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Can you lose (or gain) fat in 24 hours?

Celebrity magazines, newspapers and social media are forever blasting headlines about rapid weight-loss “miracles” achieved in just a few days. When they come for a DEXA scan, many Bodyscan customers talk of their frustration when their weight changes day-to-day and is upward rather than downward.

Let’s get one thing straight. Your weight naturally fluctuates throughout the day and from day to day. And those short-term changes are not caused by increases or decreases in body fat. If your scales show you are a kilogram heavier than yesterday, you have not – repeat NOT – put on a a kilo of body fat! Significant changes in body fat take time (weeks and months) and occur as a result of a sustained net energy (calorie) surplus.

Short-term, daily body weight fluctuations are perfectly normal. Factors that can influence movement in body weight (independent of body fat) include:

  • Fluid balance (body water)
  • Salt intake
  • Muscle damage
  • Carbohydrate intake
  • Intestinal weight (food passing through your gut)
  • Stress
  • Menstrual cycle
  • Creatine supplementation (used by many bodybuilders)

In reality, changes in fat tissue are minimal or zero during a 24-hour period.

In simple terms, one pound (454g) of fat contains about 3,500 calories of energy. This is the premise behind the popular 500 calorie per day deficit dieting protocol, to lose one pound of body fat per week (seven days x 500 cals/day = 3500 calories = one pound of fat). [In metric terms, one kilogram of fat = 7,700 calories. Thus a kilo of fat loss per week requires a daily deficit of 1,100 calories.]

Based on the ‘3,500 calories per pound rule’, it would seem that if you eat 3,500 calories above maintenance you will store one pound of body fat in one day (at least in theory).

However, for a start, eating 3500 calories in excess of what you normally eat is quite some feat – it might mean eating two-and-a-half to three times your normal daily consumption.

Second, even if you could manage the sheer quantity of food, the additional 3,500 calories won’t necessarily lead to a gain of one pound of body fat. The reason is that when you eat big or very big meals, some of the energy from a large influx of calories in a short timeframe can be released as heat, which is not stored in the body. This is known as the thermic effect of feeding.

The response to overfeeding largely depends on genetics and lifestyle, and can vary significantly from person to person, even when over-fed by exactly the same number of calories.

So, unless someone has grossly overconsumed on calories (eg, 3,500-7,000 calories above maintenance), any jump in body weight over a 24-hour period is not even close to being all from body fat. In fact, the actual amount of fat stored will be relatively small.

Significant body fat is gained over weeks or months, not in hours or even days (for the most part). The marked differences in body weight after a short period of overeating are likely due to the fluctuation factors like body water, salt intake and food in the gut listed above.

However, persistent overeating will create a long-term, cumulative energy surplus over time, which will lead to body fat gain. If you eat 500 calories above maintenance every day for two months, that is 30,000 excess calories, some of which will get stored as fat, some as lean mass and some released as heat (as explained above). If you maintain the surplus (that is, regularly eat above maintenance), the total amount of body fat steadily increases over time.

​December (Christmas) is a month where many of us over-indulge on an almost daily basis, hence we hit New Year with an expanded waistline. But for many people, the holiday season just adds a spike to a smaller, but persistent, surplus of calories consumed every week between January and December and, with it, a slow but steady increase in weight and body fat over a much longer period of time.

While you shouldn’t take daily weight changes as evidence of changes in body fat, recording your weight at the same time every day will enable you to establish a longer-term trend. For best practice, record your body weight first thing in the morning, after the toilet and before consumption of any food or drink. If you take the average of your daily weight readings every seven days, THAT figure is a better one for understanding what is happening with body fat over the medium to long term.

Take other measurements too, such as progress photos and body tape measurements, and don’t rely solely on body weight. Or better yet, measure body composition with a DEXA body scan to obtain a detailed, accurate and consistent analysis of your progress. We recommend a scan every three to four months so you can keep a close eye on how things are going. Twelve to 16 weeks between scans is long enough to know that you can keep it up (that your diet and exercise regimen is sustainable) and for a motivating change to have occurred. Changes recorded in your fat and lean mass will tell you how much of your weight loss is fat and/or how much of your weight gain is muscle. We’ve made a video specifically about how frequently you should have a DEXA scan here.

Regular DEXA scans of this order will give you the medium-to-long-term trend of how your body composition is changing, and will supplement the average weekly changes in weight only recorded on your bathroom scales.

As well as scan frequency, we discuss other DEXA topics in detail on our video page. When you’re ready to book, click here!

Kevin Garde
Nutritionist and Bodyscan Consultant 

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