There is a growing awareness, and rising use of low-carbohydrate diets for health conditions, most especially metabolic syndrome and related disorders, and increasing interest in their use for cancer treatment (covered in a previous issue of CARR). They are also becoming extremely popular in the mainstream. For example, in the first ¼ of 2019, four of five Amazon best-sellers in the ‘Diet and Weight-loss’ category were based on low-carbohydrate diet principles and a google search for “Low-Carbohydrate Diet” returned over 44,000,000 results. Despite this mainstream popularity, many important areas within this area suffer from a lack of relevant research and there are many misconceptions about the relative merits of low-carb vs high-carb diets.

There is also very conflicting advice as to who should (or should not) use either a lower- or higher-carb diet and who will benefit most from either approach. There has been limited research looking into the ‘appropriateness’ of diets more or less carb-restricted for individuals.

In this article, I summarise the available research to try to get to the bottom of who should use a low-carb diet…and who should stick to a moderate or even higher-carb one?

The relative effectiveness of low-carb vs high-carb

An idea commonly promoted is that over the long-term either low-carb or high-carb diets result in similar results for both weight and fat-loss. This is most likely true (again, based on the evidence available) but, on balance, low-carb might still provide greater benefits for certain outcomes and for many people when compared to high-carb, low-fat nutrition regimens. 

In studies up to 12 months, low-carb diets result in greater weight loss than low-fat diets

In studies up to 12 months, low-carb diets result in greater weight loss than low-fat. They also improve the key markers of future health risk; blood pressure, HDL cholesterol, and most importantly triglycerides (‘fats’ in the blood—probably the strongest indicator of future health risk of any common blood measure.) On the other hand, low-fat interventions result in improved LDL and total cholesterol,¹ which are indicators of future health risk, although in comparison to triglycerides, HDL, and other measures like blood glucose and insulin, are relatively weak. Low-carb diets also result in significant improvements in glycated haemoglobin and fasting glucose, insulin, and c-reactive protein (a key measure of systemic inflammation).²

However, after 12 months, the effect between higher- or lower-carb diets typically narrows, particularly for weight- and fat-loss, but there might be persistent benefits to cardiometabolic health (especially for improvements in triglycerides and HDL) from low-carb diets over these longer timeframes.^3-5

Bueno and colleagues in their systematic review of 13 randomised controlled trials with greater than 12 months follow-ups, did, however, note greater overall weight loss from low-carb vs low-fat diets, along with improvements in cardiometabolic markers of health.⁶

Similarly, Sackner-Bernstein et al., note  [low-carb diets are] associated with modest but significantly greater improvements in weight loss and predicted cardiovascular disease risk in studies from 8 weeks to 24 months in duration.⁷ In a review of 14 body composition change trials up to 2014, Hashimoto et al found that low-carb diets were associated with a significant reduction in body fat but sub-group analysis suggested that the results were limited to very low-carb diets.⁸

On balance, the evidence from these reviews suggest that low-carb diets are likely to be more effective for weight and fat-loss than low-fat interventions in the short-term but over the longer term, the effect between different diets narrows. This further suggests that over the long term the diet that you can stick to is likely to be the best option! BUT if you can stick to a lower-carb (vs higher carb diet) over the long term that there could be slightly greater benefits for future health risk and if you follow a very-low-carb plan, there could be a greater effect on weight and fat-loss.

On balance, low-carb is better for fat loss and weight loss, especially over the short to medium term. It may not be superior to weight loss for insulin sensitive people and athletes though…

Who benefits most from higher-carb and lower-carb?

While reviews of the scientific evidence show that when calories are restricted equally, there is little difference in outcomes (including weight, total or LDL cholesterol, and diastolic blood pressure) for those with Type 2 diabetes over the long-term,^9-13 it is also well demonstrated that the greater the carbohydrate restriction, the greater the glucose-lowering effect,¹² and low-carb diets result in significantly greater improvements in HbA1c (a proxy measure of average blood glucose levels), and cardiovascular disease markers and risk indicators, specifically; HDL, and systolic blood pressure.¹³

In preliminary randomised controlled trials that had been performed, people with relative insulin resistance (IR) respond more favourably to a low-carbohydrate diet, while those people more insulin sensitive (IS) get better results from a higher carbohydrate diet.

People with relative insulin resistance (IR) respond more favourably to a low-carb diet, while those people more insulin sensitive get better results from higher carbs

Four randomised controlled studies have looked specifically at this topic with several others offering tangential evidence. It is worth noting that these studies looked at relative insulin resistance, in other words, comparing those participants who were most insulin sensitive vs those least, not those diagnosed as insulin resistant (who are most likely to have the greatest benefits from a low-carb diet).

