Men & women & dieting

Men & women & dieting

Executive summary

* An article was published from the US DIETFITS study. This study randomised 609 men and women to either a healthy low-carb (HLC) or healthy low-fat (HLF) diet and examined weight loss and other measures over a 12-month period.

* The diet started off genuinely low carb and low fat for 8 weeks – 20g of carbohydrate or fat daily on each diet – but then participants were advised to move to the level of carb or fat intake that they believed they could sustain for the trial.

* By the end of 12 months, the low-carb dieters averaged 30% carb intake; by the end of 12 months, the low-fat dieters averaged 30% fat intake. Both groups had tended back to more of a ‘balanced’ diet.

* This study found that HLC men lost more than HLF men and HLC men lost more than HLC women. Plausible explanations were given for this.

* The study also examined adherence to each diet. Men doing the HLC diet were the most adherent and women doing the HLC diet were the least adherent. The paper adjusted for adherence and found that the difference in weight loss wasn’t explained away by adherence.

* There were other interesting findings in the paper beyond the outcomes chosen by the researchers. Calorie intake fell across all groups (to between 75-80% of baseline calories). Despite this, the average weight loss was barely a pound (less than half a kg) a month.


A study was published in the International Journal of Obesity this month (January 2021). It was called “Examining differences between overweight women and men in 12-month weight loss study comparing healthy low-carbohydrate vs. low-fat diets” (Ref 1). Healthy low-carbohydrate will be abbreviated to HLC and healthy low-fat to HLF throughout this note.

The paper was from a trial that we first looked at almost two years ago in February 2018 (Ref 2). The trial is called DIETFITS. This acronym comes from “Diet Intervention Examining The Factors Interacting with Treatment Success.” The paper we reviewed two years ago looked at the concept of dieting and genotype – was there a right diet for different people? The answer was “no” to that research question, but the trial has been re-examined for another research question.

The objectives of this latest study were two-fold. The first aim was to compare 12-month changes in weight, fat mass, and lean mass between the four groups (men HLC, women HLC, men HLF, women HLF). The second aim was to assess whether or not adherence differed by sex and the diet to which people were assigned.

The study

The DIETFITS study started in January 2013. Between that date and April 2015, 609 adults, aged 18 to 50 years (average age 38-40), were enrolled into the trial. The adults had a BMI in the range 28-40 (average BMI 33), but didn’t have other conditions such as diabetes, cancer, heart disease etc. 57% of the participants were women. The final follow-up date was May 2016.

Participants were randomly assigned to a 12-month diet described as healthy low-fat (HLF) or healthy low-carb (HLC). The numbers allocated to each diet group by sex were as follows: HLC women n =179, HLC men n = 125, HLF women n = 167, HLF men n = 138. (By the end of the trial, the numbers in each group were HLC women n =132, HLC men n = 92, HLF women n = 127, HLF men n = 98, meaning that 24%-29% of people did not complete the 12-month trial). Both groups participated in 22 small group sessions during the 12-month study (covering diet and exercise advice, emotional awareness, behavioural modification, cognitive change etc), so the intervention was quite intensive.

The random allocation of people to diet groups achieved very low differences in characteristics between the four groups (sex/diet). There were no significant differences between the four groups for age, education, ethnicity, BMI, and smoking. Weight was different, as would be expected between men and women, but average BMI wasn’t. Because weight and body fat percentage were two key measurements in the trial, they were adjusted for.

The diet

The main dietary goal was described as “to achieve maximal differentiation in intake of fats and carbohydrates between the 2 diet groups while otherwise maintaining equal treatment.” During the first eight weeks, the participants limited their daily fat or carbohydrate intake to 20 grams.

The diet specific education sessions focused on foods to be avoided to optimise each diet i.e. to maintain the maximal differentiation between fat and carb intake in the two diet groups. The low-fat group were advised to avoid oils, fatty meats, whole-fat dairy, and nuts, for example. The low-carb group were advised to avoid cereals, grains, starchy vegetables, and legumes.

After 2 months, participants gradually increased their daily carbohydrate or fat intake by 5-15 grams per week until they reached an intake that they believed they could maintain indefinitely. The goal of this phase was to find the lowest intake of fat or carbohydrates that each participant could maintain for the 12-month intervention period. No explicit calorie instructions were given. Both diet groups were advised to maximise vegetable intake and to minimise intake of added sugars, refined flours and trans fats. Both diet groups were also advised to choose minimally processed, nutrient-dense, foods and to prepare food at home wherever possible. Participants were also encouraged to follow US physical activity recommendations (Ref 3).

The results

For the primary aim (comparing 12-month changes in weight, fat mass and lean mass between the four groups), results were presented as relative to other diets, rather than absolute. These results were adjusted for differences in baseline body weight and body fat percentage. The relative results were summarised as HLC produced significantly greater weight loss and loss of fat and lean mass than HLF among men but not among women.

