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I believe your metabolism responds differently. The sugar signals your body that you've eaten a rich calorie source but fat is lacking, please save this as fat for later. If you had eaten a meal with more fiber and fat, it signals to your body that fatty meals are available now, you can use some for energy, no need to create more storage.

Some food will more likely get stored anyway and rather instruct the brain to move less to compensate, while others will be more likely to "energize" you to do things physically and expend the energy.

See Robert Lustig on sugar.

Even with all that, your body composition would still change over time from lack of protein.