Pittas and colleagues demonstrated that those with above-median insulin response (i.e. were more insulin resistant) lost more weight when consuming a low-carb diet compared with a high-carb one (p < 0.05).¹⁴ The reverse was observed in the more insulin sensitive group, who lost more weight following a high carb diet (but this difference was not statistically significant (p = 0.25)).

Similar results were demonstrated in a study comparing obese, nondiabetic insulin-sensitive women to obese, nondiabetic insulin-resistant who were randomised to receive either a high-carb, low-fat diet (60% carbs, 20% fat) or lower-carb, higher-fat (40% CHO, 40% fat). Insulin-sensitive women lost twice as much weight on a high-carb diet compared to low-carb (p < 0.002 between the groups). In contrast, insulin-resistant women lost approximately 50% more weight on the low-carb diet compared to the higher-carb (p < 0.04 between two groups).¹⁵ Likewise, in a 6-month, randomised controlled trial of 73 obese young-adults, a lower-carb diet (40% carbohydrate and 35% fat) was compared to a low-fat (55% carbohydrate and 20% fat) diet. While there was little difference between the two groups overall, those in the low-carb group that displayed higher than average insulin responses to a glucose challenge (i.e. the ‘more’ insulin-resistant participants) had a greater reduction in weight and body fat than those in the low-fat group at 18 months.¹⁶ Similarly, in another study (of 245 overweight and obese women), while there was no significant difference in weight loss between lower and higher carbohydrate diet groups overall, insulin-sensitive participants achieved greater weight loss on a higher carbohydrate diet.¹⁷ A pilot trial to investigate these effects in an ad libitum (eat as much as desired) diet over six-months found increased weight loss resulting from low-carbohydrate diets in insulin-resistant participants and improved weight loss resulting from low-fat diets for insulin-sensitive participants, with these results failing to reach significance. Also noted were (non-significant) improvements in HDL, triglycerides, fasting glucose and insulin, and blood pressure for low-carb diets versus high-carb in those more insulin resistant. In those more insulin sensitive, the low carbohydrate diet improved HDL and triglycerides more than that of the low-fat diet, whereas the low-fat diet resulted in improved fasted insulin and glucose.¹⁸

While not specifically addressing insulin resistance vs sensitivity, an RCT conducted by Tay and colleagues suggested improved outcomes for triglycerides and HDL, from a very low carb diet vs high carb diet, with no change in LDL cholesterol and no difference in weight loss, in a cohort of abdominally obese participants (who are also likely to be insulin resistant).¹⁹ While, a recent study on increased energy expenditure related to baseline insulin homeostasis by Ebbeling et al., ²⁰ suggest that the carbohydrate-insulin interaction is a modifier of dietary outcomes and that lower-carb diets might result in increased ‘burn’ of energy (thereby contributing another element to the fat-loss picture).

Taken on balance, these results suggest that those who are more insulin resistant and those with metabolic syndrome (pre-diabetes) will achieve better results from low-carb diets and conversely, those who are insulin sensitive might actually achieve better results for weight- and fat-loss and for improvement of cardiometabolic risk-markers from higher-carb and lower-fat diets. It is important to note that these studies typically compared only moderate reductions in carbohydrate (with ‘low-carb’ being ~40% of calories from carbohydrate) which most of us would not consider to be ‘functionally’ low-carb and yet even these moderate restrictions resulted in significant improvements for those people who are more insulin sensitive. Because other research (and now our research out of AUT University) has suggested that greater improvements are likely to result from the greater restriction of carbs for these people, more research needs to address this.

What about the DIETFITS study? Didn’t that put the ‘nail in the low-carb coffin’?

While the previous research suggested that people who are more insulin sensitive benefit more from higher-carb diets and those who are more insulin resistant benefit more from lower-carb, the recent DIETFITS study by Gardner et al., cast some doubt on this and was widely reported in the media as the ‘nail in the low-carb coffin’. In fact, the study concluded that there was no significant difference in weight change between a healthy low-fat diet vs a healthy low-carbohydrate diet, and neither genotype pattern nor baseline insulin secretion was associated with the dietary effects on weight loss. ²¹

