Cutting back on carbs is the norm for most people to lose body-fat. I am not opposed to cutting back on carbs such as a moderately restricted carb diet, but I am not in favor of ketogenic diets. Indeed, many people who try low-carb dieting are initially pleased by an immediate weight loss, which is mostly water and glycogen. So, in the short term, it seems like low-carb diets are superior.
But does long-term evidence support low-carb dieting?
Over the long haul, any differences between low-carb and other diets even out. One of the biggest downsides to low carbohydrate diets is that they impact training intensity. Unfortunately, low-carbohydrate diets are incapable of replenishing the glycogen depleted during high volume exercise.
Without sufficient carbohydrate to replenish both glycogen stores and maintain sufficient blood glucose, cortisol will be secreted in an effort to boost blood glucose levels through muscle breakdown and amino acid oxidation.
Low-Carb Impairs Strength Recovery After a Workout
Researchers wanted to study the impact of low carb diets on eight subjects (4 males, 4 females) were randomly assigned to a low carbohydrate (3.4 g/kg), higher protein (1.5 g/kg) diet, or a high carbohydrate (5.0 g/kg), lower protein (1.2 g/kg) diet. Both diets exceeded the recommended daily allowance (RDA) for protein. The mean weight of the subjects was 70 kilograms, so the high-carbohydrate diet consisted of 343 grams of carbohydrate, 85 grams of protein, and 62 grams of fat.
The low-carbohydrate-higher protein diet was composed of 226 g carbohydrate, 103 g protein, and 67 g fat. The diets supplied enough calories for each subject to maintain weight.
The diets were followed for five days, and then the subjects performed an eccentric exercise bout to induce muscle damage. After eccentric exercise, the researchers measured muscle soreness, creatine kinase (a marker for muscle damage), isometric strength, nitrogen retention, and whole-body protein metabolism.
The low-carbohydrate diet produced a greater strength loss and lower creatine kinase after exercise when compared with the high-carbohydrate diet. In addition, the high-carbohydrate group experienced a reduced strength loss at 24 hours post-exercise when compared with the low-carbohydrate group (8.1% versus 15.5%). This reduced strength was maintained throughout the study and averaged 28% in the low-carbohydrate group and 8% in the high-carbohydrate group on day four of recovery. The low-carbohydrate group also had a reduced protein turnover, synthesis, and breakdown during recovery.
This study suggests that a diet high in carbohydrate (at half of total calories), when protein exceeds the recommended daily allowance, will increase whole body protein synthesis and reduce muscle strength loss and enzymatic activity during recovery from eccentric exercise. Therefore, dietary carbohydrate, as opposed to protein, may be the more important nutrient when the novice weight lifter is recovering from muscle damage. Finally, the increase in dietary carbohydrate must be at least 5 – days in length and be accompanied by a protein intake above the RDA in order to be effective.
One of the other things carbohydrates are necessary for is: Carbohydrates influence testosterone production. Here are a few studies, which examine how carbohydrates impact testosterone levels:
Higher Carbohydrates Lead to Higher Testosterone Levels
Researchers examined the change in protein/carbohydrate ratio and how it influences plasma steroid hormone concentrations. The groups ate a high-carb low-protein diet, whereas the other group ate a high-protein low-carb diet. Fat intake and calories were identical. Testosterone concentrations in seven normal men were consistently higher after ten days on a high carbohydrate diet than during a high protein diet and were accompanied by parallel changes in sex hormone binding globulin. By contrast, cortisol concentrations were consistently lower during the high carbohydrate diet than during the high protein diet and there were parallel changes in corticosteroid binding globulin concentrations. These consistent and reciprocal changes suggest that the ratio of protein to carbohydrate in the human diet is an important regulatory factor for steroid hormone plasma levels and for liver-derived hormone binding proteins.
