Since the topic constantly comes up I figured I'd make some posts on low carbs vs low fat. I'll keep them short and sweet. Or long and dry, I guess I'll figure it out as I write them.
First up, metabolic advantage. Some low carb or keto advocates claim that they can lose weight on higher calories on keto, or eat more calories and maintain weight. They reference studies that show more weight loss in the low carb groups. They claim insulin stops fat oxidation so you have to lower it to lose weight.
There are studies that show more weight loss in low carb groups than the low fat groups, but they have issues such as not matching protein and calories, or relying on self reported intake data which is known to be very flawed.
When you look at studies that tightly control calories and protein and match them between groups the differences are small and not important. The best diet is whatever you adhere to best. For some that's keto or low carb, for other's its higher carbs.
But don't take my word for it, let's look at some research.
The largest difference I've seen is in this study. It was done in a metabolic ward, the subjects had no access to food apart from what they were given. They had to eat it all, and if they didn't the dietitians compensated for it in later meals. They ate a high carb baseline diet for 4wks, then an isocaloric keto diet for another 4wks. During the keto portion they ate about 35g carbs, so definitely in ketosis. Fat oxidation went up with the keto diet and peaked after about a week.They found about a 100 calorie greater energy expenditure, however they speculated it was overestimated due to the greater water loss in the keto portion. It also was highest at the beginning when glucose needs are highest before the brain transitions to using mostly ketones, and gluconeogenesis (making glucose from protein) is metabolically expensive but needed less when glucose needs lower.
"In summary, we found that a carefully controlled isocaloric KD coincided with small increases in EE that waned over time. Despite rapid, substantial, and persistent reductions in daily insulin secretion and RQ after introducing the KD, we observed a slowing of body fat loss. Therefore, our data do not support the carbohydrate–insulin model predictions of physiologically relevant increases in EE or greater body fat loss in response to an isocaloric KD."
In another excellent, very well controlled study by the same author, Kevin Hall, they fed subjects a baseline diet and then either a low fat or low carb diet equal in calories and protein. They even made sure the low fat diet didn't have less sugar or refined carbohydrates than the baseline diet, and insulin release and fat oxidation didn't change from baseline in the low fat group. Fat oxidation was much higher in the low carb group, and insulin levels were significantly lower. Those are both things that low carb advocates often say will lead to more fat loss, yet fat loss was actually slightly higher in the low fat group. It was a small difference though and the authors speculated it would even out over time. Carbs were around 140g so some would argue the results would be different with lower carbs as in a keto diet, so we'll have to look at some other studies too.
"In contrast to previous claims about a metabolic advantage of carbohydrate restriction for enhancing body fat loss (Ludwig and Friedman, 2014; Taubes, 2007, 2011; Westman et al., 2007), our data and model simulations support the opposite conclusion when comparing the RF and RC diets. Furthermore, we can definitively reject the claim that carbohydrate restriction is required for body fat loss (Taubes, 2011)."
"In summary, we found that selective reduction of dietary carbohydrate resulted in decreased insulin secretion, increased fat oxidation, and increased body fat loss compared to a eucaloric baseline diet. In contrast, selective isocaloric reduction of dietary fat led to no significant changes in insulin secretion or fat oxidation compared to the eucaloric baseline diet, but significantly more body fat was lost than during the carbohydrate restricted diet."
The next study looked at more than just weight/fat loss, it looked at the effects on visceral fat and metabolic syndrome as well. This one was about 50g of carbs in the low carb group, so ketogenic. Metabolic syndrome is closely related to insulin resistance, and low carb diets are often said to be better for the insulin resistant. They instructed the subjects to eat a high quality diet low in trans fats, sugar, highly processed foods, and omega 6 fats. They were also instructed to eat fruits and veggies with an emphasis on veggies, and veggies with every meal. Protein and calories were matched, and they were given a very detailed recipe book with over 200 meals designed to fit the study well. They were instructed to eat the same primary sources of protein, carbs and fat. It still relied on self reported intake, and the baseline diet reports were likely not exactly accurate due to some subjects eating more calories in the intervention yet losing weight, but pretty damn well controlled. I'll discuss this one more in a later article but basically there was no significant difference in results.
"In the present dietary intervention study, we found a marked reduction in visceral fat mass and improvements in most of the measured clinical variables related to metabolic function, independent of a sharp dichotomy in the fat-to-carbohydrate intake ratio...
In summary, we found similar responses to highly standardized LFHC and VHFLC diets with respect to intra-abdominal fat mass, hepatic lipid content, pericardial fat volume, and components of metabolic syndrome."