But does this tell the whole story?…

The DIETFITS study IS a great addition to the body of evidence. It was a large intervention study with over 600 participants. It was well-funded, a randomised controlled trial, and it compared two healthy diets (many studies compare standard poor diets to intervention diets) over a relatively long timeframe (12 months). However, it also used an interesting methodology; beginning with a baseline diet containing either 20 g of fat per day in the low-fat and 20 g of carbohydrate per day in the low-carb group. Participants were then instructed to gradually increase their daily intake of either fat or carbohydrate by 5 g (fat) or 15 g (carbs) per week until they found the lowest level of intake they believed could be maintained. This led to a relatively modest carbohydrate restriction overall in the low-carbohydrate group; consuming  26.5% calories from carbs compared to the low-fat diet group which consumed 50.6%. It should also be considered that the protein intake was modest at less than 1 g protein per kg of body weight per day. So, perhaps it’s fairer to say that DIETFITS compared a healthy lower-fat diet to a healthy moderately carbohydrate-restricted diet and both diets were fairly low in protein. It was also a study in relatively healthy overweight volunteers, and so,  the people who were excluded were exactly those who would be likely to  benefit most from lower-carb i.e. those with “hypertension or metabolic disease; diabetes; cancer; heart, renal, or liver disease”

The low-fat group lost 5.3 kg (95% CI, -5.9 to -4.7), while the low-carb group lost -6.0 kg (95% CI, -6.6 to -5.4). This is a small difference and while it did not meet the threshold for significance (p =0.13) the mean weight-loss in the lower-carbohydrate group was greater than the 95% CI threshold of the lower-fat group. This does suggest that the odds against chance are that there will be greater weight and fat-loss on a lower-carbohydrate diet and particularly so given the relatively modest difference in carbs between the groups. This trend towards greater fat- and weight-loss has been demonstrated in the majority of studies summarised in systematic reviews and meta-analyses, suggesting that there is a greater chance of improved fat-loss overall from carbohydrate-restricted diets. This implication is further strengthened by significantly greater improvements in BMI (0.33, 95% CI 0.01 to 0.64, p = 0.04), along with a greater reduction in waist circumference (0.67 95% CI −0.60 to 1.94), body fat %  (0.18 95% CI −0.40 to 0.75), and blood pressure (0.54, 95% CI −1.07 to 2.16), and improved respiratory quotient (0.020, 95% CI 0.006 to 0.033) resulting from the lower-carb diet compared to the higher-. While these results are, on the whole somewhat equivocal, it is interesting to note that 9 of 13 reported variables were improved more by low carb than low-fat.

Consistent with the existing research, there were significant between-group differences for LDL cholesterol favouring the low-fat group (-2.12, 95% CI -4.70 to 0.47); (low carb: 3.62, 95% CI 1.04 to 6.19). However, both triglycerides and HDL cholesterol were improved significantly more by the lower-carb diet.

There was a nearly 3-fold greater improvement in triglycerides in lower-carb vs low-fat and a 7-fold improvement in HDL for the lower-carb group!

As I have previously mentioned, significant improvements in HDL cholesterol and triglycerides are clinically more meaningful than relatively minor changes in total or LDL cholesterol. Dietary interventions that reduce serum cholesterol levels do not result in significant differences in either cardiovascular disease mortality or all-cause mortality.²² Conversely, increased triglyceride levels show a linear association with a higher risk of cardiovascular disease and all-cause mortality.²³

Overall, the multiplicity of benefits resulting from the lower-carb diet in DIETFITS shows a strong trend towards it being possibly more effective overall for both weight and fat-loss, and more effective for the improvement of the most important predictors of cardiovascular and all-cause mortality. 

Optimal carb intake is likely to be affected by how insulin sensitive vs resistant you are. More insulin-resistant = lower-carb, more insulin sensitive = higher-carb

Can baseline blood measures predict the diet you should follow?

In an upcoming publication, we will present data from a pilot study which suggests a hypothesis that a combination of baseline cardiometabolic indicators, especially triglycerides and HDL cholesterol, have predictive value for which type of diet someone should be on. In essence, the ‘worse’ your baseline lipid profile (i.e. someone in poorer metabolic health), the more likely you are to benefit from a very low-carbohydrate diet, conversely, the ‘better’ your baseline cardiometabolic profile, the more likely you might benefit from a higher-carbohydrate approach. For example, we saw a trend towards greater associations between baseline TG-HDL ratio and improvements in TG-HDL concomitant to the magnitude of carbohydrate restriction; β -0.24, -0.9, and -0.95 for diets containing 25%, 15% and 5% TE from CHO respectively over a 12-week period. This also suggests that as one’s health improves; they will become more tolerant of (and even benefit from) increased levels of carbohydrate in the diet.

It’s likely that the ‘worse’ your baseline triglyceride levels, the more you should restrict carbs.

Could a ‘Carb-Tolerance Questionnaire’ predict whether you should follow a higher or lower-carb diet?

