Two identical twins try to lose some body fat. Both reduce their caloric intake by about 500 calories.
Twin A doesn't like veggies very much, so he just makes sure that his caloric deficit is high enough as he thinks that at the end of the day, it's all about calories in vs. calories out.
Twin B doesn't mind veggies and decides to add extra veggies to each meal. This might add some extra calories but he doesn't bother counting them.
Who will lose more weight?
Twin A with a deficit of 500 calories a day or Twin B, who has a smaller deficit but eats about 20-30 grams of fiber extra?
Let's have a look at scientific studies to answer this questions.
If researchers and dietitians can agree on one thing, then its the following:
Our modern diet is deprived of fiber!
The dietary recommendation of the American Heart Association Eating Plan suggests the total dietary fiber intake should be 28 grams a day . However, a study looked at the daily fiber intake of US citizens and reported an average daily fiber intake of just 15-16 grams per day - with a significantly lower fiber intake in obese participants . Of note, paleolithic researchers found that our hunter ancestors had some days a fiber intake as high as 135 grams/day . This shows that our bodies are well adapted to far higher fiber intakes as the general recommendations.
Losing weight can be easy
Everyone knows that fiber makes you full. It was also shown that fiber helps to control the appetite . A large study looked at the nutrition composition of around 1800 Americans and found that a high fiber intake was strongly correlated to a lower BMI .
But it is simply because if we eat more fiber we eat less food that actually provides us calories?
Well, not exclusively!
If we want to understand the real power of fiber, we need to have close look inside our guts.
Fiber is critical to feed the billions of bacteria that live inside our guts! In fact, it has been shown that a high fiber intake increases the diversity of our microbiome [6, 7, 8] and many studies proved that a less diverse microbiome strongly correlates with obesity [8, 9].
Bacteria in our guts convert the fiber to short-chain fatty acids [SCFAs]. Powerful molecules that have been shown to improve diabetes, support weight loss and possess anti-carcinogenic properties (Blog will be published soon). A recent review even showed how SCFA can be used to treat Osteoporosis.
Studies have shown that SCFA promote fat burning and reduce fat storage . Addition of SCFA to a high fat diet prevented body fat gain in mice . The same was observed when obese volunteers supplemented SCFA . They reduced their body fat and also improved blood lipid levels.
Which Twin loses more weight?
Well, it is hard to give a definitive answer here. We have seen how adding extra fiber can benefit your fat loss thanks to your microbes! Calories in vs. calories out is also a ancient view that ignores how our body actually works. Here are two studies to show you how wrong the notion is:
1. A study conducted by Kekwick and Pawan compared 3 groups put on 1,000 calorie diets of 90% fat, 90% protein, or 90% carbohydrates. 
- 1,000 calories at 90% fat = weight loss of 0.9 pounds per day
- 1,000 calories at 90% protein = weight loss of 0.6 pounds per day
- 1,000 calories at 90% carbohydrate = weight gain of 0.24 pounds per day
2. In Canada, students enrolled into a study in which they were isolated from the rest of the world for 120 days and where they were only aloud to eat, sleep, watch tv, or to read. The students were over-fed with 1000 calories extra and no sports allowed. After that time period, the students gained on average 8.1kg of body weight. Which is not very surprising but what was the surprising, was the range of weight gain. While some students only gained 4.3kg, other gained up to three times more. 
These two studies pretty much show us that a calorie is not a calorie! It depends on the source of the calorie, your genes and also the composition of your Microbiome!
Fiber can do more! - The Unknown Benefits of Fiber!
... Blog follows soon
- King et al., Trends in dietary fiber intake in the United States, 1999-2008.,J Acad Nutr Diet, 2012
- Leach and Sobolik, High dietary intake of prebiotic inulin-type fructans in the prehistoric Chihuahuan Desert, British Journal of Nutrition, 2010
Shay et al., Food and nutrient intakes and their associations with lower BMI in middle-aged US adults: the International Study of Macro-/Micronutrients and Blood Pressure (INTERMAP), The American Journal of Clinical Nutrtion, 2012
Mozaffarian et al., Changes in Diet and Lifestyle and Long- Term Weight Gain in Women and Men, NEJM, 2011
Deehan and Walter. The Fiber Gap and the Disappearing Gut Microbiome: Implications for Human Nutrition. Trends in Endocrinology & Metabolism, 2016
Erica D. Sonnenburg, Samuel A. Smits, Mikhail Tikhonov, Steven K. Higginbottom, Ned S. Wingreen, Justin L. Sonnenburg. Diet-induced extinctions in the gut microbiota compound over generations. Nature, 2016
Menni et al., Gut microbiome diversity and high-fibre intake are related
to lower long-term weight gain, Nature, 2017
Turnbaugh et al., A core gut microbiome in obese and lean twins, Nature, 2009
den Besten et al., The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism., J Lipid Res, 2013
Gao et al., Butyrate Improves Insulin Sensitivity and Increases Energy Expenditure in Mice, Diabetes, 2009
Kondo et al., Vinegar Intake Reduces Body Weight, Body Fat Mass, and Serum Triglyceride Levels in Obese Japanese Subjects, Biosci. Biotechnol. Biochem, 2009
Bouchard et al., The response to long-term overfeeding in identical twins, New Eng. J. Med, 1990.
Kekwick and Paran, Calorie intake in relation to the body-weight changes the obese, The Lancet, 1956