Glucose is needed for GnRH Release
Gonadotropin-releasing hormone (GnRH), is a hormone that begins the starts the reproductive process that eventually leads to testosterone synthesis. GnRH stimulates the leydig cells to produce testosterone and GnRH is higher when glucose is plentiful; it adjusts its pulsation rate according to the glucose levels of the body. The reproductive system must sense changes in bodily energy status to prevent reproduction during times of food scarcity, and to take advantage during times of plenty. Administration of glucose restored LH pulsatility in insulin-induced hypoglycemic rats and sheep, suggesting that low glucose rather than high insulin mediates the suppression of LH. In addition to the negative effect of reduced glucose, increased glucose may positively influence GnRH/LH secretion.
Carbohydrates Affect Testosterone/Cortisol Ratio
This study examined the effect of dietary carbohydrate consumption on the free testosterone to cortisol ratio during a short-term intense micro-cycle of exercise training. The free testosterone to cortisol ratio is a proposed biomarker for overreaching–overtraining (i.e., training stress or imbalance) in athletes. The ratio was studied in two groups, control-carbohydrate (~60% of daily intake) and low-CHO (~30% of daily intake), of male subjects who performed three consecutive days of intensive training (~70–75% maximal oxygen consumption, 60 min per day) with a dietary intervention (on the day before and during training). Resting, pre-exercise blood samples were collected under standardized-controlled conditions before each day of training (Pre 1, 2, 3) and on a fourth day after the micro-cycle (Rest). Bloods were analyzed for free testosterone and cortisol. Subjects performed no additional physical activity other than prescribed training. At the end of the study, free testosterone to cortisol ratio decreased significantly from pre-study resting measurement to the final post-study resting measurement (Rest) in the low-carbohydrate group by 43%, but no change occurred in the control-carbohydrate group. Resting cortisol levels increased significantly in the low-carbohydrate group, rising from 24.1 ng/dL at baseline to 27.6 ng/dL at the end of the study. No change in resting cortisol was seen in the high-carbohydrate group. Findings suggest if the free testosterone to cortisol ratio is utilized as a marker of training stress or imbalance it is necessary for a moderately high diet of carbohydrate to be consumed to maintain validity of any observed changes in the ratio value.
Low-carb Diets Impair Muscle Growth Pathways
In addition to testosterone productions, low carbohydrate diets also seem to impair muscle growth factors. Researchers have also recently found that depleted glycogen can also impair muscle growth signaling pathways as well.
Eight experienced male cyclists underwent two trials, a low carbohydrate and a high carbohydrate trial. On Days 1 and 2, they performed exhaustive leg and arm cycling workouts to deplete muscle glycogen. Early in the morning of Day 3, they arrived at the lab following an overnight fast and performed 3 sets of 10 repetitions of leg extensions with 2 minutes’ rest between sets.
During the two-day period leading up to the experimental trial, subjects were fed an isocaloric (mean intake of 5200 calories) diet containing 18% protein/80% fat/2% carbohydrate, or 13% protein/7% fat/80% carbohydrate. This equates to a daily carbohydrate intake of 26 grams during the low-carb trial and 1042 grams during the high-carbohydrate trial.
Pre- and post-exercise leg muscle glycogen was far higher in the high-carbohydrate trial than in the low-carbohydrate trial, while intramuscular triglyceride concentration was 40% higher in the low-carb when compared to the high-carb trial prior to exercise. Akt phosphorylation (i.e. a critical regulator of muscle hypertrophy) was similar in both groups prior to exercise and immediately post-exercise.
After ten minutes of recovery, Akt activity increased 1.5 fold in the high-carbohydrate trial only. During the low-carbohydrate trial, Akt activity remained unchanged at all time points.
The researchers concluded that due to this lack of Akt response, “adaptations to an acute bout of exercise may be blunted”.
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Lane AR, et al. Influence of dietary carbohydrate intake on the free testosterone: cortisol ratio responses to short-term intensive exercise training. European Journal of Applied Physiology, 2010; 108: 1125–1131.
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