Clinicians often utilise a variety of questions, based on commonly reported effects of poorer ‘carb-tolerance’ like dips in energy after eating a high-carb meal, excessive cravings or desire for sugar and high-carb foods, mood or mental disturbance from higher-carb foods, and self-reported weight gain (especially middle-weight gain) as a result of eating higher-carb foods. But, at this time, there is little research available to support the use of questionnaires to indicate whether a lower- or higher-carbohydrate diet is a ‘best-fit’ for an individual. For example, ‘metabolic type’ has been suggested in books and articles as a predictor of which type of diet someone should follow,²⁴ but a pilot trial of rugby players in New Zealand found that the metabolic type questionnaire results did not match up with laboratory analysis of fat and carbohydrate oxidation rates.²⁵

We recently published the results of a pilot study on a proposed ‘Carbohydrate Tolerance Questionnaire’.²⁶ This questionnaire used the following questions:

• When I gain weight, I tend to put it on my tummy / around my middle
• If I don’t eat regularly / every few hours I suffer energy ‘crashes’, or mood / mental disturbance [i.e. ‘hangry’],
• I crave sweet and/or starchy foods often

Answers were ranked on a 5-point scale of: Not at all, Seldom, Occasionally, Often, Almost always.

Overall, we demonstrated that people with higher baseline ‘Carb Intolerance Scores’ (who we suspected would do ‘worse’ on a higher carb diet) had the best results for improvements in key measures like triglycerides and HDL cholesterol from a low-carb diet intervention. However, results between different low-carb diets (ranging between 25% calories from carbs, down to 5% calories from carbs) didn’t differ significantly. So, although those with worse self-reported responses to carbs tended to do better on a low-carb diet intervention, we can’t be sure at this time whether they benefit most from a greater restriction of carbs, or whether perhaps they simply benefited most from following a good diet because they were most sensitive to higher intakes of ultra-refined and processed foods and/or were more metabolically challenged. In a retrospective analysis, I also found that there was no meaningful or significant association between baseline, self-reported ‘carb-intolerance’ and any body, or blood measurement.

we can’t be sure at this time whether they benefit most from a greater restriction of carbs, or whether they simply benefited from following a good diet

Questionnaires to predict macro allocation require further research.

How does adherence affect which diet you should follow?

It is a common narrative in the mainstream AND scientific media that low-carbohydrate diets are difficult to stick to. However, this is actually not matched by the evidence.

A systematic review of long-term (6-36 month) low-carbohydrate diets vs low-fat, calorie-restricted diet interventions showed an overall attrition rate of 36%, with a higher rate of attrition in low-fat, high-carbohydrate interventions (Figure 1.)³ In these studies, 11 of 13 compared ad libitum lower-carbohydrate (and higher-protein) diets to calorie-restricted higher-carbohydrate diets. The higher adherence rates to low-carb were not likely to be due to simply eating more, as the data favoured improved weight and fat-loss results in the low-carb diets, and improved results for HDL, triglycerides, and blood pressure. Because the participants ‘ate as much as they desired’ and yet achieved better results for fat-loss, this supports the hypothesis that lower-carb, with higher-protein, provides an ‘auto-regulation’ effect that helps people to eat to comfort levels and yet not overeat.

It has also been demonstrated that insulin-resistant people are less likely to adhere and therefore to lose weight on a low-fat diet, compared to insulin-sensitive people. However, adherence and weight-loss were similar between both insulin resistant and sensitive participants allocated to a low-carb diet.²⁷ So, while low-carb is equally well adhered to, low-fat is more difficult to adhere to for those at increased risk of future cardio-metabolic events. Many of the (especially earlier) studies included in reviews and meta-analyses compare calorie-restricted high carbohydrate diets to ad libitum lower carbohydrate diets (such as those by Brehm et al.,²⁸ Ebbeling et al.,²⁹ and others).

insulin-resistant people are less likely to adhere and therefore to lose weight on a low-fat diet

People tend to adhere to low-carb diets better than low-fat, especially if insulin resistant.

Behavioural types and adherence

In a qualitative study arising from a ketogenic diet, controlled trial,³⁰ my colleagues and I reported a finding of very different behavioural patterns which we characterised as ‘abstainers vs moderators’.

Some participants, while having enjoyed the study and the diet, sought advice for how they could moderate the diet post-intervention to make it more flexible, others were happy to continue with a strict compendium of foods (low-carbohydrate) that allowed relative freedom of choice within the compendium, as they felt that if they resumed eating carbohydrate foods, they would not be able to moderate. This could help to explain, conceptually and behaviourally, why different people respond better or worse to moderated diet plans vs inclusion/exclusion strategies.

