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I have friends who are trying to lose weight and they keep saying they can't eat certain foods because it is high in carbs and carbs are fattening.

This doesn't make sense to me, I lost weight myself but all I did was look at the calorie content of foods, if it was carbs or anything else it didn't really matter. Although I do find that foods that are high in carbs are generally also high in calories but this doesn't mean that carbs are more fattening does it?

Is there any truth in what my friends are saying; that carbs are fattening?

But it's not just my friends that are saying this, I've heard that carbs are fattening from many different sources: A quick search on how to lose weight shows multiple results mentioning limiting of carbs in order to lose weight: 1 where they say calorie restricted diets do not work, 2 low carb diet lost more weight than low fat group, because they replaced carbs with protein

  • The high energy that carbs provide is fattening enough. Maybe your friend is saying that carbs are fattening because they are high in calories? Or are you sure your friend is saying that carbs are especially fattening? – busukxuan Jan 17 '16 at 2:07
  • @busukxuan but carbs aren't particularly calorie dense, it is the carbs they are saying is fattening, as they say that an all meat diet is much better. meat is fat and protein so is higher in calories, but has no carbs and hence are better for losing weight (according to them). See edit for more (comment too long) – Aequitas Jan 17 '16 at 2:33
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    Also, it is relatively easy to eat large amounts of calories from carbs - most people can eat 3000 to 6000 in cakes and sweets in a day if they try. Overeating on fat or protein calories is more difficult, you get 'full' and disinterested much quicker (try to eat a pound of chicken...). I have no references for that, so consider it 'anecdotal'. – Aganju Jan 17 '16 at 5:32
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    @Aganju I find overeating on "fat calories" pretty easy as well. – YviDe Jan 17 '16 at 8:32
  • @user19679 Please do not answer in the comments. If you have an answer leave it below in the answer section backed by reliable sources. – michaelpri Jan 17 '16 at 16:32
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Note: This explanation does not concern itself with body health, wellbeing, the ability to follow/sustain the diet short/long term, impact to nervous system or the psychological impact of satiety that low-carb diets can offer. It only concerns itself with the imaginary scenario of two identical subjects following the same total calorie diet but with macro-nutritional differences.


Are Carbohydrates fattening?

The question asks if carbohydrates lead to more fatty mass gain than the other macronutrients: fat and protein (and alchohol), if consumed at the same calorie level.

To put it another way: For weight change, does the macro-nutritional profile of a diet affect the rate and total amount of fat mass gained?

The common tautology employed by people proving that macro-nutritional profile is not important when it comes to weight loss is "a calorie is a calorie".

Evidence For, or "A calorie is a calorie"

Several metabolic ward studies have shown that there is no difference in weight loss when protein intake was held constant. If you're really looking for a metabolic advantage through manipulating macronutrient, you'd be far better off putting your money on protein. There's actually some evidence that higher intake levels do convey a small metabolic advantage.

Unsurprisingly, the studies into macronutrient impact on mass change are numerous but by no means perfect. One has to cast a very critical eye over all the conflicting evidence (and mud-slinging) and make their own decision.

A good meta analysis of the above tautology by the well-respected Buchholz AC & Schoeller DA. concluded that:

...Neither macronutrient-specific differences in the availability of dietary energy nor changes in energy expenditure could explain these differences in weight loss. Thermodynamics dictate that a calorie is a calorie regardless of the macronutrient composition of the diet...

Buchholz AC, Schoeller DA. Is a calorie a calorie?. Am J Clin Nutr. 2004;79(5):899S-906S.

This referenced a ward study (amongst others which also concluded the same):

Both the high-carbohydrate and high-protein groups lost weight (-2.2+/-0.9 kg, -2.5+/-1.6 kg, respectively, P <.05) and the difference between the groups was not significant (P =.9).

Sargrad KR, Homko C, Mozzoli M, Boden G. Effect of high protein vs high carbohydrate intake on insulin sensitivity, body weight, hemoglobin A1c, and blood pressure in patients with type 2 diabetes mellitus. J Am Diet Assoc. 2005;105(4):573-80.

To continue the evidence-train for there being no win in carbs-vs-fat:

In a 2003 study by Bravata DM, et al. the conclusion was that nutritional-profile really doesn't affect total weight change at a significant level.

There is insufficient evidence to make recommendations for or against the use of low-carbohydrate diets. [...] Among the published studies, participant weight loss while using low-carbohydrate diets was principally associated with decreased caloric intake and increased diet duration but not with reduced carbohydrate content.

Bravata DM, Sanders L, Huang J, et al. Efficacy and safety of low-carbohydrate diets: a systematic review. JAMA. 2003;289(14):1837-50.

