Metabolic Effects of Total Fasting Suggest: You Better Eat "Nothing" Than "Some" If You Want to Make Weight - The Metabolic Adaptation to Dieting is Yet Not All That Counts

It is hard and it will cost you muscle, but dieting as in simply eating nothing for a few days won't kill your metabolism and it will shed a lot of weight (assuming you're as obese as the subjects in a new study).

Common sense dictates: The more you reduce your energy intake, the more your resting and total energy expenditure will decline. That's a normal adaptational process, right? Right, at least in the short run, however, the equation isn't that easy.

Scientists from the from the Newcastle University just published the results of an interesting experiment, in which they aimed to investigate whether three groups of obese men, exposed to different levels of negative energy balance (fasting, very low calorie diet (VLCD, 2.5MJ/day) and low-calorie diet (LCD, 5.2MJ/day)) in experimental controlled conditions, were characterised by distinct changes in resting and total EE after losing a similar amount of body weight (5% and 10%WL).

As the scientists point out, "[t]he study also provided the opportunity to test if the rate of WL and weight lost as FFM [fat free mass] were associated with the level of adaptive thermogenesis" (Siervo. 2015). The significance of the results for athletes and wanna-be athletes should thus be obvious.

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To collect the necessary data, the scientists recruited 18 obese men who were randomly assigned to one out of three groups. Group 1 did a total fast for 6 days (=no food!). Group 2 was meant to achieve a 5% weight loss in 3 weeks on a very low calorie diet containing only 2.5MJ/day (that's only 597.51kcal/day). Group 3 had an "easier" (?) job, as they were meant to lose 10% of their body weight in six weeks during which they consumed a low calorie diet containing 5.2MJ/day (that's 1 242.83 kcal/day).

Figure 1: Macronutrient intake in grams in the 3 diet groups (Siervo. 2015)

"[D]uring the 6-day baseline period subjects consumed a fixed maintenance diet (13% protein, 30% fat and 57% carbohydrate). After the 7-day baseline period, each group followed the specific diet to lose 5% and 10% of their baseline body weight. However, the duration of the fasting was of 6 days as ethical constraint allowed to fast subjects to lose 5% of their baseline body weight. The duration of the WL phases to achieve a 10%WL was of 3 and 6 weeks for the VLCD and LCD groups, respectively. Throughout the study, participants were residential in the Human Nutrition Unit at the Rowett Institute of Nutrition and Health (RINH), Aberdeen, UK.

All food and drinks consumed by each participant during the study were supplied by the dietetics staff in the Unit. The participants were requested not to undertake any other strenuous physical activity during the study and they were asked to record their individual exercise sessions" (Siervo. 2015).

The actual energy intake (EI) of the subjects was measured daily. While the participants in the starvation group had access to water only, the diets of the other groups contained 32% of the energy as protein, 35% as carbohydrate and 33% as fat. More specifically,

• the LCD weighed 1260g, with an energy content of 5.2kJ/g coming from protein 50.3g (17%), carbohydrate 155.7g (50%), and fat 45.4g (33%), while
• the VLCD weighed 642g, with an energy concent of 2.55kJ/g coming from protein 49.4g (32%), carbohydrate 52.8g (35%), and fat 23.1g (33%) and was thus - in science terms - a high "protein diet", because it contained >30% of the energy from protein.

The resting energy expenditure (REE) was measured at baseline and at the end of each WL phase (5% and 10%WL) by indirect calorimetry over 30–40 min using a ventilated hood system (Deltatrac II, MBM-200, Datex Instrumentarium Corporation, Finland).

Figure 2: Changes in body composition in the three diet groups according to weight loss (Siervo. 2015).