(Following correspondence, the researchers have very kindly shared all raw data with me. I am interested in: i) weight loss as a percentage of starting weight – this would enable comparison between the four groups without need for adjustment; ii) did the people who stuck closer to the original 20g diet instruction fare better overall? Those are just two research questions of interest; some may arise from looking at the data. I will do a follow-up note if the data reveal anything interesting.)

Among men, the HLC diet achieved, on average, 2.98kg higher weight loss than men following the HLF diet. It also resulted in, on average, 1.51kg greater fat mass loss and 1.33kg greater lean mass loss (the latter is generally not seen as positive) (Note 4). Among women, there were no significant differences between HLC and HLF for weight loss, fat mass loss or lean mass loss. Women lost, on average, approximately 5kg in weight during the 12 months on either diet.

Comparing the sexes, HLC men lost significantly more weight (2.32kg on average) than HLC women. The loss of weight, fat mass, and lean mass were not significantly different between HLF women and HLF men.

For the second aim, adherence was calculated based on deviation from the initial 20g of fat or carbohydrate and the grams of each macronutrient the person was consuming at 3, 6 and 12 months. Overall differences in adherence by diet-sex group were not significantly different. However, HLC men and HLC women were the most and least adherent of all groups, respectively. The difference in adherence to HLC between men and women was statistically significant.

It might be argued that men did better on HLC because adherence to that diet was better. The researchers examined this hypothesis by adjusting for adherence. Their conclusion was: “Weight loss estimates within group remained similar after adjusting for adherence, suggesting adherence was not a mediator.”

Before we look at why men did better than women on low carb (and touch on why low-carb men did better than low-fat men), there were a few other interesting observations from the study:

Calorie and macronutrient intake

Table 2 in the paper reported calories, carbohydrate, fat, and protein intake at baseline, at 3 months, at 6 months and at 12 months. This provided a comprehensive review of what was consumed by each of the four groups during the study. The most interesting findings were:

1) All groups reduced calorie intake while following their allocated diet.

After 3 months, calorie intake was approximately 70-71% of what it had been at baseline. After 12 months, calorie intake was 75-80% of what it had been at baseline. Calorie intake increased between 3 and 12 months of the trial, but it stayed below four fifths of what it had been at the start. (This is shown in the table below – with the calories extracted from Table 2 in the paper and the percentage of these calories from baseline calculated by me.)

2) The 3,500 calorie theory would have concluded that those calorie deficits should have led to between 49lb (HLF men) and 65lb (HLC men) being lost across the groups (Note 5). That would also be fat alone with more on top from water and lean mass. The actual average weight loss across all groups was nearer 11lb (and that includes fat, water, and lean mass). That’s interesting to reflect on in itself. Participants in a diet trial – who had more incentive and help than the average individual dieter to stick to the diet and do well – lost, on average, about a pound a month for 12 months. There are not many dieters who would consider that a good result.

3) The low-carb groups started off consuming an average of 237g/day (women) and 261g/day (men). They were instructed to stick to 20g/day of carbohydrate for 8 weeks and then to move to what they thought they could maintain. By 3 months, women were averaging 99g/day and men 93g/day. This had increased to 129g/day and 137g/day for women and men respectively at 12 months. Both groups were close to the definition of moderate carb, not low carb, by the end of the trial (Ref 6).

4) The low-fat groups started off consuming an average of 79g/day (women) and 96g/day (men). They were instructed to stick to 20g/day of fat for 8 weeks and then to move to what they thought they could maintain. By 3 months, women were averaging 41g/day and men 44g/day. This had increased to 54g/day and 61g/day for women and men respectively at 12 months. The low-fat groups stayed lower in fat grams, on average, than the low-carb groups did in carb grams.

5) The typical macronutrient composition across both sexes and both diet groups at the start of the trial was 45% carbohydrate, 36% fat and 18% protein (rounding errors).

The typical macronutrient composition at the end of the low-carb diet (averaging both sexes) was 30% carbohydrate, 45% fat and 23% protein. The typical macronutrient composition at the end of the low-fat diet (averaging both sexes) was 49% carbohydrate, 29% fat and 21% protein.

The macronutrient data provided in grams revealed that grams of carbohydrate and grams of fat fell from baseline to 12 months in every group. i.e. carb intake in grams fell in the low-carb and low-fat groups and fat intake in grams fell in the low-carb and low-fat groups. Grams of protein also fell in every group except men following low-carb diets where grams of protein increased fractionally from 104 to 106 g/day. This is perhaps not surprising given that calorie intake dropped so much across all groups. Inevitably if calories are 20-25% lower, grams of all macronutrients are lower.

Why might men do better than women?