A 2009 study directly comparing the weight loss "fad" diets concluded that provided you reduce calories, the method you do this is not of importance:

Reduced-calorie diets result in clinically meaningful weight loss regardless of which macronutrients they emphasize.

Sacks FM, Bray GA, Carey VJ, et al. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates. N Engl J Med. 2009;360(9):859-73.

A 1996 study also concluded the same thing:

The results of this study showed that it was energy intake, not nutrient composition, that determined weight loss in response to low-energy diets over a short time period.

Golay A, Allaz AF, Morel Y, De tonnac N, Tankova S, Reaven G. Similar weight loss with low- or high-carbohydrate diets. Am J Clin Nutr. 1996;63(2):174-8.

An Australian study put them head-to-head over 12 months and didn't find a large difference:

Under planned isoenergetic conditions, as expected, both dietary patterns resulted in similar weight loss and changes in body composition. The LC [low carbohydrate] diet may offer clinical benefits to obese persons with insulin resistance. However, the increase in LDL cholesterol with the LC diet suggests that this measure should be monitored.

Brinkworth GD, Noakes M, Buckley JD, Keogh JB, Clifton PM. Long-term effects of a very-low-carbohydrate weight loss diet compared with an isocaloric low-fat diet after 12 mo. Am J Clin Nutr. 2009;90(1):23-32.

A 2010 study went a step further and did a 2-year study across over 300 participants; patients lost an average of 7 kg or 7% of body weight, and no differences between the 2 groups were found:

Successful weight loss can be achieved with either a low-fat or low-carbohydrate diet when coupled with behavioural treatment.

clearly no difference

Foster GD, Wyatt HR, Hill JO, et al. Weight and metabolic outcomes after 2 years on a low-carbohydrate versus low-fat diet: a randomized trial. Ann Intern Med. 2010;153(3):147-57.

Taking a different view and looking at weight gain, there are fewer studies but the evidence points to the same outcome.

There was no significant difference in fat balance during controlled overfeeding with fat, fructose, glucose, or sucrose.

Mcdevitt RM, Poppitt SD, Murgatroyd PR, Prentice AM. Macronutrient disposal during controlled overfeeding with glucose, fructose, sucrose, or fat in lean and obese women. Am J Clin Nutr. 2000;72(2):369-77.

This can be also seen in this small 2000 study:

...fat storage during overfeeding of isoenergetic amounts of diets rich in carbohydrate or in fat was not significantly different, and carbohydrates seemed to be converted to fat by both hepatic and extrahepatic lipogenesis

Lammert O, Grunnet N, Faber P, et al. Effects of isoenergetic overfeeding of either carbohydrate or fat in young men. Br J Nutr. 2000;84(2):233-45.

A important point to note is that dietary fat is what is stored as bodily fat, when a caloric excess is consumed. For dietary carbohydrate to be stored as fat then they must undergo a conversion through 'de novo lipogenesis' which will occur when the carbohydrate portion of someone's diet alone must approach or exceed ones total daily energy expenditure (TDEE). You can read more here

In addition, for a comprehensive primer on insulin and how it functions please check out this post on weightology which is summarised in layman on reddit by /u/ryeguy, here.

Evidence Against, or "A calorie is not just a calorie"

I cannot find evidence to support the opposite viewpoint. However there is criticism of the studies done these are found in the "Dear Sir"'s in the ASfCN/

A post by Anssi H Manninen is critical of a Bravata study:

Bravata DM, Sanders L, Huang J. Efficacy and safety of low-carbohydrate diets: a systematic review. JAMA 2003;289:1837-50.

She states that:

...In the true low-carbohydrate group, the mean weight loss in trials was 17 kg, whereas in the higher-carbohydrate group it was only 2 kg. Oddly, the authors did not consider this significant. Only by intermingling the results of trials of low- to medium- and high-carbohydrate diets could the authors have reached the misleading conclusion quoted above.

In another article Richard Feinman and Eugene Fine dispute the assertation of "a calorie is a calorie" using the first law of thermodynamics by stating that the second law must also be taken into account.

Conclusions

What should I eat for weight loss?

Eat less. Different diets can make this easier, so pick whichever one best fits your lifestyle. Ultimately, you need to reduce your caloric intake.

https://examine.com/nutrition/what-should-i-eat-for-weight-loss/

Many diets, fad or not, do work. This is mainly because they reduce calories.

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Scientists still aren't sure, but it seems carbohydrates may be easily converted into fat, depending on the form, or promote fat storage through stimulating insulin.

Nutrition Science and Applications (2nd E) by Smolin and Grosvenor, in Chapter 4, page 140 covers this question well.

Carbohydrates in and of themselves are not “fattening.” They provide 4 kcalories per gram compared with 9 kcalories per gram provided by fat. In fact, it is the fats that we often add to our high-carbohydrate foods that increase their kcalorie tally. A medium-sized baked potato provides about 160 kcalories, but the 2 tablespoons of sour cream you add brings the total to 225 kcalories...