If we take a look at the changes in body composition in Figure 1, we'll see that the Weight loss in the fasting group was 6.0 kg over 6 days. The VLCD group, who dieted far longer, lost 5.2 and 9.2kg over 11 and 21 days and the LCD group lost 7.2 and 12.6 kg over 21 and 42 days, respectively. Thus, the "[m]ean rates of WL during the 5% WL period were different between the fasting (-1.01 kg/d), VLCD (-0.52 kg/d) and LCD (-0.35 kg/d) groups" (Siervo. 2015).

Women watch out! It is not just possible, but in view of the association between the magnitude of daily energy deficit and the frequency of menstrual disturbances (Williams. 2015), women may see significantly more more side effects on harsh (fasting) diets.

What is far more important, though, is the fact that the allegedly "sanest" way of dieting, i.e. the LCD (=moderate deficit), produced the most significant fat mass loss. Accordingly, the "slow" diet had the upperhand in terms of fat free mass (FFM) losses, as well:

"The fraction of FFM to total WL after 5%WL was 46, 30 and 18% for the fasting, VLCD and LCD groups respectively. At 10% WL, the VLCD losses were 20% FFM and 80% FM compared with 9% FFM and 91% FM in the LCD group (Siervo. 2015).

Against that background it's quite surprising that (a) the VLCD and LCD showed a similar degree of metabolic adaptation for total EE (VLCD=-6.2%; LCD=-6.8%) and that (b) the metabolic adaptation for resting EE was greater in the LCD (-0.4MJ/day, -5.3%) compared to the VLCD (-0.1MJ/day, -1.4%) group.

Likewise noteworthy: The resting EE did not decrease after short-term fasting and no evidence of adaptive thermogenesis (+0.4MJ/day) was found after 5%WL. The rate of WL was inversely associated with changes in resting EE (n=30, r=0.-42, p=0.01).

Figure 3: Metabolic Adaptation - Percent of total and resting energy expenditure not accounted by changes in body composition (FFM and FM) after 5% and 10% weight loss (WL) in obese assigned to three different WL interventions (Siervo. 2015).

So what to make of these study results? If you scrutinize the results in Figures 2 & 3 and, most importantly, the ratio of fat free mass to fat mass loss, it's quite astonishing that the study confirms the common wisdom that slow weight loss will allow for a greater fat-specificity than fast weight loss. After all, the subjects losing 5% of their total body weight lost 88% of it in form of lean muscle (I wonder how much glycogen and thus water loss this was) in the total fast group, 42% in the very low calorie diet group (VLCD) and only 20% in the "moderate" = low calorie diet group. In this context, it's also noteworthy that "biggest losers", i.e. those who lost weight most successfully (cf. Tremblay. 2013) and achieved a 10% weight loss in the VLCD group had a sign. better fat free / fat mass ratio (23% vs. 42%) than those who managed to lose only 5% in three weeks.

Eventually, it may thus seem that the study at hand would confirm what we already know: Slow and steady is best, ... and that's true, but the fact that "slow and steady" produces the greatest reduction in resting metabolic rate makes me question whether the weight rebound after longer, but less severe dieting phases is actually smaller or not. Previous studies suggested there's no difference, but these studies used different protocols and stand in contrast to a plethora of studies like Sénéchal et al. (2012 | learn more). Whether it would be the same for the study at hand is thus questionable | Comment on Facebook! 

References:

• Siervo, Mario, et al. "Imposed rate and extent of weight loss in obese men and adaptive changes in resting and total energy expenditure." Metabolism (2015): Accepted Article.
• Sénéchal, Martin, et al. "Effects of rapid or slow weight loss on body composition and metabolic risk factors in obese postmenopausal women. A pilot study." Appetite 58.3 (2012): 831-834.
• Tremblay, A., et al. "Adaptive thermogenesis can make a difference in the ability of obese individuals to lose body weight." International journal of obesity 37.6 (2013): 759-764.
• Williams, Nancy I., et al. "Magnitude of daily energy deficit predicts frequency but not severity of menstrual disturbances associated with exercise and caloric restriction." American Journal of Physiology-Endocrinology and Metabolism 308.1 (2015): E29-E39.