore proved that complex carbs do not cause weight gain.

Both arguments are of course utterly baseless, as they have not established causation.

When you test thousands of people, carefully measure the evidence, using advanced statistical methods, you find that in fact, for the vast majority of people — today and throughout history — complex carbs in themselves do not contribute to weight gain or weight loss.