Any energy source consumed in excess of requirements can cause weight gain ... even though carbohydrates are not [as] high in kcalories [compared to fat], the type of carbohydrate affects the impact that carbohydrates have on body weight.

Fructose metabolism in the liver favors fat synthesis, which in part contributes to fat production. Studies in mice indicate that dietary fructose produces a greater increase in body fat than the same amount of sucrose [1]

The rationale behind consuming a low carbohydrate diet for weight loss is that foods high in carbohydrate stimulate the release of insulin, which is a hormone that promotes energy storage. It is suggested that the more insulin you release, the more fat you will store. High-glycemic index foods, which increase blood sugar and consequently stimulate insulin release, are therefore hypothesized to shift metabolism toward fat storage. Low-carbohydrate diets... cause less of a glycemic response and less insulin release, which is suggested to promote fat loss."

So there is the possibility that carbs aren't actually that fattening by themselves. It could be just that people eat a lot of fat with their carbs. On the other hand, some studies seem in indicate that high-energy intake through sugary drinks, desserts, and large quantity consumption play a role in fattening.

[1] US Dept. of Health and Human Services. US Public Health Service. Oral Health in America: A Report of the Surgeon General. Rockville, MD: National Institutes of Health, 2000



The following links may provide more insight. They suggest that carbohydrate intake is indeed correlated with obesity:

http://www.hsph.harvard.edu/nutritionsource/carbohydrates/low-carbohydrate-diets/ http://www.nejm.org/doi/full/10.1056/NEJMoa1200303#t=articleDiscussion http://www.ncbi.nlm.nih.gov/pubmed/22735432
http://annals.org/article.aspx?articleid=1900694

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The reason why some people say this is because consuming carbs causes insulin levels to rise and insulin inhibits fat metabolism. In the opposite case when type 1 diabetic patients don't take insulin (e.g. when they are ill and not eating well), they are at risk of diabetic ketoacidosis. This is caused by the fat metabolism going in overdrive due to lack of insulin, the waste products poisoning the body as a result.

However, the mere fact that there exists such a mechanism is not proof that it has a relevant role to play in energy management of the body. There is no clear evidence in favor of low fat or low carb diet for weight loss from trials. Also any result from a trial has to be evaluated on whether the observed weight loss is sustainable. Take e.g. this study comparing low fat to low carb diet:

Cardiologist Dr Aseem Malhotra was concerned about the health impact of such a low-fat diet. "The total fat intake of 7% is way too low for this to be sustainable and would likely lead to nutritional deficiencies for the essential fatty acids and the fat soluble vitamins. "For the best health, even in the short term, eat real unprocessed foods, concentrate on good nutrition and stop counting calories!"

As pointed out by Paparazzi in the comments, we also have to also note that whole grains won't lead to a large insulin spike as simple carbs. Also, simple sugars such as fructose are not going to lead to a sharp insulin spike when consumed from whole foods such as fruits compared to when consumed in refined form, as pointed out here.

  • It's a bit strange that this answer was downvoted multiple times and then deleted while another answer given here invokes the same fundamental mechanism referred to in this answer. Unbelievable! – Count Iblis Feb 18 '16 at 4:34
  • Not my downvote, but it might be due to reliability of resources you used? – Lucky Feb 18 '16 at 6:59
  • I think insulin is more tracked to simple carbohydrates (sugar). – paparazzo Jan 29 '17 at 17:56
  • "There is no clear evidence in favour of low fat or low carb diet for weight loss" To clarify, the trials I have read attribute the success of either method to what the diet-ee is most likely to be able to maintain, based on their cultural background and current diet. For example, a traditional Indian diet is complemented by low-fat fairly well. I agree that the point about good-health-eat-balanced but as a method for losing weight, counting calories has been long-proven to be the most effective. The laws of thermodynamics are quite hard to prove wrong. – Gunge Mar 2 '17 at 13:45
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Carbs are not fattening if consumed within the allowed macronutrient ratios. Human body follows the law of energy in vs energy out. Calories that you consume is your energy in and calories that you expend is your energy out. If energy in > energy out, you will gain weight. The opposite, you will lose weight.

People go wrong in understanding the use of carbohydrates in the body. Carbs are your body's preferred source of energy (not the only source of energy). If consumed well in the limit and understanding your body's response to carbs, you can lose weight while eating carbs as well.

Now lets say you are consuming carbs throughout majority of your calories, then yes in some cases your body's ability to tap into fat sources for deriving energy drops down as there is a lot of glycogen (storage form of carbs in body) available already. So in short, carbs have to be regulated and had smartly.

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