Behavioural preferences (i.e. ‘abstainers’ or ‘moderators’) affect adherence.

Do activity and sport affect the type of diet I should be eating?

There is a finite amount of carbohydrate able to be stored in the body as glycogen. So, it has commonly been assumed that athletes should eat a high-carbohydrate diet to provide optimal stores of fuel for activity, especially intense ‘glycolytic’ (carb-burning) exercise.

Louise Burke, a renowned nutrition leader, famously called the ‘nail in the coffin’ for low-carb and performance several years ago and recently walked that back in response to growing anecdotal and clinical evidence that at least some athletes benefit from low-carbohydrate diets and that newer modifications to these diets might be appropriate for performance.³¹

Overall, the evidence shows little if any detriment to strength and power athletes from lower-carb regimens,³² while studies have noted no-change or slight decrements in performance in endurance athletes over relatively short time-frames (i.e. circa 10-12 weeks), but with improvements in wellbeing, inflammation, and body composition.³³ Stephen Phinney and others have suggested that given time for adaptation, optimised sodium and potassium provision and sufficient fat intake to provide for energy-sufficiency, there should be unimpaired endurance performance even if in ketosis resulting from a very-low-carb approach.^{34, 35}

Are glycogen levels depleted in low-carbohydrate diets?

Reductions in glycogen have been noted in short-term studies on low-carb and keto diets,³⁶ but more recently in a long-term (20 month) study comparing a high carb (~60% calories from carbs) vs low-carb (~10% calories from carbs) diet in ultra-endurance athletes fat oxidation was higher overall and persisted at higher intensities of exercise [(figure X)] AND most importantly there were no appreciable differences in either resting muscle glycogen or the level of depletion after 180 min of running (− 64% from pre-exercise) and 120 min of recovery (− 36% from pre-exercise). The authors concluded, “Compared to highly trained ultra-endurance athletes consuming a high-carbohydrate diet, long-term keto-adaptation results in extraordinarily high rates of fat oxidation, whereas muscle glycogen utilization and repletion patterns during and after a 3-hour run are similar.”³⁷

Metabolic efficiency

Metabolic efficiency refers to the ability to use different fuels to maximise the ability of the body to efficiently perform during activities of differing intensities. Low-carb diets even without supplementation of either carbohydrate or ketones might be appropriate for recreational athletes or those involved in low-volume sports or training. For elite athletes, especially endurance athletes, periodised training with low carbohydrate diets, combined with carbohydrate and/or ketone supplementation is likely to be most appropriate.³⁸ The effect of a ketogenic diet in athletes is equivocal and performance benefits vary substantially.³⁹

Low-carb diets can result in equal glycogen storage HOWEVER all athletes, whether high or low-carb can deplete glycogen stores and high volumes of activity benefit from the addition of additional fuel overall, and additional carbohydrate (i.e. during and after long exercise bouts) to fuel benefit exercise.

Summary and conclusions

The evidence considered ‘as a whole’ shows that low-carb is likely to be superior for fat loss over low-fat diets. However, this may not apply to those who are very insulin sensitive and to athletes who are likely to be both insulin-sensitive and very active. On the other hand, muscle gain and retention benefit from the addition of carbohydrate to the diet.

Low-carb diets are also likely to be superior for the improvement of markers of future health risk, especially for those who are insulin resistant and who have ‘worse’ measures at baseline (such as poor glucose control or poor blood lipid readings).

Adherence might be better for low-carb over low-fat and this is especially true for those who are insulin resistant. Behavioural and psychosocial factors are also likely to play a key role in being able to comply with any diet too.

On balance, the best diet for YOU is still one that is calorie and protein sufficient, nutrient-dense, and that YOU can stick to over the long term. Over time, any differences between otherwise healthy diets are small and the biggest variations in results people derive from diet have a greater tie to behaviours, mindset, and the psychology of eating.

On balance, the best diet for YOU is still one that is calorie and protein sufficient, nutrient-dense, and that YOU can stick to over the long term

It’s important to recognise that there is far more than just physical measures of outcomes too. How you feel on a particular diet and how it works within your psychosocial environment are also critically important. To paraphrase words from my friend and colleague, Professor Grant Schofield; “At the end of the day, the most important thing for most people is how they feel”. How someone feels is not only important for their sense of wellness, satisfaction, health, and happiness in the moment, but will also likely affect adherence to a diet that will help them reduce their risk of future ill-health, and thus, has important implications for societal health. That’s why differing diets have such avid devotees, because they have found something (for better, or for worse) that works for them, and for them, the n = 1 is all that matters!

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