This study looked at a low carb diet versus a very low carb keto diet. They provided all food and beverages to the subjects for the first 6 weeks, and gave them meal plans for the last 4. They tracked body weight, fat mass, hunger, and 6 different mood states weekly for the first 6wks, and body weight and fat mass at the end of 10. Protein was matched, and carbs started at 20g for the keto group and was raised by 5g a week weeks 3-6 and ~150g in the other group. The keto group was instructed to eat ~40g the last 4wks. I'll be discussing this study more in depth later too, so I'm just gonna leave some quotes from the authors here for now. Lots of interesting stuff in this study.
"These data show that, under isocaloric conditions, total weight loss and fat loss did not differ significantly by diet treatment. Yet, according to weight-loss averages during the “self-monitored” follow-up period, dietary compliance may be more easily achieved with NLC than with KLC diets."
"As in other weight-loss trials (14-16), insulin resistance (HOMA index units) decreased in both diet groups (30%; P < 0.05), and body mass explained nearly 20% of the variance in insulin resistance."
"Weight-adjusted REE increased in both diet groups over the 6-wk trial, but blood β-hydroxybutyrate concentrations were not correlated with REE (r = −0.014, P = 0.921), which indicates that the protein content of the diet, rather than the severity of the carbohydrate restriction, likely contributed to the elevations in REE. These data support the contention that calorie-reduced diets high in protein facilitate weight loss, in part, by preserving the metabolic rate (7, 8, 18)."
"Insulin sensitivity was significantly improved by both LC diets"
"Weekly ratings of perceived hunger did not differ by diet group during the trial, which suggests, as discussed by others (31, 32), that it is the protein content of the diet and not the severity of dietary carbohydrate restriction that affects perceived hunger."
"In the current study, weekly fatigue-inertia scores, representing a mood of weariness, inertia, and low energy level, did not differ significantly by diet treatment or time; however, vigor-activity scores, representing a mood of vigorousness, ebullience, and high energy, were significantly higher in NLC dieters than in KLC dieters. These data suggest that, in the context of high-protein diets, small differences (as little as 50–60 g/d) in dietary carbohydrate may affect emotion, mood state, and, potentially, the desire to be physically active. "
"In the current study, the KLC diet did not offer any significant metabolic advantage over the NLC diet. Both diets were effective at reducing total body mass and insulin resistance, but, because blood ketones were directly related to LDL-cholesterol concentrations and because inflammatory risk was elevated with adherence to the KLC diet, severe restrictions in dietary carbohydrate are not warranted. Furthermore, the NLC diet was associated with feelings of high energy and a more favorable mood profile than was the KLC diet. Practitioners should advise patients who wish to follow an LC diet to choose low-fat meats and dairy products, 8–9 daily servings of fruit and vegetables, and a dietary carbohydrate limit near 100–125 g/d. Patients should know that there is no apparent metabolic advantage associated with ketosis during dieting."
This study looked directly at whether the benefits seen in low carb diets are due to the reduction in carbs or increase in protein that typically comes along with them. After a two week baseline diet the subjects were divided into 4 groups and fed different diets with the same amount of relative calories. The 4 diets were high protein low carb, high protein normal carb, normal protein low carb, and normal protein normal carb. They were given menus designed to induce weight loss for 3 months, and then to maintain for 9 months. They also went to a group program that included members of all 4 groups, and focused on healthy diet and eating behavior in general, and answered questions about the diets individually. Again, I'm just gonna quote the authors here:
"The study showed irrefutably, that, despite the success all-over with all four diets, the answer is that it is the relatively high-protein content per se, that supports the even greater success, and not the relatively lower carbohydrate content"
"the two HP diets, namely HPNC vs. HPLC and the two NP diets, namely NPNC vs. NPLC did not show significantly different effects"
"Significant differences in loss of body-weight were observed when comparing HP diets with NP diets, namely HPLC vs. NPLC and HPNC vs. NPNC over the complete period of time, i.e. over the 3 months and over the 12 months. In parallel significant differences were observed in decreases in FM when comparing these 4 diets. When in focusing on ‘low-carb’ diets, larger bodyweight loss and body-weight maintenance was achieved when ‘low-carb’ was in the presence of high-protein, but not when it was in the presence of normal-protein"
"Relationships between body-weight changes, or changes in FM, or in FFM and energy percentage of fat in the diet were not statistically significant, nor were relationships between changes in any of the metabolic parameters and energy percentage of fat in the diet."
"Overall, the metabolic profile was improved in all diet groups. Several metabolic parameters were related to BW loss, indicating a larger effect of weight loss on these parameters than of the diet itself, as was also shown previously [19,37]."