Men tend to lose more weight on any diet than women because, on average, they have more weight to lose. They have greater body size, higher muscle-to-fat mass ratio, and higher resting and total energy expenditure. Additionally, women tend to have done more diets than men and have been dieting over a longer period of time and there is evidence that subsequent diets are less successful than the first one. The women in this study are likely to have undertaken, on average, more diets during their lifetime than the men in this study.

The paper offered a few explanations as to why men might do better on low carb than women. The paper gave 12 references for large population studies indicating that women express a greater preference for low-fat foods and a greater concern about high-fat foods than men. Because women have been dieting more and for longer than men, they are likely to have been indoctrinated with the view that “fat makes you fat” for longer than men. Women may find it more difficult than men to embrace a diet that embraces fat.

Those were the themes explored in the paper. I have a few more thoughts. In our experience of running a diet forum for over 10 years, women tend to have a more emotional relationship with food. This is a sweeping generalization and not intended to dismiss any struggles that men have with their weight, but, in our experience, men eat food – women have a relationship with food. This may make it more difficult for women to adhere to a diet and lose weight generally. It may also make it more difficult for women to adhere to a low-carb diet (as was shown in this study). Comfort/emotional foods are carbohydrates. One does not turn to a hard-boiled egg for comfort – one turns to cake, chocolate, biscuits, muffins and so on.

Interestingly, the founders of the best known low-fat diets – Weight Watchers (Jean Nidetch), Slimming World (Margaret Miles-Bramwell), Rosemary Conley (her own diet), Lighter Life (Jackie Cox) – are female. The leading Paleo/low carb pioneers are male – Robb Wolf, Loren Cordain, Eric Westman, Gary Taubes, Arthur de Vany, and Mark Sisson.

Men – again a generalization from our experience – tend to diet for different reasons. Women are more typically driven by a ‘gain’ motivation – they want to look and feel better. Men are more typically driven by a ‘pain’ motivator – they want to avoid type 2 diabetes or need to do something after a health scare. Pain is a far more effective motivator than gain. Those with a pain motivation invariably achieve better results.

The hormonal issue is critical too. If one believes, as I do, that hormones are a significant factor in weight and weight loss, then the participants in this study (with an average age of 38-40) were not on a level playing field. While the male hormones would have stayed relatively stable over the year, the female hormones would have followed large cyclical swings each month. Women know that the time of the month is the toughest time to stick to a diet. The average woman faced this toughest time 12 times over the year. The men faced nothing similar.

Closing thoughts

This was another interesting paper from a valuable and well-designed and executed study. The headlines were that men lost more on a low-carb diet than men on a low-fat diet and men on a low-carb diet lost more than women on a low-carb diet. The differences may have been statistically significant, but they weren’t large – a couple of kilos over 12 months. There were plausible explanations for these observations.

Perhaps more interesting takeaway messages were:

i) even taking part in a clinical trial, with all the support that this brings, approximately one quarter of participants dropped out of the trial and those remaining lost about one pound (less than half a kilogram) a month.

ii) dieters who started on genuinely low-carb and low-fat diets moved to moderate carb and moderate fat diets when advised to move to intakes that they felt they could sustain.

It could be the case that very low-carb and very low-fat diets achieve the best weight loss, but that both extremes are not adhered to well enough and for long enough for this to happen.

Until the next time

All the best – Zoë


Ref 1: Aronica et al. Examining differences between overweight women and men in 12-month weight loss study comparing healthy low-carbohydrate vs. low-fat diets. Int J Obes (Lond). 20
Ref 2:
Ref 3: Physical Activity Guidelines Advisory Committee, US Office of Disease Prevention and Health Promotion. 2008 physical activity guidelines for Americans.
Current US recommendations for adults are to do one of the following:
– 150 minutes (2 hours and 30 minutes) each week of moderate-intensity aerobic physical activity (such as brisk walking or tennis)
– 75 minutes (1 hour and 15 minutes) each week of vigorous-intensity aerobic physical activity (such as jogging or swimming laps)
– An equivalent combination of moderate- and vigorous-intensity aerobic physical activity.
Note 4: HLC induced significantly greater weight loss than HLF [-2.98 kg (95% CI -4.47 to -1.50); P < 0.001]. This was also observed for fat mass [-1.51 (95% CI -2.79 to -0.23); P = 0.02], and, for lean mass [-1.33 (95% CI -1.97 to -0.68); P < 0.001] (Fig. 1). Note 5: – I did the calculations by taking the calorie intake at 6 months as a guide of the average for the year. Subtracting this from baseline calories gave the average daily deficit. This multiplied by 365 (days) and divided by 3,500 (the calorie theory) gives pounds that should be lost – in fat alone – according to the theory. Ref 6: Noakes TD, Windt J. Evidence that supports the prescription of low-carbohydrate high-fat diets: a narrative review. Br J Sports Med 2017.

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