You can't stop with sucrose in the intestines. Once that sucrose is split into fructose, which goes to the liver for additional processing, and glucose, which goes into the bloodstream, you have to deal with the individual's hormonal responses. You have insulin, leptin, ghrelin, and a host of other signaling chemicals, and they all interact with each other and with specific areas of the nervous system.

Give 10g sucrose to one person, and they may bounce off the walls for a few minutes (increasing caloric expenditure). Give 10g to someone else, and they may feel an urge to eat more (increasing caloric intake). Try it on a third person, and they may want to take a nap (decreasing caloric expenditure).

The chemicals in foods can act upon the body's biochemistry in as drastic a manner as pharmaceutical drugs, together with side effects. People that just consumed large amounts of ethanol have different eating and activity patterns than those who have not. The drunkenness is obvious, but the Buffalo wing consumption still varies.

Sugar intake influences metabolism, just like drugs such as caffeine, nicotine, ethanol, or epinephrine, though those are active at far lower doses.

Thus, the thermodynamic equation of "energy_in - energy_out = delta energy_stored" is certainly valid at a technical level, but it has no practical utility for weight loss. Both energy in and energy out are black box functions that vary by individual, and include types and quantities of foods eaten and current fat stores as input parameters.

The black boxes are not currently well understood. The genome has already been sequenced, but that only led to epigenetics and proteomics. As research in those areas proceeds, we might see a magic pill for fat people that reprograms their black boxes to be more like athletic people. Until then, their effort to lose weight will be a conscious struggle against the natural inclinations of their own body.

And they will continue to be judged as deficient by people who do not need to consciously oppose the biochemical signals of their own bodies, and are therefore ignorant of what that may entail. This is why weight loss is a touchy subject for some people. If you can't be unambiguously supportive and encouraging to someone trying to get fit, it is usually better to just maintain your distance and pretend to not have an opinion. A person who has lost 100 kg will have a far greater understanding of what is involved than someone who has never even tipped the scales past 100 kg in total. And even that person may not understand all that another 200 kg person has to overcome to lose half their body weight.

So here's what you say: "Good for you! You're already a winner! I know you have the ability to reach your goals!"

You don't suggest that they could do better by buying more accurate or more precise scales. They aren't fat because they were lax in their data collection protocols. The stoichiometry of inhaled oxygen and exhaled carbon dioxide won't help. Taking body temperature and recording urine and sweat volume won't help. Anecdotes about how you lost your weight won't help. If you aren't currently activating genes or folding proteins as part of your regular job or primary hobby, you're better off simply providing the psychological support to just keep trying, without getting into specifics.