"Lowered carbohydrate intake per se had no effect on decrease in BW and FM during energy restriction, while daily elevated absolute protein intake of 1.1 vs. 0.7 g/kg BW promoted BW loss while reducing FM during the weight-loss phase. Additionally, we observed that a HPNC diet did not promote BW- or FM‐regain, and improves diastolic blood pressure."
Another study by Kevin Hall and Juen Guo included a systematic review and meta-analysis of diets varying in carbs and fat but equal in protein. It included 32 studies with 563 subjects, and found a very small benefit to the low fat groups, but not enough to be considered significant. Some quotes:
"Figure 2A shows the daily energy expenditure differences between isocaloric diets with equal protein but differing in the ratio of carbohydrate to fat. The pooled weighted mean difference in energy expenditure was 26 kcal/d greater with lower fat diets (P < .0001). Figure 2B shows differences in the rate of body fat change between diets with the pooled weighted mean difference of 16 g/d (P < .0001) greater body fat loss in favor of the lower fat diets (P < .0001). These results are in the opposite direction to the predictions of the carbohydrate-insulin model, but the effect sizes are so small as to be physiologically meaningless. In other words, for all practical purposes “a calorie is a calorie” when it comes to body fat and energy expenditure differences between controlled isocaloric diets varying in the ratio of carbohydrate to fat."
"long-term weight loss diet studies targeting different macronutrients demonstrate similar mean body weight trajectories corresponding to a similar exponential decay of diet adherence with all diets"
Another systematic review and meta-analysis done for patients with type 2 diabetes came out earlier this year. I'll be discussing this one later also so just gonna quote here:
"We found that iso-caloric low to moderate carbohydrate diets and HCD had similar effects on BMI or body weight throughout the trials. This is consistent with recent studies showing that LCD does not increase energy expenditure to a relevant extent."
"No effect is reported in subgroups of diabetic subjects7 8and in meta-analyses of randomized trials in type 2 diabetes."
It's pretty clear there isn't a metabolic advantage to low carbs.
Just for the fuck of it I'll include some more studies, in case anyone isn't convinced. These don't even all match protein, and some are done in diabetics. Normally studies on special populations aren't very applicable to the general population, but in this instance diabetics are a demographic who are said to benefit greatly from low carb diets by low carb advocates, making them relevant.
A randomized trial comparing low-fat and low-carbohydrate diets matched for energy and protein.
"There was no significant difference in weight loss (p 0.509) between the LF and VLC diets."
Long-term effects of a very-low-carbohydrate weight-loss diet and an isocaloric low-fat diet on bone health in obese adults.
"Weight loss was similar in both groups"
Long-term effects of a low carbohydrate, low fat or high unsaturated fat diet compared to a no-intervention control.
"As observed in previous isocaloric trials of up to 6 months, the VLC diet did not result in greater weight loss compared to the other diet groups in the present study. Although it was thought that VLC diets may have some metabolic advantage on weight loss, this present finding adds to the increasing evidence that it may not result in additional weight loss especially over longer periods of time [15,22,23]. However, when reported intakes were compared to weight outcomes, we found that higher reported protein intakes (% energy) were associated with greater weight loss."
Long-term effects of weight loss with a very-low carbohydrate, low saturated fat diet on flow mediated dilatation in patients with type 2 diabetes: A randomised controlled trial.
"After 52 weeks, both groups experienced similar overall reductions in weight (-10.6±0.7kg; p<0.001) and HbA1c (- 1.05±0.10%; p<0.001) with no differential effect of diet"
Metabolic effects of weight loss on a very-low-carbohydrate diet compared with an isocaloric high-carbohydrate diet in abdominally obese subjects.
"Under isocaloric conditions, VLCHF and HCLF diets result in similar weight loss. Overall, although both diets had similar improvements for a number of metabolic risk markers, an HCLF diet had more favorable effects on the blood lipid profile."
Low Carbohydrate Diets and Type 2 Diabetes: What is the Latest Evidence?
"Overall, low carbohydrate diets failed to show superiority over higher carbohydrate intakes for any of the measures evaluated including weight loss, glycemic control, lipid concentrations, blood pressure, and compliance with treatment."
For further reading I highly recommend the ISSN Position Stand on diets and body composition written by Alan Aragon, Brad Schoenfeld, Susan Kleiner and Jose Antonio, among others.
I also highly recommend the new project Adam Tzur, Brandon Roberts and Alex Leaf are working on.
Last but not least The Ketogenic Diet by Lyle Mcdonald is THE book regarding the keto diet and the one I recommend most often for those interesting in a complete overview of it.