The chemistry of the body does not react quite at the speed that you would think and many signs are false positives.

For example: Go on a fast for a day. Energy out will exceed energy in. Will you loose fat that day? No . The fat is the final reserve of the body and it will take all the other glygogen from where it can before it starts on the fat.

Another example: Binge on carbs and calories on a day. Really go overboard will you put on a couple of pounds of fat overnight? No you won't. You will feel blosted and retain a fair amount of water but it will not be fat.

Similarly it takes several days of very low carbs before a ketostix will change colour and even that is dependent on a number of variables. The reverse of that is also true if you consumed your 100 carbs throughout the day but the last meal before you tested had dietry fat in it on you had consumed any even the smallest amount of alcohol then the ketostix may still show trace.
The stix can be negative in the morning, positive in the evening, and negative the following morning. The strips are only indicative of the likely metabolic changes.

So the first answer is that in the microcosm that is a day in the life of a diet, it is virtually impossible to accurately know what the metabolism is doing. Moreover it will be different for each person depending on their genetic makeup.

But your question I think could be a more fundamental question and strikes right at the confusion about low calorie, low fat and low carb. Can you eat carbs and just lower calories instead is what you are asking. Isn't it all just low calorie?

Lets break the question down a bit.

Will you lose weight on a low calorie diet?, Yes you will. Millions of pounds have been lost by low calorie diets, Millions of pounds sterling have been made by selling programmes to tell you how. Wouldn't it be nice to have the weightwatchers franchise in your wallet. Low fat works as well.(heresy I here you cry) There are over twice the calories in a gram of fat than a gram of carbohydrate. This is undeniable.

The tragedy is that dieting in a low calorie way when stopped, and everybody does, leads to the immediate regain of weight and often to a level above that of the previous level. We diet again and it goes down. We stop and it goes up. I and the majority of the people here have spent a lifetime at this continual up and down yo yo existence. It is because we have not understood the fundamentals of how the body produces fat.

So what makes us gain weight. Is it too much calories? The answer is ... No. (and yes) Is it the calories from carbohydrate that are the primary cause of making you fat? The answer is...Yes (and no) My best friend eats carbohydrate and does not gain weight why should I? Its not fair. Why have I missed breakfasts, gone on fasts, eaten what I believe is quite normal amounts, eaten my fruit as I had been told(bananas were my favorite)only to gain weight and why are some around me for ever slim.

The reason is ( I am sorry it has taken so long to get here) that I and millions like me have a genetic propensity (probably inherited) to react to sugar consumption in a certain way or our eating lifestyle has affected the way in which the body deals with sugar. (Just a reminder here that all carbohydrate is either a simple or refined chain of sugar molecules.)

It is how our bodies produce and react to Insulin that makes us fat or makes us thin, Not the number of calories consumed, not the number of carbs, and not the amount of fat. Insulin is the master controller. If your body's metabolism becomes clever at resisting the job of insulin -- to reduce blood sugar levels-- this is what makes you fat.

So if you want to lose fat and then stay thin forever you have to control the sugar intake in the first place!. The body itself is having a hard time with it. It is because my own body has developed this resistence to Insulin that makes me fat and not my friend. It is not the french fries that we both ate. This is why I said yes and no to the carb question.

We often lose sight of why we are following a low carb lifestyle and demonise carbs. There is nothing inherently wrong with a carbohydrate, there is nothing dangerous about a banana. Half the world would starve without rice.The problem with the western world is that we have become over familiar with processed and refined carbohydrate. We love it. It has become habit forming. Carbohydrate does that. The more we have the more we want it. The more we eat it the more of a resistence to insulin is built up.

Fortunately the reverse is also true. The less we have the less we want it. For us poor insulin resistent souls we have to permantly kiss good by to sugar in all its forms. This restores the receptivity to Insulin, and restarts the metabolism burning the fat.

So we eat enough calories to sutain normal living, we eat enough protein to not lose muscle or lean body mass. We eat good fats, which in the absence of carbohydrate is neutral in our diet.

Overdosing on carbs on a day will only be a little blip in a life time. It may not do anything at all. You could even see the scales move downward. But when we overdose we set off the sugar cycle again and the potential is to overdose again and again and again and that is the slippery slope.

So if you overdose on carbs, reducing other calorie intake is pretty meaningless in the short term. There is no need. All you do is just go back the next day to lower carb levels. What that level is for you is what you need to find out by experiment. What level of carbs can you eat and still maintain a steady loss.40? 60? 100 even? And later what level of carb consumption will keep you at a given chosen weight.